EUSKAL HERRIKO UNIBERTSITATEA/
UNIVERSIDAD DEL PAÍS VASCO
Zien zia e a Teknologia Depa amen ua/
Facul ad de Ciencia y Tecnología
S udy o Li me al anode su ace: in e ac ion
wi h a mosphe ic gases and impac o
impu i ies in elec ochemis y
Feb ua y 2020
Ane E xeba ia Dueñas
Thesis Supe iso : D . Miguel Ángel Muñoz Má quez
EUSKAL HERRIKO UNIBERTSITATEA/
UNIVERSIDAD DEL PAÍS VASCO
Zien zia e a Teknologia Depa amen ua/
Facul ad de Ciencia y Tecnología
CIC ENERGIGUNE
S udy o Li me al anode su ace: in e ac ion
wi h a mosphe ic gases and impac o
impu i ies in elec ochemis y
Feb ua y 2020
A disse a ion submi ed o he Uni e si y o he Basque Coun y
in pa ial ul illmen s o he equi emen s o he deg ee o Ph.D.
Ane E xeba ia Dueñas
Thesis Supe iso : D . Miguel Ángel Muñoz-Má quez
UPV/EHU Tu o : D . F ancisco Ja ie Zúñiga Laga es
(cc)2020 ANE ETXEBARRIA DUEÑAS (cc by 4.0)
‖ iii
Con en s
Con en s
Acknowledgemen s --------------------------------------------------------------------------- ix
Abs ac /Resumen/Labu pena ------------------------------------------------------------ xi
1. In oduc ion ------------------------------------------------------------------------------- 1
Mo i a ion ------------------------------------------------------------------------------ 1
Li-ion echnology ---------------------------------------------------------------------- 3
1.2.1 Li-ion echnology basics ---------------------------------------------------------- 3
1.2.2 Li-ion ba e y main componen ma e ials ----------------------------------- 5
1.2.3 Solid Elec oly e In e phase (SEI): a key pa ame e ----------------------- 8
Nex gene a ion Li me al ba e ies (LMB): ole o Li me al --------------- 10
1.3.1 Li-sul u and Li-ai ba e ies --------------------------------------------------- 11
1.3.2 Li me al su ace ins abili y ----------------------------------------------------- 13
1.3.3 A i icial solid elec oly e in e phases o Li me al anodes ------------ 16
1.3.4 Conside a ions o Li me al-based ene gy demand ---------------------- 19
Scope o he hesis ----------------------------------------------------------------- 22
2. Expe imen al echniques ------------------------------------------------------------- 23
Thin ilm g ow h --------------------------------------------------------------------- 23
2.1.1 The mal e apo a ion ----------------------------------------------------------- 23
2.1.2 Magne on spu e ing ---------------------------------------------------------- 25
2.1.2.1 Spu e ing ins umen --------------------------------------------------- 25
Su ace modi ica ion --------------------------------------------------------------- 26
i ‖
2.2.1 Ion bomba dmen --------------------------------------------------------------- 26
2.2.1.1 Ion sou ce ins umen --------------------------------------------------- 27
Su ace Cha ac e iza ion --------------------------------------------------------- 27
2.3.1 X- ay pho oelec on spec oscopy ------------------------------------------ 27
2.3.1.1 XPS spec a main ea u es ---------------------------------------------- 30
2.3.1.2 Collec ed in ensi y and o e laye a enua ion -------------------- 32
2.3.1.3 XPS ins umen ------------------------------------------------------------ 35
2.3.1.4 Spec a simula ion ------------------------------------------------------- 37
2.3.2 Ambien p essu e X- ay pho oelec on spec oscopy ----------------- 37
2.3.2.1 Synch o on adia ion --------------------------------------------------- 39
2.3.2.2 APXPS ins umen -------------------------------------------------------- 41
2.3.3 Ul a iole pho oelec on spec oscopy ----------------------------------- 42
2.3.3.1 UPS ins umen ----------------------------------------------------------- 44
2.3.4 Scanning Elec on mic oscopy ----------------------------------------------- 44
2.3.4.1 SEM ins umen ----------------------------------------------------------- 45
Elec ochemical cha ac e iza ion ----------------------------------------------- 46
2.4.1 Full cell elec ochemical cha ac e iza ion --------------------------------- 46
2.4.2 Symme ic cell elec ochemical cha ac e iza ion ----------------------- 48
3. Li hium su ace in e ac ion wi h pu e a mosphe ic gases ------------------ 53
In oduc ion ------------------------------------------------------------------------- 53
3.1.1 Li e a u e e iew ---------------------------------------------------------------- 53
3.1.2 Wo k unc ion o moni o li hium su ace s abili y --------------------- 56
Spec a measu ing condi ions and da a analysis guidelines ------------- 58
3.2.1 XPS and UPS da a analysis guidelines -------------------------------------- 58
Li oil su ace cleaning ------------------------------------------------------------- 60
O2, CO2 and N2 gases e ec s on clean li hium su aces ------------------- 62
‖
3.4.1 Oxygen in e ac ion -------------------------------------------------------------- 62
3.4.2 Ca bon dioxide in e ac ion ---------------------------------------------------- 68
3.4.3 Ni ogen in e ac ion ------------------------------------------------------------ 75
Conclusions --------------------------------------------------------------------------- 80
4. S udy o Li ca bona e e olu ion on Li me al su ace ------------------------- 83
In oduc ion -------------------------------------------------------------------------- 83
Spec a measu ing condi ions and da a analysis guidelines ------------- 84
4.2.1 Da a analysis guidelines -------------------------------------------------------- 85
Li oil su ace cleaning ------------------------------------------------------------- 87
4.3.1 Cha ac e iza ion o Li oil su ace cleaned in A a mosphe e --------- 87
4.3.2 Cha ac e iza ion o Li oil su ace cleaned in UHV ----------------------- 90
Li2CO3 e olu ion on Li me al su ace ------------------------------------------- 95
4.4.1 E olu ion o ca bon-based compounds ------------------------------------ 95
4.4.2 Li2CO3 g ow h kine ics -------------------------------------------------------- 102
4.4.3 Dep h p o iling o li hium-based compounds -------------------------- 104
4.4.4 Insigh s in o he eac ion mechanism ------------------------------------ 107
4.4.5 O2 gas e ec on Li2CO3 g ow h --------------------------------------------- 108
Conclusions ------------------------------------------------------------------------- 114
5. Li hin ilm g ow h -------------------------------------------------------------------- 117
In oduc ion ------------------------------------------------------------------------ 117
5.1.1 S uc u e De elopmen o a hin ilm ------------------------------------ 118
Expe imen al p ocedu e -------------------------------------------------------- 119
S udy o li hium hin ilm deposi ion ----------------------------------------- 121
5.3.1 Li sou ce deposi ion a e calcula ion a 8 A ----------------------------- 121
xii ‖
impu i ies, i s s ep has been o g ow a li hium hin ilm, which has been
cha ac e ized using a scanning elec on mic oscope. Then, he elec ochemical
pe o mance and in e nal esis ance o a s anda d polyme elec oly e sys em
wi h Li symme ic elec odes has been analyzed. In his s udy, i has been
concluded ha a oiding Li oil na i e su ace impu i ies s ongly modi ies
in e acial p ope ies ha de e mine he elec ochemical pe o mance o a
sys em. This emphasizes he need o gaining knowledge abou he ini ial s a e o
me allic li hium su ace used in ba e ies.
‖ xiii
RESUMEN
Aumen a cuo a del me cado de ehículos eléc icos es esencial si se p e enden
mi iga las consecuencias del e ec o in e nade o causadas, en e o os ac o es,
po las emisiones de gases de ehículos de mo o é mico. Sin emba go, la
ecnología que suminis a ene gía a los ehículos eléc icos, la ecnología de Li-
ion, no es su icien emen e compe i i a debido a sus limi aciones en ca ga ápida,
au onomía, segu idad y du abilidad. Pa a que se p oduzca una comple a
implemen ación del anspo e eléc ico en el me cado, el desa ollo de la
ecnología de Li-ion es i al. Además, su a ance asegu a á la e olución de los
apa a os elec ónico po á iles, ambién alimen ados po ba e ías de Li-ion. De
en a las di e en es al e na i as exis en es pa a mejo a la densidad ene gé ica de
las ba e ías de Li-ion, una de las es a egias más p ome edo es es el cambio del
ánodo ac ual, g a i o, po li io me álico. Es o se debe a la al a capacidad del Li
(unas diez eces supe io al g a i o) y a que posee el meno po encial de educción
conocido (-3.040 V s elec odo de hid ógeno es ánda ). Aun así, la g an
eac i idad de la supe icie del li io imposibili a ene una supe icie es able en e
el ánodo y el elec oli o, pe diendo con inuamen e ma e ial ac i o. Además, la
deposición de li io en e los p ocesos de ca ga y desca ga en el ánodo no es
homogénea, y se o man y c ecen dend i as. És as pueden llega a alcanza el
cá odo, causando a ios p oblemas de segu idad.
En es e abajo de esis, la supe icie del li io ha sido es udiada con el obje i o de
adqui i mayo conocimien o sob e su es abilidad. Pa a ello, p ime amen e, se ha
analizado cómo los gases a mos é icos secos más comunes (O2, CO2 y N2)
modi ican la composición química y las p opiedades elec ónicas de la supe icie
del li io. Es e es udio se ha ealizado po medio de las écnicas espec oscópicas
de o oemisión de ayos X y de ayos ul a iole a. Se ha concluido que el gas más
eac i o es el O2, y que los es gases educen la unción de abajo del li io
me álico. En el siguien e es udio, la e olución del ca bona o de li io en la
supe icie del li io se ha analizado in si u po medio de la écnica espec oscópicas
de o oemisión de ayos X de p esión ambien e. El ca bona o de li io inc emen a
la uni o midad de la deposición del li io me álico cuando es á p esen e en la
in e ase en e el elec odo y el ánodo. Po ello, es de g an in e és e i a
xi ‖
condiciones en las que el c ecimien o de es e compues o es á a o ecido. En es e
es udio, se ha ob enido in o mación que con ibuye al escla ecimien o del
mecanismo de eacción, además de p opo ciona es udios ciné icos del
c ecimien o del Li2CO3.
Finalmen e, se ha analizado el e ec o de las impu ezas na i as de la supe icie de
una lámina de li io come cial en un sis ema elec oquímico. Pa a ello, con el in de
e i a es as impu ezas, el p ime paso ha sido c ece una capa ina de li io, la cual
ha sido ca ac e izada po medio de un mic oscopio elec ónico de ba ido. A
con inuación, se ha analizado el endimien o elec oquímico y esis encia in e na
de un sis ema o mado po elec odos simé icos de li io y un elec oli o
polimé ico es ánda . En es e es udio, se ha obse ado que e i a las impu ezas
na i as de las láminas de li io come ciales modi ica no o iamen e las p opiedades
in e aciales, las cuales de e minan la ejecución elec oquímica de un sis ema.
Es o en a iza la necesidad de adqui i mayo conocimien o sob e el es ado inicial
de la supe icie de li io que se u iliza en las ba e ías.
‖ x
LABURPENA
Ene gia so zeko egun e egai osilekiko dagoen menpeko asunak ondo io
zuzenak di u ingu ugi oan, bes ak bes e CO2 isu iek a eago zen du en be o egi
e ek ua dela e a. Honakoa i au e egi eko, ene gia be iz aga ien e abile ak e a
ibilgailu elek ikoe a ako jauziak be ebiziko ga an zia dauka e. Bi e emu haue an,
ene gia en me ake a ako gailu e aginko ak beha ezkoak di a. I u i
be iz aga iek so zen du en ene gia baldin za klima ologikoen a abe akoa da, ez
du e ene gia denbo an i aunko ki so zen. Ho i dela e a, eskain za e a eska iak
ba egin deza en, so zen du en ene gia me a uko duen gailuen menpe daude.
Bes e ik, ibilgailu elek ikoak lehiako ak izan dai ezen, eskain zen du en
au onomia mo o e e mikoko ibilgailuekiko alde aga ia izan beha da. Be az,
hauek e e, ene gia me ake a gailu e aginko en beha ean daude.
Ene gia me ake a ako gailu desbe dinen a ean, ba e iak di a a un ene akoak.
Ba e ia ba hainba zelda elek okimikoz osa ua dago, e a be aie ako bakoi zean
ene gia elek ikoa ene gia kimiko gisa me a zen da e edox e eakzioen bi a ez.
Zelda elek okimikoek honako osagai nagusiak di uz e: ka odoa edo elek odo
posi iboa, anodoa edo elek odo nega iboa, elek oli oa e a elek odo bakoi zeko
ko on e kolek o eak. Ka odo e a anodoen a eko e edukzio po en zial
desbe din asuna e edox e eakzioen inda e agilea da. Elek oli oa en bi a ez,
elek odoek ioiak elka uka zen di uz en, e a p ozesu honen ondo ioz elek oiak
kanpo zi kui u ba en bidez elek odo ba e ik bes e a doaz, elek izi a ea so uz.
Anodoa i e epa a uz ge o, li io me alikoa eo ikoko oso auke a ap oposa da. Izan
e e, kapazi a e espezi iko eo iko al ua dauka (3860 mAh/g), den si a e baxua
(0.53 g/cm) e a ezagu zen den po en zial elek okimiko nega ibo xikiena (-3.040
V hid ogeno es anda a e e e en zia za ha u a), azken hau ba e iak ene gia
handiagoa ema ea en e an zulea dela ik. Hala e e, li ioa en gainazal
ezegonko ak be e me ka u a zea zaildu du. Elek oli oa ekin e engabe
e eakziona zen du, ma e ial ak ibo asko galduz, e a ezinezkoa du
elek odo/elek oli o gainazal a e egonko ba lo u. Hone az gain, ka ga e a
deska ga a ean, li ioa ez da e a homogeneo ba ean jalki zen anodoa en
gainazalean, e a dend i a an zeko mik oes uk u ak so zen di a. Hauek hazi
x i ‖
egi en di a e a, ka odo a helduz ge o, zi kui u labu ak e agin di zake e, honek
daka zan a iskuekin.
Li ioa en ezegonko asuna en a azoa i au e egi eko, 90. hama kadan anodo
bezala Li-ioiak i zulga iki a eka u li ezkeen ma ize ba e abil zea p oposa u
zen, non ma izea en e a elek oli oa en a eko gainazal a ea egonko a izango
zen. Ma e ial honen au kikun zak 1991. u ean Sonyk lehendabizikoz Li-ioi
eknologia me ka u a zea ahalbide u zuen. Ba e ia haie ako anodoa pe olio
ja o iko kokea izan zen, ka odoa LiCoO2 oxido lamina a e a elek oli oa
disolba zaile o ganiko ka bonikoe an disolba u iko Li ga za. Egun, Li-ioi
ba e ie ako anodoa pe olio ja o iko kokea iza e ik g a i oa iza e a pasa da.
Teknologia hau so u zene ik me ka uko lehiako ena da, ene gia den si a e al ua
eskain zen duelako e a segu u e a e aginko ba ean. Ho i dela e a, me ka uko
ibilgailu elek ikoek Li-ioi eknologian oina i u ako ba e iak di uz e. Gailu
elek oniko e amanga iek e e, hazkunde e engabean dagoen me ka uak,
eknologia mo a be dina e abil zen du ba e ie an. Hala e e, g a i oa en kapazi a e
(372 mAh/g), li io ena ekin alde a uz hama ba aldiz xikiagoa. Be az, li io
me alikoa ekiko in e esak bizi ik ja ai zen du, e a be au egonko zeko bide
desbe dinak p oposa u di a azken u ee an; hala nola, gainazala en molda zea
au e a amenduen bidez edo elek oli o solidoen e abile a zi kui u labu ak
ekidi eko. Hala e e, o aindik ez da au ki u li io me alikoa egonko uko duen epe
luze ako konponbidea.
Honako esian li ioa en gainazala en egonko asuna az e u da, e a li io
kome ziala en be ezko ezpu u asunek sis ema elek okimiko ba ean du en
e agina neu u da. Hone a ako, lehendabizi a mos e an uga iak di en O2, CO2 e a
N2 gasek li ioan zein ondo io di uz en az e u da 3. kapi uluan, e a Li2CO3
konposa ua en bilakae a ja ai u da Li gainazalean 4. kapi uluan zeha . Ja aian,
ezpu u asunik gabeko li io/elek oli o gainazal a ea so ze bidean, li io ge uza
ina hazi e a ka ak e iza u da 5. kapi uluan. Azkenik, 6. kapi uluan, elek oli o
polime ikoa duen sis ema elek okimiko ba ean li io kome ziala en
ezpu u asunek elek okimikan du en e agina ike u da.
Li ioa en gainazalean a mos e an au ki zen di en O2, CO2 e a N2 gasek so zen
di uz en aldake ak az e zeko o oigo pen espek oskopia eknikak e abili
di a: XPS (X-Ray Pho oelec on Spec oscopy) e a UPS (Ul a iole
Pho oelec on Spec oscopy). CIC Ene giguneko Gainazalen Az e ke a
Pla a o man au ki zen den eknika ani zeko ekipoan bu u u di a bi
‖ x ii
espek oskopia hauek. Lehenengo eknika en bidez gainazalean so zen di en
konposa u kimikoak zehaz u di a. Biga en eknika en bidez gainazalen lan-
un zioa (w , wo k unc ion) de e mina u da. Pa ame o honek hu s maila en
a abe ako Fe mi maila en posizioa adie az en du, e a elek oi ba
gainazale ik a e a zeko beha ezko ene gia zenba den adie az en du.
Fo oigo pen espek oskopia eknika hauekin li ioa en gainazala ex-si u
az e u da; hau da, li ioa en gainazala molda u os ean neu u da hu s al uko
egoe an (UHV, Ul a High Vacuum).
Gasen e agina ike u au e ik, lehendabizi a goi a mos e an go de ako li io
xa la kome ziala en gainazala az e u da, ba e ie a ako anodo bezala
e abil zen dena. Xa la honen gainazaleko li io guz ia oxida ua dagoela
konp oba u da, Li2O e a Li2CO3 konposa ue an be eziki. Ho i dela e a,
a mos e ako gasek be agan du en e agina az e zeko, li io gainazala A
ioiekin bonba da u da UHV egoe an. Me odo hau e aginko a izan da
gainazaleko ezpu u asunak ken zeko: ga bi u iko gainazalak %(93.6 ± 1.9) Li
me alikoz osa uak daude, gaine akoa Li2O dela ik.
O2, CO2 e a N2 gasen a ean, oxigeno gasa da li ioa ekin bo i zen
e eakziona u duena. 9 L O2 gas (non 1 L 10-6 To p esiopean segundo ba ez
eginiko dosi ikazioa en baliokidea den) nahikoa di a gainazaleko 8.6 nm- ako
li io guz ia oxida ua iza eko. O2 gasa en p esioa 10-4 mba azpi ik denean,
e eakzio hone ako p oduk u baka a Li2O izan da. Ho ik go ako p esioe an,
Li2O2 e e neu ua izan da gainazalean. CO2 gasa en in e akzioa i dagokionez,
Li2O, Li2CO3 e a bes elako ka boi oina idun p oduk uak iden i ika u di a.
E eakzio hau askoz mo elagoa da, 8·108 L CO2 gas e e ez di a nahiko
gainazaleko 8.6 nm- ako li io me aliko guz ia oxida zeko. Ni ogenoa i
dagokionez, li ioak ez du gas honekin e eakziona zen 10000 L-e ik behe a.
E a 10000 L- an, soilik gainazalen %1.2 dago osa ua ni ogeno oina ia du en
konposa uekin. Li3N lo zeko modu baka a li io gainazala ni ogeno ioiekin
bonba da zea izan da. Modu hone an lo u iko gainazala honako
konposa uez osa ua dago: %68.4 Li0, %19.8 Li3N, %8.1 Li2O e a %3.7
ezpu u asun.
Lan un zioa dagokionez, hi u gasek be e balioa en xikiago zea daka e. Li0-
en ba ez bes eko lan- un zioa 3.01 ± 0.08 eV da. 1000 L O2- en ondo ioz, lan
un zioa 2.12 eV- a xiki zen da, e a 1000L CO2- en e aginez 2.30 eV- a
mu iz en da. Li3N konposa uak e a lan un zioa en xiki zea daka , 2.49 eV-
x iii ‖
a jai siz. Be eziki, gainazala Li2O and Li0 konposa uez osa ua badago, lan-
un zioak Li2O kon zen azioa en a abe ako e o ke a esponen ziala ja ai zen
duela ondo ioz a u da. Be az, hi u gas hauek molda uko li io gainazalek li io
me ilkoak baina e az asun handiagoaz galduko du e elek oi ba , anodo
bezala e abil zeko ezauga i kal ega i za jo dena.
Ja aian, li io ka bona o en ga apena en az e ke a egin da li io me alikoa en
gainazalean. Izan e e, li io ka bona oa kal ega i za ha ua dago li io anodo
gainazala en egonko asune ako. Konposa u honek gainazaleko bes elako
konposa u ba zuekin alde a uz Li-ioi konduk ibi a e xikiagoa dauka, e a honek
li ioa en deposizio ez homogeneoa bul za zen du. Ike ke a hone a ako li io
gainazala en bilakae a neu u da CO2 a mos e apean APXPS (Ambien P essu e X-
Ray Pho oelec on Spec oscopy) eknika en bidez sink o oi bidezko e adiazioa
e abiliz. Neu ke a hauek ALS (Ad anced Ligh Sou ce) azele agailuan egin di a,
LBNL (Law ence Be keley Na ional Labo a o y) labo a egian. Au e ik e abili ako
XPS- ekin alde a uz ge o, APXPS eknika en aban aila nagusia neu ke ak in-si u
egin dai ezkeela da; hau da, e eakzioa ema en den bi a ean gainazala en
eboluzioa ja ai ua izan dai eke. Gaine a, sink o oia i eske , e adiazioa alda u
dai eke, sakon asun p o ileko neu ke a ez-sun si zaileak egi ea ahalbide uz.
Au eko kasuan bezala, hemen e e a goi a mos e an go de ako e a ga bi u ako
li io xa la en hasie ako egoe a ike u da, non be i o konp oba u den li ioa en
gainazal osoa oxida ua dagoela. Sakon asun p o ileko neu ke ek bidez Li2CO3 Li2O-
en gainean koka zen dela ikusi da. Kasu hone an, li ioa en gainazala ga bi zeko
bes elako eknika e abili da: gainazala isikoki u a ua izan da UHV egoe an,
ma aza ba en bidez.
Li io ka bona oa en eboluzioa az e ze akoan, be a ekin ba e a bes e konposa u
ba en bilakae a e e neu ua izan da: li io oxala oa, Li2C2O4. Konposa u hau
au e iaz bi a eka i gisa p oposa ua izan zen Li2CO3 so zeko, baina ez zegoen
be e hazkun za en ebiden zia espe imen alik. Be az, oxala oa en neu ke ak
ka bona oa so zeko mekanismoa a gi ze bidean in o mazio oso baliaga ia
eskain zen du. Ka bona oa en hazkunde mo a i dagokionez, bi a e iden i ika u
di a: e eakzioak kon ola u ikoa e a di usioak kon ola u iko. Lehenengoak
hazkunde lineala dauka, e a biga enak pa abolikoa. Li ioa CO2 gasa en pean
ego ea en ondo ioz, Li2O konposa ua e e so zen da gainazalean. A al hone an
lo u iko in o mazioa ekin e eakzio mekanismo ba p oposa u da. CO2
a mos e a i O2 gasa gehi zeak di uen ondo ioak e e az e u di a, non ikusi den
‖ xix
oxigenoak li io ka bona oa en bilakae a bul za zen duen, oxala oa so zea
ekidinez. Az e u iko gainazal guz iek es uk u a be dina dauka e: Li2O Li
me alikoa en gainean koka zen da, e a Li2CO3 oxidoa en gainean oxala oa ekin
ba e a, baldin e a oxala oa so zen bada.
Behin li io xa la kome ziala az e u a, li io ge uza ina so u e a ka ak e iza zea i
ekin zaion. Hone a ako, bapo izazio e miko eknika e abili da. Li i u i kome zial
ba e ik abia uz, so u ako gainazalak elek oien mik oskopia bidez ka ak e iza u
di a CIC Ene giguneko Gainazalen Az e ke a Pla a o man au ki zen den SEM
(scanning elec on mic oscope) e abili a. Honakoa ekin i u i kome ziala en
deposizioa abiadu a neu u e a ge uza en hazkun za en mo ologia beha u di a.
I u i ik 8 A-ko ko on ea pasa zean, deposizio abiadu a 120 – 400 nm/h-koa da
e a ge uzak mendixkak e a zuloak di u. P ozesu hone an zeha subs a ua en
enpe a u a 42.3 °C-koa da. Ko on ea 10 A denean, be iz, deposizio abiadu a 730
– 1400 nm/h-koa da, e a subs a ua en enpe a u a 51.5 °C- a igo zen da, zeinak
gainazala en mo ologia homogeneiza zen duen. Li ioa en hazkundea hainba
subs a u an az e u da: Si monok is alinoa, Ti ge uza, al zai u he doilgai za, PET
(Polye hylene e eph hala e) polime oa e a S TiO3 monok is alinoa. Haue a ik, Si
monok is alinoan ez da lo u li ioa ge uza moduan haz ea. Ho en o dez, li ioak
mik oes uk u a ez homogenoa ja ai zen du, dend i a e akoa. Ezin izan da
hazkunde mo a hau silizioa en p opie a e jakin ba ekin e laziona u.
Azkenik, li io xa la kome ziala en ezpu u asunek sis ema elek okimiko ba ean
du en e agina az e u da. Az e ke a hone a ako LiTFSI (li hium
bis( i luo ome hanesul onyl)imide) ga za duen PEO (Poly(e hylene oxide))
elek oli o solido polime iko es anda a sin e iza u da. Polime o honekin
Li/PEO:LiTFSI/Li sis ema elek okimikoa en po ae a az e u da bi kasu an, CIC
Ene giguneko elek okimika ka ak e izazio baliabideen bidez. Lehenengo kasuan,
Li xa la kome ziala e abili da. Au e ik ikusi bezala, xa la honen gainazalak Li2O
e a Li2CO3 konposa uak di u, be az elek odo/elek oli o gainazal a ean
ezpu u asun hauek egongo di a. Biga en kasuan, li ioa zuzenean bapo a u da
polime oa en gainean UHV egoe an, gainazal a eko ezpu u asunak minimiza uz.
70 °C- an, non elek oli o polime ikoak konduk ibi a e ap oposa daukan, li io
xa len ezpu u asunak ekidi eak gainazal a ean ba neko e esis en zia %26
mu iz ea daka . 45 °C- an, oso enpe a ua baxua elek oli oa en un zionamendu
egoki ako, mu izke a hau a e e a naba iagoa da, %92-koa. Pola izazio
gal anos a ikoan e e e agina dauka ezpu u asuna ekidi eak, gain-bol aia en
xx ‖
mu iz ea bai aka . Emai za hauekin Li gainazala en egoe ak zelda
elek okimikoa en ja due an e an zukizun zuzena daukala konp oba u da,
ma e ial honen e eak ibo asun al ua ule ze bideko espe imen uen ga an zia
azpima a uz.
‖ xxi
6 ‖ 1. In oduc ion
spinel s uc u e phase ansi ion[23]. In he las decades, he in e es in mixed
ansi ion me al oxides combining Ni, Mn and Co (NMC) has been g owing, due o
he syne ge ic bene i s o me ging hem. These ma e ials can o e a capaci y o
200 mAh/g when cha ging be ween 2.5 V and 4.5 V[24].
Apa om he abo e-men ioned ca hode ma e ials ha ely on he in e cala ion
o Li in laye ed oxide channels, h ee-dimensional s uc u es also ep esen a
compe i i e al e na i e: e.g. he LiMn2O4 spinel and he LiFePO4 (LFP) oli ine
s uc u es. The mos ecen ad ances a e explo ing bo h high ol age li hium-ion
ca hode ma e ials, as he spinel LiMn1.5Ni0.5O4 which can ope a e a 4.7-4.8 V, and
high capaci y ca hodes, such as he so-called Li- ich laye ed oxides; deno ed as
xLi2MnO3(1-x)LiTO2 (T=Mn, Ni, Co), hey can each capaci ies highe han 250
mAh/g[22].
Mos common s anda d elec oly e in Li-ion ba e ies a e composed by LiPF6 sal
in a mix u e o o ganic ca bona e sol en s. Gene ally, he sol en includes
e hylene ca bona e (EC) and dialkyl ca bona es[25]. The ad an ages o o ganic
liquid elec oly es a e he ela i ely high po en ial window a which hey can
ope a e wi hou deg ada ion (s able un il 4.4 V) and he high ionic conduc i i y.
Howe e , hese elec oly es a e lammable, co osi e and he mally uns able,
which could cause explosions and i e acciden s when no used p ope ly.
Fu he mo e, LiPF6 sal s is highly oxic[26]. Despi e wa e -based elec oly es[27]
could be a sui able op ion o emo e o ganic sol en s, main al e na i e
elec oly es o a oid he sa e y issues o o ganic liquids a e he solid elec oly es
and ionic liquids.
Solid elec oly es can be di ided in wo main amilies: polyme elec oly es and
ce amic elec oly es[28]. In o de o be compe i i e, bo h o hem should possess
high ionic conduc i i y (abo e 10-4 S/cm) a oom empe a u e, ha e negligible
elec onic conduc i i y wi h high ionic ans e ence numbe and emain s able in
a wide elec ochemical window[29]. Polyme based elec oly es a e ela i ely easy
o p ocess a oom empe a u e and ha e a good adhesion, bu hei conduc i i y
a oom empe a u e is below he desi ed one[30]. Among he di e en
al e na i es, polye hylene oxide-based a e he mos s udied ones. Ce amic
elec oly es ha e a high mechanical igidi y, hey a e s able a high empe a u e
imp o ing sa e y and kine ics, and possess a e y high Li+ anspo numbe , close
o one. Howe e , c acking and delamina ing due o high empe a u e p ocessing
cons i u es a mayo p oblem, and s ill su e om a lowe ionic conduc i i y han
1.2 Li-ion echnology ‖ 7
liquid elec oly es[31]. Some examples o ac i ely esea ching ce amic elec oly es
a e NASICON ype (Na1+xZ 2SixP3-xO12, 0<x<3) Li-ion conduc o s[31] and ga ne ype
elec oly es, de i a i es om he Li3Ln3M2O12 (M = T,W; Ln = Y, P , Nd, Sm, Eu, Gd,
Tb, Dy, Ho, Tm, Yb, Lu) ga ne disco e ed in 1968[32], among o he s. Cu en ly,
hyb id polyme -ce amic elec oly es a e unde in ense s udy wi h he aim o ind
a solid elec oly e ha will ul il all he equi emen s o be in eg a ed
compe i i ely in a Li-ion ba e y[33].
Ionic liquid elec oly es a e also an a ac i e al e na i e o o ganic liquid
elec oly es due o hei negligible apo p essu e and almos negligible
lammabili y, which enhanced sa e y o he ba e y[34]. Fu he mo e, hey also
show a wide elec ochemical window s abili y (up o 6 V o ce ain combina ions),
high ion densi y and wide liquidus phase ange. These oom- empe a u e mol en
sal s ha e asymme ical, la ge and bulky anions and ca ions. Typical ionic liquid is
comp ised o a qua e na y ammonium ca ion, such as imidazolium, py idinium o
py olidinium, combined wi h an o ganic o ino ganic coun e anions such as BF4-
, PF6-, [(FSO2)(CF3 SO2)N]- o [(CF3SO2)(CF3CO)N]-, o ins ance. Howe e , issues as
hei lowe ionic conduc i i y compa ed wi h o ganic liquid elec oly es, and some
incompa ibili ies wi h common ac i e ma e ials s ill ep esen majo challenges
o hei implemen a ion in Li-ion ba e ies[35].
The inding o an app op ia e anode was he main p omo e o he de elopmen
o Li-ion echnology by Sony Ene ge ic o Japan in 1991. P e ious a emp s o
echa geable ba e ies used Li me al as anode. Conside ing i s ligh ness, high
heo e ical capaci y (3860 mAh/g) and lowes educ ion po en ial known (-3.040
V s s anda d hyd ogen elec ode), i was a e y a ac i e anode ma e ial. Indeed,
in he 1960s, he concep o li hium seconda y ba e ies was p esen ed[36]. In he
nex decade, i s comme cial Li me al echa geable ba e ies appea ed[37].
Howe e , he highly eac i e na u e o li hium made i impossible o ha e a s able
in e ace be ween he anode and he elec oly e, hus me allic li hium anode
echa geable ba e ies we e quickly disca ded. Mo e de ails abou he
p oblema ic cha ac e is ic o cycling a me allic li hium anode a e explained la e
in his chap e (sec ion 1.3.2). In he 90s, he p oposed solu ion o add ess he
ins abili y o li hium was he use o a ma e ial whe e Li ions could in e cala e
e e sible and he in e ace be ween he anode and elec oly e could be s able.
The inding o a ma e ial ha me hese equi emen s ga e bi h o li hium-ion
echa geable ba e ies. The i s chosen ma e ial was pe oleum coke, a so
8 ‖ 1. In oduc ion
ca bon ma e ial wi h a ce ain amoun o s uc u al diso de ha enabled he
comme cializa ion o Li-ion ba e ies[38]. Nowadays, g aphi ic ca bon ma e ials
ha consis o g aphene laye s held oge he by weak an-de -Waals o ces[39] a e
s ill used as anode due o hei ou s anding s abili y[40]. This ma e ial can
in e cala e Li+ a 0.1 V s Li/Li+, and i has a heo e ical capaci y o 372 mAh/g when
LiC6 is o med.
1.2.3 Solid Elec oly e In e phase (SEI): a key pa ame e
Ha ing a s able solid elec oly e in e phase (SEI) o e he g aphi e anode, a laye
ha was able o s abilize de in e ace be ween he anode and he o ganic liquid
elec oly e, was de e mining o he de elopmen o Li-ion ba e y echnology.
The SEI o ms because he chemical po en ial o he anode is ou side he
elec ochemical s abili y window (ESW) o he elec oly e (Figu e 1.3a). The
o ganic elec oly e will educe un il he anode elec oly e eac ion is blocked by
he SEI laye [19], which p e en s he anode om u he educ ion he elec oly e,
p o iding s abili y o he elec ochemical sys em (Figu e 1.3b).
Figu e 1.3. Rela i e ene gies o a liquid elec oly e and he elec odes in an elec ochemical cell a)
when anode po en ial lies ou side he elec ochemical s abili y window o he elec oly e and b)
once SEI laye passi a es he anode su ace, s abilizing he in e ace be ween anode and he
elec oly e. (: chemical po en ial o he anode, : chemical po en ial o he ca hode, LUMO:
lowes unoccupied molecula o bi al, HOMO: highes occupied molecula o bi al, ESW:
elec ochemical s abili y window).
1.2 Li-ion echnology ‖ 9
This i e e sible laye mus be bo h ionic conduc i e and elec onic insula ing o
a oid he con inuous educ ion o elec oly e[25], and i also needs o be adhe ed
o he elec ode and be insoluble in elec oly e, specially a high empe a u es[41].
Fu he mo e, SEI mus be bo h mechanically s able and lexible enough o expand
and con ac du ing cycling wi hou b eaking[42]. The composi ion o he SEI will
a y depending on elec oly e and ac i e ma e ial composi ions. Using classical
o ganic elec oly es and LiPF6 sal , a ious o ganic and ino ganic componen s ha e
been iden i ied in he SEI o g aphi e anodes: Li2CO3, LiOH, LiF, Li2O, ROCO2Li and
RCOLi, among o he s[43]. Same esea che s ha named he elec ode-elec oly e
in e phase as SEI in 1979 p oposed a he e opolymic ophase mosaic ype
mo phology o ep esen he SEI laye in bo h g aphi e and li hium anodes[44]
(Figu e 1.4).
Figu e 1.4. Schema ic ep esen a ion o a solid elec oly e in e phase (SEI) o med upon g aphi e o
li hium anode and composi e solid o ganic elec oly e. [Rep oduc ion wi h pe mission om[44],
Copy igh (2019) Jou nal o Elec ochemical Socie y].
I has been p o ed ha his mosaic model, unde ce ain condi ions, shows a
bilaye s uc u e: an inne laye domina ed by ino ganic compounds and an ou e
laye domina ed by o ganic compounds[45]. The inne laye is assumed o be dense.
The o ganic laye , in con as , is assumed o be po ous. Howe e , his bilaye
s uc u e in e p e a ion could be mo e complex in gene al, so he mosaic o mic o
phase model is belie ed o be a mo e app op ia e model[42]. Indeed, mode n
10 ‖ 1. In oduc ion
in e p e a ions a e s ill based on his model[46]. The es ima ed hickness o a
s anda d SEI laye is assumed o be om a ound 20 Å o se e al ens o
nanome e s[45,47].
The SEI is di ec ly in luencing he ba e y pe o mance, i e e sible cha ge, a e
capabili y, cyclabili y, and sa e y, as well as p e en ing g aphi e ex olia ion when
using his anode[43]. I has been p o ed ha some addi i es enhance he ba e y
p ope ies, since hey induce he o ma ion o a mo e s able SEI[41]. A p o ound
unde s anding o SEI o ma ion, composi ion and e olu ion du ing cycling is
essen ial i wan o imp o e he pe o mance o Li-ion ba e y. Howe e , despi e
all he e o s o each such le el o unde s anding, he elucida ion o he
o ma ion and e olu ion o SEI emains elusi e[25,41].
1.3 Nex gene a ion Li me al ba e ies (LMB): ole
o Li me al
Li-ion echnology, despi e being he bes echa geable ene gy s o age op ion so
a , has an s a e o he a g a ime ic ene gy densi y o a ound 260 Wh/kg[30], one
o de o magni ude lowe han ha o pe ol. Mo ing o Li me al anode, Li me al
ba e ies (LMB), is he only possible way o each e y high ene gy densi y sys ems
based on Li chemis y. Li me al, as ea lie men ioned, has a capaci y o 3860
mAh/g, en imes la ge han ha o ac ual g aphi e anode, and i also ope a es a
he lowes educ ion po en ial known. Bo h pa ame e s will inc ease he o e all
ene gy densi y o he ba e y acco ding o equa ion (1.1). By eplacing cu en
anodes o Li-ion ba e ies wi h Li me al, his echnology will be able o deli e ≈
440 Wh/kg (Li-LMO ba e ies in Figu e 1.5, whe e LMO e e s o LiMO2, M = Co,
Mn, Ni).
Besides, eme ging echnologies pos ula ed as nex gene a ion ene gy s o age
sys ems such as Li-ai and Li-Sul u ba e ies also ely on he use o Li me al as
anode. Wi h hese echnologies, he ene gy densi y o ba e ies could inc ease up
o 650 Wh/kg o Li-S and 950 Wh/Kg o Li-ai [15]. The compa ison o he
g a ime ic and olume ic ene gy densi y deli e ed by hese echnologies is
illus a ed in Figu e 1.5.
1.3 Nex gene a ion Li me al ba e ies (LMB): ole o Li me al ‖ 11
Figu e 1.5. Compa ison o p ac ical speci ic g a ime ic and olume ic ene gy densi y ob ained by
pe ol, s a e o he a Li-ion ba e ies and nex gene a ion Li me al-based (LMB) ba e ies: Li me al
- LMO ba e ies, Li hium-sul u ba e ies and Li hium-ai ba e ies. (LMO: LiMO2, M = Co, Mn, Ni)
[Rep oduc ion wi h pe mission om[15], Copy igh (2019) Na u e Nano echnology].
1.3.1 Li-sul u and Li-ai ba e ies
Bo h Li-sul u and Li hium-ai ba e ies ca hodes a e no based on in e cala ion
eac ions as in Li-ion ba e ies, bu in con e sion eac ions. In a li hium-sul u
ba e y (Figu e 1.6 o main componen s schema ics o his ba e y), he eac ion
om sul u (S) o li hium sul ide (Li2S) inco po a es wo elec ons pe sul u a om,
which esul s in a ca hode capaci y up o 1672 mAh/g[48]. Se e al in e media es
a e o med du ing his eac ion, summa ized in Figu e 1.6.
One o he bigges challenges o his echnology is ela ed o he o ma ion o
in e media e polysul ides ha a e soluble in he liquid elec oly e and can eely
mo e om he ca hode o he anode. This so-called shu le e ec esul s on he
passi a ion o elec odes, loss o ac i e ma e ial and sel -discha ge. Mo eo e ,
bo h Li2S and S a e insula ing ma e ials, so conduc i e addi i es need o be added
o he ca hodes in o de o ensu e elec on pe cola ion. Ano he p oblem o his
echnology is he olume expansion o abou 80% happening when sul u con e s
o Li2S[49]. Nowadays, g ea e o s in esea ch a e ca ied ou o o e come all
hese challenges o Li-Sul u ba e ies[50,51]. Some niche echnologies ha use Li-S
ba e ies can be ound, as he Zephy High Al i ude Pseudo-Sa elli e (HAPS)
Ai c a [52]. In his echnology, he low cyclabili y o Li-S ba e ies is no a ma e o
12 ‖ 1. In oduc ion
conce n, he in e es is o ha e high capaci y du ing ope a ion. Howe e , o
common applica ions his echnology is s ill no compe i i e i compa ed wi h Li-
ion ba e ies[53].
Figu e 1.6. Schema ic ep esen a ion o a Li-sul u ba e y and edox eac ions occu ing in he
ca hode du ing he discha ge. [Rep oduc ion wi h pe mission om[49], Copy igh (2019) Jou nal o
Powe Sou ces].
Li-ai ba e ies a e e y a ac i e due o hei heo e ical high ene gy densi y
(Figu e 1.5) and also due o ha ing a eely a ailable ca hode uel: O2 gas, which
al hough being a con enien gas, i s il e ing and handling needs o be sol ed. The
ypical p oduc o he ba e y discha ge in he ca hode is Li2O2, whe e an oxygen
educ ion eac ion akes place. Du ing he cha ge, oxygen e olu ion eac ion akes
place in he ca hode (Figu e 1.7).
The ca hode o his ba e y consis s o a po ous ma e ial, ypically ca bon wi h
binde ma e ial such as he s anda d Li-ion ba e ies binde Poly inylidene
luo ide, o highe s abili y binde al e na i es such as polye hylene[54]. In hese
ca hodes special a chi ec u es wi h an adequa e po ous s uc u e a e necessa y
o a oid mass anspo limi a ions. Se e al p oblems a ise in hese ba e ies,
mainly ela ed o he pa asi ic eac ions ha decompose bo h ca bon elec ode
and elec oly e: echa geabili y becomes poo , cha ge ol ages high, e iciency
low, and he cell ends up dying wi hin a ew cycles[55]. Finding new ca hode designs
o o e come hese issues is becoming a big challenge due o he un esol ed ac i e
eac ion in e ace o elec ochemical oxida ion o li hium pe oxide[56,57]. E en wi h
all hese p oblema ics, he po en ial o his ba e ies is so high ha esea che s
a e s ill pu ing hei e o s o ind p ac ical solu ions[58]. Wha is mo e, he s udy
o how o deal wi h he ai componen s besides O2 o a oid he pu i ica ion o he
ai is being se iously conside ed in he de elopmen o he ba e ies[59].
1.3 Nex gene a ion Li me al ba e ies (LMB): ole o Li me al ‖ 13
Figu e 1.7. Schema ic ep esen a ion o a Li-ai ba e y and he edox eac ion happening du ing
discha ge and cha ge in bo h anode and ca hode. [Rep oduc ion wi h pe mission om[57], Copy igh
(2019) Na u e Ene gy].
1.3.2 Li me al su ace ins abili y
In addi ion o he abo emen ioned in insic p oblems o li hium-sul u and
li hium-ai ba e ies, ano he conce n needs o be added o hese echnologies:
ha ing a Li me al anode ha will esul in handling and s abili y p oblems. Ge ing
o e he ins abili y haza ds a ising om he high eac i i y o li hium su ace ha
hinde ed i s comme cializa ion back in 1980s is s ill one o he majo d awbacks
o achie e a eal de elopmen o LMB.
In con as o he cu en g aphi e anode, solid elec oly e in e phase (sec ion
1.2.3) o med on me allic li hium is no s able du ing he cycling o a cell, di ec ly
a ec ing he pe o mance o he cell[60]. The low educ ion po en ial o me allic
li hium will educe he elec oly e (p ac ically any o hem[61]) a he su ace o he
me al, o ming an uns able SEI ha will b eak du ing pla ing and s ipping p ocess
due o olume changes o Li anode[15]. F esh li hium will be hen exposed o he
o ganic elec oly e, o ming a new SEI laye . The i s SEI model, he mosaic one
om Figu e 1.4, was p oposed bo h o ca bonaceous and li hium me al anode.
Analogously o he g aphi e anode case, o Li me al anodes wo laye s we e also
iden i ied in his mosaic SEI: an inne compac laye close o he elec ode mainly
including ino ganic species and an ou e po ous laye mainly composed by o ganic
14 ‖ 1. In oduc ion
ma e ial[62]. He e also, mode n s udies based hei SEI in e p e a ion in he mosaic
model, as show in he Figu e 1.8[63].
Signi ican su ace esea ch esul s summa ized in a Li me al SEI e iew[47] ha e
ound ha , using se e al o ganic sol en s and sal s, majo ino ganic compounds
a e Li2O, Li2S/Li2S2, LiOH, LiF, LiI, Li3N and Li2CO3; whe eas he o ganic ones a e
ROLi, RCOOLi, ROCOLi, RCOO2Li and ROCO2Li (R = alkyl g oups).
This complex he e ogeneous na u e o SEI is ende ing e y di icul a p ope
quan i a i e cha ac e iza ion o SEI chemical composi ion, s uc u e and
mechanical p ope ies. S ill, bo h expe imen al and heo e ical s udies keep ying
o elucida e he na u e o he SEI due o he di ec impac o his in e phase in he
pe o mance o he cell[64,65].
Figu e 1.8. Schema ic ep esen a ion o Li pla ing and s ipping e ec on li hium me al su aces
based on he mosaic model o SEI in e p e a ion, using o ganic ca bona e liquid elec oly e (LE)
wi h LiNO3 (LNO). [Rep oduc ion wi h pe mission om[63], Copy igh (2019) Chemis y o Ma e ials].
The o he big issue ela ed o he me allic li hium anode is he non-uni o m
elec odeposi ion o li hium in he anode du ing elec ochemical cycling. When
deposi ing, i g ows o ming whiske ype s uc u es, named as dend i es (Figu e
1.9a). Al hough he ami ied me allic elec odeposi ion om dilu e sal solu ions
in high elec ic ield was al eady conside ed an old subjec in 1990[66], he di icul
in e acial chemis y o li hium su ace makes he explana ion o dend i e g ow h
complex. The he e ogeneous SEI en ails inhomogeneous nuclea ion due o
di e en ion conduc i i y o he se e al compounds, and he c acks in he non-
s able SEI inc emen he non-uni o m deposi ion. In o de o explain he sel -
enhanced na u e o he dend i ic g ow h se e al heo ies ha e been p oposed.
1.3 Nex gene a ion Li me al ba e ies (LMB): ole o Li me al ‖ 15
One o hem ocused on he highe elec ic ield a he ip o he bulges due o
hei cu a u e, which a ac s mo e Li ions and hus o ms u he p o usions,
e ol ing in o dend i es[67]. I he dend i es g ow pe pendicula o he anode and
pie ce he sepa a o , he mal unaway and explosion could occu due o he sho
ci cui [68].
Ano he nega i e consequence o he dend i es is he loss o ac i e li hium. When
he dend i e de aches om he anode, i disconnec s elec ically. This li hium,
su ounded by SEI, becomes inac i e, and is usually called dead li hium (Figu e
1.9b). Fu he mo e, he con inuous accumula ion o he dead Li c ea es o uous
di usion pa hways ha a ec s he di usion o Li ions[69].
a) b)
Figu e 1.9. a) Dend i es o ma ion as a consequence o non-uni o m elec odeposi ion o li hium. b)
Inac i e dead li hium as a consequence o dend i e de aching om he anode, which dec eases he
amoun o ac i e ma e ial o he anode. [Rep oduc ion wi h pe mission om[70], Copy igh (2019)
Cell P ess].
F om bo h SEI c acks and dead li hium ha cause he loss o ac i e ma e ial, main
con ibu ing ac o o he low Coulombic e iciency is belie ed o be he dead
li hium[71]. This pa ame e is de ined as he a io o he amoun o cha ge ha exi s
he ba e y du ing he discha ge and he amoun o cha ge ha en e s he ba e y
du ing cha ge. No mally, in con en ional ca bona e o ganic elec oly e, he
Coulombic e iciency is lowe han 90%[70]. Bu e en when eaching 99% o
Coulombic e iciency wi h ad anced elec oly es[72], he ine iciency emains being
a p oblem. The goal o applicabili y ha will allow o ha e mo e han 1000 cycles
needs a Coulombic e iciency o 99.98%[73]. All hese in e acial issues, besides he
low coulombic e iciency, ha e also a di ec impac in o he pa ame e s ha
22 ‖ 1. In oduc ion
1.4 Scope o he hesis
In o de o mo e o nex gene a ion li hium me al ba e ies, he uns able in e ace
be ween li hium and elec oly e needs o be add essed. Mos e o s pu suing his
objec i e a e ocused on inding an app op ia e a i icial SEI. Howe e , li le
a en ion has been paid so a o he in luence li hium na i e su ace exe s in he
s abili y o he in e ace. The aim o his disse a ion is o unde s and how
a mosphe ic gases modi y he su ace o me allic li hium and analyze which is he
eal p is ine su ace o a ba e y g ade comme cial li hium oil o inally see o
which ex en he p eexis ing impu i ies a e a ec ing he in e ace, which will
ul ima ely d i e he elec ochemical pe o mance o he cell. In o de o do ha ,
he in e ac ion o li hium oil wi h main pu e a mosphe ic gases (O2, CO2 and N2)
is s udied using spec oscopic echniques, and he elec ochemical pe o mance
o an impu i ies ee su ace and a s anda d su ace is compa ed in a symme ic
solid elec oly e sys em.
2.1 Thin ilm g ow h ‖ 23
2. Expe imen al echniques
CHAPTER 2
Expe imen al echniques
In his chap e , undamen s o he expe imen al echniques used o de elop he
wo k p esen ed in his hesis a e in oduced. Chap e is di ided in ou sec ions.
Fi s ly, echniques employed o g ow hin ilm a e p esen ed. In he second
sec ion, he me hod applied o modi y su aces is explained, and a e ha su ace
cha ac e iza ion echniques a e de ailed. Las ly, a desc ip ion o he
elec ochemical cha ac e iza ion echniques can be ound.
2.1 Thin ilm g ow h
Two physical apo deposi ion (PVD) p ocesses we e used o g ow hin ilms.
These echniques a e based on mo ing a oms in gas phase om he a ge
ma e ial in solid phase o he g owing ilm, also solid phase. The apo iza ion o
he a ge a oms in his hesis has been p oduced ei he by applying hea ( he mal
ene gy) o by ca hodic pul e iza ion (spu e ing).
2.1.1 The mal e apo a ion
Vacuum he mal e apo a ion is he mos basic physical apo deposi ion p ocess.
The elemen o be e apo a ed is placed in a me allic c ucible, which is hea ed by
passing a cu en () h ough i , acco ding o Joule e ec . The amoun o hea
gene a ed is hen:
=
(2.1)
24 ‖ 2. Expe imen al echniques
whe e is he pa allel esis ance o he c ucible and e apo an combina ion a he
e apo a ion empe a u e.
Unde pe ec acuum condi ions and conside ing a single-componen e apo an
ma e ial, he maximum mola lux o subs ance om he solid phase o i s gaseous
o m is exp essed by he He z-Knudsen equa ion[96]:
,
=
√
2
(2.2)
whe e is he molecula weigh o he e apo a ing compound, is he uni e sal
gas cons an , is he absolu e empe a u e a he e apo an su ace and is he
s anda d apo p essu e o he e apo an , which is a unc ion o he absolu e
empe a u e. The ela ionship be ween he e apo a ion lux and maximum
e apo a ion lux is co ela ed by he e apo a ing coe icien () acco ding o:
=
,
(2.3)
Mos me als ha e a omic apo s and e apo a ing coe icien is equal o one.
When e apo a ing an alloy, which is a solid solu ion o a mix u e o solid phases,
he e apo a ed lux will be iche in he mo e ola ile elemen o any
composi ion, so he mel ing will con inue o deple e in ha elemen as
e apo a ion p oceeds. Compounds ha e a e y di e en e apo a ion beha io
compa ed o alloys. In con as wi h alloys, hey ha e a speci ic a io o elemen s,
ha is, hey ha e a speci ic s oichiome y, and du ing e apo a ion hey can
e apo a e as molecules, pa ially dissocia ed o dissocia ed comple ely upon
e apo a ion. This las beha io is e y p ac ical when e apo a ing alkaline me als,
because hei low sublima ion poin makes hem inapp op ia e o use in high
acuum e apo a o s which a e usually baked ou a empe a u es abo e 100 °C.
Wi h an in e me allic compound, he e y low sublima ion empe a u e can be
signi ican ly inc eased by high mel ing in e me allic alkali compound o high
en halpy o o ma ion[97].
One o he main p oblems o he mal e apo a ion is he con amina ion, bo h
c ucible ma e ial and e apo an s elease con aminan apo s om hei su aces
and om he bulk. Much o he ola ile impu i y con en in he e apo an can be
emo ed be o e ilm e apo a ion, which includes adso bed gases and dissol ed
elemen s o highe apo p essu e han he e apo an . Fo hei emo al, c ucible
is hea ed a a empe a u e below e apo a ion empe a u e o e apo an , whe e
2.1 Thin ilm g ow h ‖ 25
he dissol ed impu i ies will p og essi ely deple e ela i e o he e apo an ,
pu ging he e apo an .
2.1.2 Magne on spu e ing
In his p ocess, ep esen ed in Figu e 2.1, a solid a ge is spu e ed by ene ge ic
ions o ine gases (e.g., a gon) om a magne ically enhanced glow discha ge. The
spu e ed ma e ial is deposi ed on he subs a e, which is placed opposi e he
a ge . A c osswi e magne ic ield inco po a ed o e he a ge aps seconda y
elec ons nea he a ge su ace. Then, elec ons pa h leng h is g ea ly inc eased
be o e hey inally escape o he subs a e. When he subs a e is elec ically
insula ing, adio equency (RF) bias ins ead o di ec cu en (DC) bias mus be
used.
Figu e 2.1. Schema ic c oss sec ion ep esen a ion o a magne on spu e ing p ocess.
2.1.2.1 Spu e ing ins umen
The spu e ing ins umen used in his hesis is a P ei e Classic 500 SP, which
consis o a p ocess chambe wi h 5 magne on heads: 3 DC powe supplies and 2
RF powe supplies. The sys em is pa o su ace analysis uni o CIC Ene gigune
and i can each a base p essu e o 10-8 mba . The ins umen is equipped wi h a
as en y chambe ha allows keeping good acuum le els. In addi ion, he as
en y chambe has a modi ica ion ha enables he a achmen o an ai igh
ans e sys em o sensi i e samples.
26 ‖ 2. Expe imen al echniques
2.2 Su ace modi ica ion
In his hesis wo k, su aces we e modi ied by ion bomba dmen o wo pu poses.
On one side, his echnique was used o emo e impu i ies om he su ace. On
he o he side, su ace chemis y was modi ied bomba ding i wi h eac i e ions.
2.2.1 Ion bomba dmen
The impingemen o ene ge ic ions o a oms upon a solid su ace p oduces a
a ie y o e ec s ela ed o he high e iciency ene gy ans e o his p ocess. The
amoun o kine ic ene gy ans e ed om he ion o he a ge a om is de ined
by:
=
4
(
+
)
=
(2.4)
whe e is he mass o he impinging pa icle, is he mass o he a ge a om,
is he kine ic ene gy o he impinging ions and de ines he e iciency o he
ene gy ans e p ocess be ween he bomba dmen ions and a ge a oms. I he
masses a e wi hin wo imes each o he , is > 0.9.
The ways in which bomba ding ions can mo e su ace a oms can be g ouped in
su ace and subsu ace p ocesses. Su ace p ocesses a e usually in he ange om
ew eV o ens o eV. One o he su ace displacemen p ocess is e y use ul i he
aim is o emo e con aminan s om he su ace o ou sample. Du ing his p ocess
an ine gas is used o bomba d he sample su ace, and he adso bed impu i y
ecei es enough ib a ional ene gy o b eak i s bond o he su ace and deso bs
(Figu e 2.2a). Special ca e has o be aken wi h he ene gy o he ions, because i
i is oo high, he con aminan , ins ead o being emo ed, can be implan ed on he
subsu ace (Figu e 2.2b).
When he ion bomba dmen ene gy exceeds a ew ens o eV, hen pa icle
pene a ion in o he bulk ma e ial begins, and one o he mos impo an
subsu ace phenomena ha appea s a his poin is he ion implan a ion (Figu e
2.2c). When wo king wi h an ine gas, implan a ion is gene ally undesi ed, main
pu pose is he emo al o su ace con amina ion. Howe e , i can be used o
inco po a e a desi ed dopan o e en o o m a compound ilm i he impinging
ion is eac i e. This las one is called eac i e implan a ion.
2.3 Su ace Cha ac e iza ion ‖ 27
In he ion sou ces, gas ions a e p oduced, ocused, accele a ed and emi ed as a
na ow and in ense beam owa ds he sample. In all ypes o ion sou ces, he ions
a e gene a ed by an elec ic discha ge ha goes h ough he gas a low p essu e.
Ions a e p oduced by elec on collision inside ioniza ion ca i y o ming an elec on
ion plasma.
Figu e 2.2. Typical su ace and subsu ace p ocess gene a ed by ion bomba ding. a) adso ba e
emo al p ocess, b) knock-on implan a ion o an impu i y a om and c) ion implan a ion.
2.2.1.1 Ion sou ce ins umen
Ions sou ces used in his hesis a e IQE 11 and IQE12/38, bo h om SPECS GmbH.
Bo h sou ces gene a e and ex ac ions, bu second ype also ocus and de lec he
ion beam using a double lens sys em and de lec ion pla es. IQE 11 is used o he
eac ion implan a ion p ocesses and IQE12/38 o clean su aces by A ion
spu e ing.
2.3 Su ace Cha ac e iza ion
Se e al echniques we e used in o de o cha ac e ize he su aces s udied in his
hesis. Chemical composi ion was de e mined by X- ay pho oelec on
spec oscopy. To s udy he elec onic con igu a ion, ul a iole pho oelec on
spec oscopy was used. Las ly, in o ma ion abou he mo phology and hickness
o hin ilm samples was ob ained by scanning elec on mic oscopy.
2.3.1 X- ay pho oelec on spec oscopy
X- ay pho oelec on spec oscopy (XPS) is widely used o de e mine he chemical
composi ion o he su ace. I consis s o an X- ay sou ce ha i adia es he sample
unde s udy wi h pho ons ha exci e elec ons om he co e le els o he a oms
o a solid sample in o he acuum. Thus, XPS p obes he elec onic s uc u e o
ma e wi h elemen al sensi i i y and chemical s a e speci ici y. The pho on
28 ‖ 2. Expe imen al echniques
pene a es in o he sample su ace and is abso bed by a co e elec on wi h a
binding ene gy below he ene gy di e ence be ween he pho on ene gy and he
acuum le el. Then, elec on eme ges om he solid wi h a gi en kine ic ene gy
as de e mined by he pho oelec ic e ec . The kine ic ene gy (KE) o emi ed
elec ons in he sample is de e mined by:
=
ℎ
−
−
,
(2.5)
whe e ℎ is he pho on ene gy, , he wo k unc ion o he sample and he
binding ene gy o he exci ed elec ons.
The kine ic ene gy o hese elec ons is measu ed, in ou case, by a pho oelec on
hemisphe ical analyze (HAS). Figu e 2.3 shows he main pa s o he HAS analyze
sys em. I consis s o wo me allic hemisphe ical pla es concen ically a anged. A
se o elec os a ic lenses collec s he emi ed pho oelec ons and ocuses hem
on o he analyze en ance sli . Elec ons a e e a ded by a po en ial di e ence R
inside he lens sys em un il hey ma ch he elec os a ic ield o he hemisphe ical
analyze . This ield is called he pass ene gy and i is applied be ween he inne
and ou e hemisphe es o he analyze so ha ajec o y o he incoming elec ons
is ben in o a cu e. A channel on ype elec on mul iplie (de ec o ) is si ua ed
behind he exi sli o he analyze and coun s he eme ging elec ons. The e o e,
by scanning R, spec um o he pho oelec on in ensi y as a unc ion o kine ic
ene gy can be eco ded, he measu ed kine ic ene gy being he sum o R and pass
ene gy. One way o inc ease he ene gy esolu ion o he analyze is dec easing
he pass ene gy; howe e , he collec ed in ensi y will decay. The hemisphe ical
analyze and ans e lenses can be ope a ed in wo modes, namely Fixed Analyze
T ansmission (FAT) and Fixed Re a d Ra io (FRR). In FAT mode, he pass ene gy is
held cons an , ans e s lenses a e in cha ge o e a d he kine ic ene gy channel
o he one accep ed by he analyze . In FRR, he cons an alue is he ini ial
elec on ene gy and analyze pass ene gy a io. Fi s one is mos used in XPS
sys ems because he ene gy esolu ion is cons an o he whole spec um.
The HAS is cha ac e ized by i s own wo k unc ion. A con ac po en ial exis s
be ween he sample and he analyze when bo h a e elec ically connec ed, i he
sample is elec ically conduc ing, he Fe mi ene gies o sample and analyze a e
aligned. Consequen ly, kine ic ene gy (´) o elec ons collec ed in he analyze
is a ec ed by he con ac po en ial, yielding in:
´
=
ℎ
−
−
,
+
,
−
,
=
ℎ
−
−
,
(2.6)
2.3 Su ace Cha ac e iza ion ‖ 29
whe e , is he wo k unc ion o he sample. Then, he measu ed kine ic ene gy
is de e mined by he pho on ene gy, he binding ene gy and he wo k unc ion o
he analyze . A schema ic o ene gies o co e le el pho oelec on spec oscopy is
shown in Figu e 2.3.
Figu e 2.3. Ene gy le el diag am o co e le el pho oelec on spec oscopy om a solid. Ene ge ic
le el e ms a e explained in he ex . A schema ic o a hemisphe ical analyze is also d awn, showing
he pa h o elec ons om he sample o he de ec o . The e is an example o a ypical XPS spec um
in he igh side o he igu e.
The measu ed KE spec um is a supe posi ion o p ima y elec ons and seconda y
con inuum elec ons. The p ima y elec ons esul om elas ic collisions and a e
ea u ed as dis inc spec a (peaks), hese p ima y elec ons a e he ones ha
con ain he in o ma ion abou he co e le els o he sample elemen s including
hei oxida ion s a e. Seconda y elec ons a e p ima y elec ons ha unde go
inelas ic collisions esul ing in a educ ion o hei kine ic ene gy. They ha e a
mo e o less con inuous ene gy spec um.
As said p e iously, he pho oemission p ocess elays on he in e ac ion be ween
one X- ay pho on and one co e le el elec on. Con en ional X- ay sou ces a e
based on he bomba dmen o a solid a ge wi h high ene gy elec ons. The
30 ‖ 2. Expe imen al echniques
emission om his a ge consis s o cha ac e is ic X- ay emission lines associa ed
wi h he illing o co e holes c ea ed by he inciden high ene gy elec on beam.
Elec on ene gy is gene ally chosen o c ea e holes o K-shells. The ideal ene gy
pho on sou ce should ha e low backg ound and na ow cha ac e is ic line
emission, he nea es o a monoch oma ic sou ce. Ano he impo an ea u e o
he a ge is i s capaci y o dissipa e hea , which will acili a e he cooling down
p ocess needed because o he inciden elec on beam bomba ding he a ge .
Mos used ma e ials ha ha e app op ia e cha ac e is ic o ul ill he p e ious
men ioned c i e ia a e Mg and Al. Bo h o hem ha e a dominan Kα1,2 X- ay
emission line, a 1253.6 eV o Mg and 1486.6 eV o Al. These emissions also ha e
o he lines associa ed wi h doubly and mul iply ionized a oms. Table 2.1 shows
he ene gies o main emission lines when using Mg as a ge o p oduce X- ays.
Table 2.1. X- ay emission line ene gies o a Mg sou ce. Mos p onounced cha ac e is ic line is Kα1,2.
Rela i e heigh o seconda y lines is less han 9% o main line.
Kα
1,2
(eV) Kα
3
(eV) Kα
4
(eV) Kα
5
(eV) Kα
6
(eV) K
β
(eV)
1253.6 1262 1263.7 1271.2 1274.2 1302.3
One way o o e come he non-monoch oma ic na u e o a ge s is adding a
monoch oma o , which is a se o sui able c ys als o c ea e single o mul iple
B agg e lec ions. The p ocess leads in ha ing jus a pa o he dominan Kα1,2;
howe e , his ene gy esolu ion imp o emen su e s om a conside able
in ensi y loss.
2.3.1.1 XPS spec a main ea u es
The emi ed elec ons a e eco ded acco ding o hei kine ic ene gy. To ob ain
chemical in o ma ion, BE (ob ained om equa ion (2.6)) is used o co ela ed he
peaks wi h abula ed co e le els o elemen s. Since he binding ene gy o a
pho oelec on is sensi i e o he chemical su oundings o he a om: when
changing he chemical en i onmen , he e will be a shi in he binding ene gy,
which p o ides in o ma ion o iden i y indi idual chemical s a e o an elemen .
These peaks a e named as nlj, whe e n is he p incipal quan um numbe , l he
o bi al momen um quan um numbe and j he o al angula momen um quan um
numbe . s le els (l=0) gi e ise o a single peak (Figu e 2.4a o Li 1s), bu p, d and
le els (l>0) o a double (Figu e 2.4b o Cu 2p), which a ises om spin-o bi
coupling (spli ing) e ec s in he inal s a e.
2.3 Su ace Cha ac e iza ion ‖ 31
Figu e 2.4. Example o di e en spec a ea u es collec ed when measu ing XPS using Mg Kα non-
monoc oma ic sou ce. Sa elli e om b) co esponds o Mg Kα3 and Mg Kα4 emission lines.
When an a om has unpai ed elec ons in he alence band, emission o an elec on
om co e le el a ises in mul iple spli ing: his is he esul o coupling be ween
he unpai ed elec on in he co e wi h he unpai ed elec on in he ou e shell,
c ea ing a numbe o inal s a es which will be e lec ed in he measu ed
pho oelec on spec um (Figu e 2.4c)
O he impo an ea u es ha also appea on he spec um a e X- ay sa elli es,
shake up lines, plasmon loss peaks and Auge elec ons. X- ay sa elli es (Figu e
2.4b) a e a consequence o i adia ion wi h a non-monoch oma ic X- ay sou ce,
whe e i adia ion has no only he cha ac e is ic X- ay bu also some mino
componen s a highe pho on ene gies, as shown in Table 2.1 o Mg. Thus, hese
mino componen s exci e also co e le el elec ons ha appea s a lowe binding
ene gies. Shake up peaks (Figu e 2.4b) appea when he ou going pho oelec on
in e ac s wi h a alence band elec on and exci es i o a highe ene gy le el
changing he kine ic ene gy o he emi ed pho oelec on. Shake up peaks ha e
in ensi ies o up o 5-10% o he main peak. The plasmon loss peak (Figu e 2.4a) is
a ypical ea u e o some me als, whe e emi ed elec ons loss a speci ic amoun
o ene gy due o he in e ac ion be ween he pho oelec ons and delocalized
elec ons in he conduc ion band ha a e ypically in ol ed in collec i e
oscilla ions, he so called plasmons.
38 ‖ 2. Expe imen al echniques
which elec ons and gas molecules escape. The p essu e di e ence ac oss his
ape u e depends on he size o he ape u e, ype o he gas, gas empe a u e
and pumping e iciency, and i is ypical o he o de o 102 and 104 To . Small
ape u es spaced a la ge dis ance imp o e di e en ial pumping bu dec ease he
e ec i e solid angle o ansmi ed elec ons. In his kind o sys em, maximum
ope a ion p essu e in sample en i onmen is abou 1 To . In o de o be able o
ope a e a highe p essu es, elec os a ic lenses a e used o ocus elec ons
h ough he ape u es (Figu e 2.9) allowing hen he measu emen s o samples in
en i onmen s wi h a p essu e up o 10 To [105].
Figu e 2.9. Schema ic o APXPS di e en ial pumping sys em. Sample is placed in a high-p essu e
chambe and elec ons each he analyze unde UHV condi ions hanks o he di e en ial pumping
sys em be ween hem. The diame e o he ape u e (d) de ines he op imal dis ance be ween
su ace and ape u e (z)[105].
Simila o he a enua ion p oduced by an o e laye men ioned in p e ious
sec ion (sec ion 2.3.1.2), he a enua ion o he pho oelec on yield in gas
en i onmen has also an exponen ial decay acco ding o:
=
·
/
(2.13)
whe e is he pho oelec on in ensi y om he ma e ial unde s udy a e he gas
a enua ion, is he pho oelec on in ensi y ha we would obse e wi hou he
a enua ion, z is he dis ance he elec ons a el in gas a mosphe e and is he
mean ee pa h o elec ons in gas en i onmen , which is de ined as:
=
(2.14)
2.3 Su ace Cha ac e iza ion ‖ 39
is he Bol zmann cons an , he empe a u e o he gas, he p essu e o he
gas and he elec on sca e ing c oss sec ion.
The e o e, a way o dec ease he a enua ion o ejec ed pho oelec ons is placing
he sample nea he ape u e o di e en ial pumping sys em. Howe e , he e is a
minimum dis ance a which he sample should be kep in o de o ensu e a
homogeneous p essu e on he sample su ace. This dis ance is co ela ed wi h he
ape u e dimension o he di e en ial pumping sys em[105]. I bo h ape u e and
dis ance om sample o ape u e a e he same, hen p essu e a sample su ace
is 95% o chambe p essu e and, i he dis ance is double ha om he ape u e
dimension, he p essu e a he su ace is 98%. The e o e, he ocal dis ance z a
which elec ons su e less a enua ion and su ace p essu e is same as chambe
p essu e is simila o he ape u e dimension (Figu e 2.9). Then, he smalle he
ape u e he mo e is educed he pa h elec ons need o a el unde gas
a mosphe e. S anda d ape u es o APXPS sys em a e less han 1 mm o he on
ape u e and 2 mm o he es o ape u es be ween di e en ial pumping s ages.
Wi h his echnique i is also possible o collec he pho oelec on signal om he
gas phase; his is because X- ay i adia es no only he sample bu also he gas.
Besides all he possibili ies his echnique o e s o s udy solid gas in e ace,
nowadays i s design is being pushed o s udy also he solid liquid in e ace, using
ins umen s ha can wo k a p essu es up o 110 To [106].
2.3.2.1 Synch o on adia ion
When X- ay spec oscopies use synch o on adia ion ins ead o labo a o y-based
X- ay ube as inciden X- ay, mo e in o ma ion abou he su ace can be ob ained.
B illiance is a pa ame e ha de ines he pho ons gene a ed pe second di ided
by he ligh sou ce oo p in , di e gence and bandwid h (BW). When compa ing
he b illiance ob ained om each sou ce, i is a ound 107pho ons/(s mm2 m ad2
0.1%BW) o a labo a o y X- ay ube, whe eas i is a ound 1022 pho ons/(s mm2
m ad2 0.1%BW) o a hi d-gene a ion ligh sou ce whe e he gene a ed pho on
beam is highly collima ed. Ano he impo an p ope y o synch o on adia ion is
i s pola iza ion and cohe ence, he emi ed ligh is linea ly pola ized in he o bi
plane and i can p oduce de ec able wa e-like e ec s. Fu he mo e, gene a ed X-
ay co e s a wide spec um, om mic owa es o ha d X- ays.
Mos common synch o on adia ion sou ces a e based on s o age ings, whe e a
beam o highly ene ge ic elec ons is s o ed and kep a eling on a ci cula pa h.
40 ‖ 2. Expe imen al echniques
Rela i is ic accele a ions on he elec ons will esul on he emission o an
elec omagne ic ield, he so-called synch o on adia ion used as a ligh sou ce
o expe imen s (Figu e 2.10).
Figu e 2.10. Schema ic o a synch o on adia ion acili y.
Elec ons a e p oduced inside he elec on sou ce and ini ially accele a ed by a
high ol age o adio equency ield, hese elec ons a e hen eeded in o he
Linac (linea accele a o ). The elec ons a e packaged in bunches and accele a ed
enough o injec ion in he boos e synch o on. This is a p e-accele a o whe e
elec ons a e accele a ed o hei inal ene gy in he o de o GeV be o e being
inally injec ed in o he s o age ing. The boos e only wo ks when he s o age ing
has o be e illed. In he s o age ing, elec ons a el a a cons an ela i is ic
speed. In o de o eci cula e he cha ged pa icles along a ci cula pa h, a
magne ic ield pe pendicula o he ho izon al o bi al plane is used.
As he elec ons a el a ound he ing, adia ion is emi ed whene e hey a e
o ced o de ia e om a s aigh -line mo ion. Bending magne s we e he i s
a ailable sou ces o apply a magne ic ield used o de ia e elec ons and,
subsequen ly, gene a e synch o on adia ion. A way o inc ease he in ensi y o
adia ion gene a ed by bending magne s is using wiggle s, whe e a se ies o
bending magne s a e lineup enhancing he in ensi y simply by he numbe o
2.3 Su ace Cha ac e iza ion ‖ 41
magne poles. The spec um gene a ed by a wiggle is ha o a bending magne
bu wi h a highe b illiance, because he indi idual emissions o each magne
o e lap and he in ensi y adds up. Mos mode n way o c ea e synch o on ligh
is using undula o s ins ead o wiggle s, hese a e mos powe ul gene a o s. They
consis o a pe iodic a angemen o dipole magne s gene a ing an al e na ing
s a ic magne ic ield which de lec he elec on beam sinusoidally, esul ing in
adia ion wi h he wa eleng h o his pe iodic mo ion, di e ing om bending
magne s and wiggles spec um. Figu e 2.11 compa es he spec a b illiance o a
bending magne , wiggle and undula o o he Sp ing-8 synch o on acili y.
Figu e 2.11. B illiance o he SP ing-8 synch o on bending magne , wiggle and undula o . The solid
cu e o he undula o shows he ou pu a a ixed gap be ween op and bo om poles, he dashed
lines he a ia ion in he ha monic peaks as he gap is a ied om 25 o 8 mm. B illiance o sun has
also been indica ed in he igu e. [Modi ied om[107]].
2.3.2.2 APXPS ins umen
APXPS expe imen s p esen on his wo k we e ca ied ou using a Scien a R4000
HiPP APXPS sys em, which is placed a Beamline 9.3.2 Law ence Be keley Na ional
Labo a o y’s (LBNL) Ad anced Ligh Sou ce (ALS). This sys em is based on a Scien a
R4000 wi h a wo-dimensional de ec o consis ing o wo mul ichannel pla es
coupled o a phospho sc een and cha ge-coupled de ice (CCD) came a. I has ou
pumping s ages, and he base p essu e o he analyze is low 10-9 To . The
sepa a ion be ween he high-p essu e chambe and i s pumping s age is a
emo able Ti cone wi h a 0.425 mm ape u e adius on he ip (Figu e 2.12). The
app oxima e ocal dis ance o his ins umen is 0.8 mm and i can eco d spec a
abo e 2 To .
42 ‖ 2. Expe imen al echniques
The 9.3.2 bending magne beamline gene a es so X- ays wi h an ene gy be ween
250 and 850 eV. A Si3N4 window isola es he UHV X- ay ube om he high-
p essu e chambe . The UHV sys em has also a p epa a ion chambe which
includes an ion gun o ion spu e ing p ocesses. An ai sensi i e ans e ool was
used o mo e samples unde a gon a mosphe e om an a gon glo e box o he
load lock o he UHV sys em.
Figu e 2.12. Pic u e o main chambe o APXPS sys em placed in beamline 9.3.2 a Ad anced Ligh
Sou ce. The sample holde is a The mionics STLC pla e.
One o he main ad an ages o measu ing APXPS spec a using synch o on
adia ion, apa om he high- esolu ion spec a, is he capabili y o une he
ene gy o he sou ce. When changing he pho on ene gy, kine ic ene gy o ejec ed
elec ons om he same co e le el is also changed so pho oelec ons gene a ed
a di e en dep hs in he sample su ace can be measu ed and compa ed.
The e o e, a nondes uc i e dep h p o ile can be measu ed, which is essen ial o
unde s and he dis ibu ion o he compounds on he su aces unde analysis.
Fu he mo e, i is also use ul o measu e di e en co e le els a he same kine ic
ene gy o quan i ica ion easons, because in ha way we a e ensu ing ha all
elec ons a e coming om same dep h.
2.3.3 Ul a iole pho oelec on spec oscopy
The basis o ul a iole (UV) pho oelec on spec oscopy (UPS) a e he same as o
XPS al eady explained in sec ion 2.3.1, he di e ence elays on he i adia ion
2.3 Su ace Cha ac e iza ion ‖ 43
sou ce: ins ead o using X- ays, pho oelec ons a e gene a ed a e exci a ion by
ul a iole ligh . Typical UV sou ce is a He gas discha ge line which can be ope a ed
o maximize he ou pu o ei he He I (hν = 21.2 eV) o He II (hν = 40.8 eV). Because
o his low ene gy, only alence le els can be p obed, he ones ha ing lowe
binding ene gies. These include he occupied band s a es o a clean solid su ace
as well as he bonding o bi al s a es o adso bed molecules. This echnique is
su ace sensi i e, bu acco ding o he a enua ion o he low kine ic ene gy
elec ons (Figu e 2.6), his a enua ion is smalle han ha o high kine ic ene gy
elec ons. In summa y, UPS can p obe deepe egions han XPS.
Apa om he s udy o alence band s uc u e, ano he in o ma ion ha can be
ob ained by his echnique is he alue o he ma e ials wo k unc ion, which
s ands o he minimum ene gy equi ed o wi hd aw an elec on om a bound
s a e in o he acuum le el. A de ailed explana ion o he wo k unc ion and i s
use ulness o s udy su ace p ope ies can be ound in Chap e 3sec ion 3.1.2. The
alue o he wo k unc ion co esponds o he di e ence in he pho on ene gy and
he ene gy o he seconda y cu o (es ima ed wi h a linea i ing) ela ed o he
Fe mi edge, as indica ed in Figu e 2.13.
Figu e 2.13. Wo k unc ion (w ) ex ac ion om a UPS He I spec a. The UPS spec a co esponds o
a clean li hium me al su ace and i was ob ained wi h he UV pho ons emi ed by Helium gas wi h
an ene gy o 21.8 eV (He I). The igu e shows di e en egions o he spec a and how we can use i
o ob ain he wo k unc ion o he su ace. The inse is he enla ged spec a in he egion o he
Fe mi edge. Du ing da a collec ion he sample was pola ized -12 V o ob ain a sha p seconda y edge,
he binding ene gy scale is calib a ed acco ding his pola iza ion.
44 ‖ 2. Expe imen al echniques
The de e mina ion o seconda y edge can be icky because elec ons om sample
a low kine ic ene gy o e lap wi h elec ons gene a ed on he analyze i sel : hese
a e gene a ed when pho oelec ons om he sample hi he in e nal su ace o
he analyze which is ypically coa ed wi h g aphi e. The analyze elec ons a e no
in luenced by he con ac po en ial be ween sample and analyze , and hey o m
a spec um supe imposed o he seconda y edge o he sample spec um. An easy
way o a oid his o e lap is applying a po en ial be ween he sample and he
analyze . Elec ons om sample a e going o be accele a ed, sepa a ing he
seconda y edges. In Figu e 2.13, he binding ene gy has been co ec ed, bu in
o de o ge a sha p seconda y edge sample was pola ized -12 V.
2.3.3.1 UPS ins umen
UPS spec a we e aken wi h a He I emission lamp (hν = 21.2 eV), SPECS UV10/35,
and he same pho oelec on analyze used o XPS measu emen s om he UHV
mul i echnique su ace analysis sys em a CIC Ene gigune (Figu e 2.7). The helium
gas used in he UPS lamp had a pu i y o 99.99% (P axai ). To inc ease he pu i y
o he gas, he gas line was guided h ough a liquid ni ogen ap which ac s as a
c yopump educing he amoun o impu i ies in he gas; especially hose wi h a
condensa ion poin abo e he empe a u e o liquid ni ogen.
2.3.4 Scanning Elec on mic oscopy
In a scanning elec on mic oscope (SEM) an elec on beam gene a ed by an
elec on gun is ocused using elec omagne ic lenses la e accele a ed on o he
sample su ace. UHV is needed o a oid in e ac ion o elec ons wi h ai . When
scanning he beam o e he sample, seconda y and backsca e ed elec ons
ejec ed by he incoming elec on beam a e collec ed in a speci ic de ec o o each
ype o elec on, hence ob aining a magni ied image o he su ace. Seconda y
elec ons a e elec ons ejec ed om he sample when he inciden beam
elec ons ans e ene gy o he a om. Usually, hei kine ic ene gy is lowe han
50 eV. The image ob ained is a magni ica ion o he su ace mo phology.
Backsca e ed elec ons a e elec ons om he inciden elec on beam a e
in e ac ion wi h sample a oms. The kine ic ene gy o backsca e ed elec ons goes
om 50 eV o almos he ene gy o he inciden beam elec ons[108]. Then,
backsca e ed elec ons a e coming om deepe egions o he sample han
seconda y elec ons. In con as o he seconda y elec ons, backsca e ed
elec ons also con ain in o ma ion abou he chemical di e ences o he su ace
2.3 Su ace Cha ac e iza ion ‖ 45
compounds: hea ie elemen s can de lec inciden elec ons mo e s ongly, hence
hose elemen s appea b igh e in he images when compa ed o ligh elemen s.
When he sample is no conduc i e, an o e cha ging o he su ace happens due
o elec on accumula ion ha canno be d ain o g ound. Be o e measu ing SEM,
non-conduc i e samples a e usually spu e coa ed wi h a conduc i e and ine
me al, like Au. Enhanced spa ial esolu ion o scanning elec on mic oscope
depend on design o he sys em, bu hey can ypically achie e spa ial esolu ions
below 1 nm owing o he sho e wa eleng h o elec ons i compa ed o isible
ligh . Hence, SEM allows o ob ain highe esolu ion images han wi h an op ical
mic oscope.
2.3.4.1 SEM ins umen
FEI Quan a-200FEG mic oscope om CIC Ene gigune has been used o he
mic oscopy s udies. In he ield emission gun (FEG), elec ons a e emi ed om
he ca hode by applying a high elec ic ield nea he ilamen ip. This echnology
gene a es elec ons wi hou hea ing o he gun which can induce p oblems.
An ai sensi i e ans e ool (Figu e 2.14) wi h a speci ic coupling o he load lock
o his ins umen was used o deal wi h ai sensi i e samples and o mo e hem
om he ine a mosphe e o a glo e box o he acuum condi ions o he SEM.
Figu e 2.14. Ai igh ans e ool o mo e samples om an ine a mosphe e o he SEM
mic oscope.
Ano he SEM mic oscope was also used o he measu emen s p esen ed in his
hesis wo k: Helios NanoLab 450S – FEI, om CIC Nanogune. The pa icula i y o
his SEM is ha is has a Focused Ion beam (FIB) inco po a ed. The FIB is used o
p ecisely e ch o cu he sample, hen he new exposed su ace is measu ed by
SEM. An ad an age o his mic oscope is ha a clea e c oss sec ion can be
measu ed i cu ing he sample by o he me hods p esen s di icul ies.
46 ‖ 2. Expe imen al echniques
2.4 Elec ochemical cha ac e iza ion
Fo he elec ochemical measu emen s, coin cells we e assembled using a manual
clampe in an a gon a mosphe e glo e box. The ype o coin cells used a e CR2032
(20 mm diame e and 3.2 mm heigh ). Di e en pa s o a coin cell a e speci ied
in Figu e 2.15.
Figu e 2.15. CR2032 ype coin cell elemen s.
Case, cap, sp ing and space s (cu en collec o s) a e made by 316L s ainless s eel,
and he p opylene gaske a oid he sho ci cui o he cell. No e ha , when a solid
elec oly e is used, he e is no need o sepa a o .
Du ing his wo k, wo ypes o CR2032 we e assembled. In expe imen s in ol ing
ull cells, con en ional elec ode con igu a ion was used wi h posi i e and
nega i e elec odes ha deli e an open ci cui ol age (OCV) which is he
di e ence be ween he educ ion po en ial o he elec odes. The second ype o
CR2032 assembly we e symme ic cells. In his case, bo h elec odes a e made o
he same ma e ial, consequen ly, OCV o symme ic cells should be ze o.
All he elec ochemical measu emen s we e pe o med using a Biologic VMP3
po en ios a es e om CIC Ene gigune. Following he elec ochemical
cha ac e iza ion me hods used in each ype o cell a e explained.
2.4.1 Full cell elec ochemical cha ac e iza ion
The elec ochemical cha ac e iza ion echniques used in con en ional wo
elec ode sys ems we e cyclic ol amme y (CV) and gal anos a ic cycling. In a
cycling ol amme y expe imen , he in ensi y esponse o a wo king elec ode
2.4 Elec ochemical cha ac e iza ion ‖ 47
( he elec ode unde s udy) is measu ed while applying a ol age sweep using a
cons an scan a e (Figu e 2.16). I p o ides in o ma ion abou he edox
eac ions, he ol age a which hey occu and hei e e sibili y.
Figu e 2.16. Example o a cyclic ol amme y expe imen o one edox p ocess o elemen A. a)
Applied cyclic po en ial sweep o he wo king elec ode and b) esponse o he wo king elec ode
esul ing in a cyclic ol amme y. Eu indica es he uppe limi o he ol age and EL is he lowe limi
o he ol age, which co esponds o he OCV a discha ged s a e.
In a gal anos a ic cycling expe imen , in con as o he p e ious me hod, he
cu en is con olled and held cons an un il eaching he uppe and lowe ol age
window limi s, and he po en ial becomes he dependen a iable, which is
ollowed as a unc ion o ime (Figu e 2.17). Wi hin his echnique, we can also
obse e he ol age a which he edox eac ion is happening: ep esen ed by a
pla eau in he plo .
Figu e 2.17. Example o a gal anos a ic cycling expe imen o one edox p ocess o elemen A. a)
Applied cons an in ensi y un il eaching he desi ed ol age in he wo king elec ode and b)
esponse o he wo king elec ode. Eu is he uppe limi o he ol age, and he pla eau indica es a
edox eac ion p ocess. Time needed o cha ge and discha ge is no he same ela ed o i e e sible
eac ions.
54 ‖ 3. Li hium su ace in e ac ion wi h pu e a mosphe ic gases
su ace[113-141]. In hese esea ch wo ks, su ace is analyzed by one o he ollowing
su ace speci ic echniques: auge elec on spec oscopy (AES)[113–122], ul a iole
pho oelec on spec oscopy (UPS)[123–127], in a ed spec oscopy (IR)[128,129], X- ay
pho oelec on spec oscopy (XPS)[116,118,134–138,119,120,124,127,130–133], elec on ene gy
loss spec oscopy (EELS)[122,134], abso p ion spec oscopy(XAS)[118], me as able
deexci a ion spec oscopy[125], ellipsome y[121], su ace X- ay di ac ion (XRD)[139],
densi y unc ional heo y (DFT) combined wi h molecula dymanics (MD)[140],
selec ed a ea elec on di ac ion[141] and ene gy il e ed ansmission elec on
mic oscopy[141]. Figu e 3.1 shows he dis ibu ion o published scien i ic a icles
pe decade (Figu e 3.1a) and pe s udied gas (Figu e 3.1b). We ound ha , a e
an in e es decay in he i s decade o he 21s cen u y, he numbe o published
pape s in he las decade (2011-2020) inc eased (Figu e 3.1a), his sugges s he e
a e s ill unsol ed ques ions ela ed o he in e ac ion o me allic li hium wi h
a mosphe ic gases. In ollowing, we will discuss he e ec s O2, CO2 and N2 p oduce
on he li hium su ace as epo ed in he wo ks om Figu e 3.1 ag eed on, in
addi ion, we will also emphasize he con o e sial issues ha en ail us o u he
in es iga e he gas-li hium in e ac ion.
Figu e 3.1. Dis ibu ion o he numbe o published a icles ha analyze he in e ac ion o O2, CO2
and N2 wi h he su ace o me allic li hium; a) pe decade and b) pe s udied gas, om e e ences[113-
141].
Acco ding o ou li e a u e e iew (Figu e 3.1b), he mos s udied eac ion is he
one be ween me allic li hium and oxygen gas. All s udies, wi hou excep ion,
co obo a e ha li hium su ace is e y eac i e o oxygen, being Li2O he eac ion
p oduc . Mos o he au ho s ag ee ha his eac ion does no c ea e a s able
passi a ion laye on he li hium su ace. Indeed, he oxida ion eac ion con inues
in o he bulk o he me al. Za adil e al.[134] explained his phenomenon as a
3.1 In oduc ion ‖ 55
consequence o a combina ion o ela i e he modynamic s abili ies, he solubili y
o ze o alen li hium in i s own oxide and he ac ha li hium is a highly iscous
liquid a oom empe a u e ha allows o a con inuous s uc u al ea angemen .
Howe e , besides he modynamical and solubili y conside a ions, li hium has a
mel ing poin o 180.5 °C and he only me al ha is conside ed liquid a 1 a m and
oom empe a u e is me cu y[142], hen i is mo e app op ia e o say ha li hium
is a so me al a he han a highly iscous liquid. An al e na i e explana ion
p o ided some yea s la e a ibu e he con inuous oxida ion p ocess o he
di e ence in he a omic densi y o Li and Li2O: being ou imes la ge o Li2O han
o me allic li hium[121]. I was claimed ha he densi y di e ence p oduces a
con ac ion o he su ace whe e esh me allic li hium will be con inuously in
con ac wi h he a mosphe e. By means o ellipsome y, i was concluded ha Li2O
laye is po ous, so i has ee pa hways o oxygen o each me allic li hium. In
con as , a ecen s udy sugges s ha pu e oxygen will o m a passi a ion laye i
he gas has no aces o mois u e and only a e ce ain exposu e ime[141]; his
nm- hick laye blocks he di usion o oxygen molecules p e en ing u he
oxida ion o he unde lying li hium.
The eac ion be ween li hium and CO2 gas was comp ehensi ely s udied by
Zhuang e al.[137], whe e he eac ion mechanism was in es iga ed by combining
XPS, UPS and Ab ini io Ha ee-Fock sel -consis en calcula ions. These au ho s
concluded ha he eac ion o CO2 gas wi h clean li hium leads o a mix u e o
CO32- wi h O2-.
Ou o he h ee in e ac ions, he one wi h ni ogen gas is o special in e es due
o he epo ed s a egies based on ni ide ma e ials chemis y o s abilize li hium
me al anode[143]. Despi e i s impo ance, we ind some con o e sial esul s
epo ed in he li e a u e. Some au ho s belie e ni ogen gas is, oge he wi h
oxygen and wa e , he mos eac i e esidual gas o me allic li hium in UHV
sys ems[123]. Indeed, se e al imes, i has been epo ed he o ma ion o Li3N by
di ec chemical eac ion be ween he me al and ni ogen gas wi h he aim o
c ea ing a passi a ion laye ha p o ec s li hium upon elec ochemical
cycling[139,144–146]. In con as , heo e ical s udies by Koch e al.[140] epo ed ha
di ec exposu e o N2 o a clean li hium su ace does no a o he dissocia ion o
N2 gas. A he same ime, some o he in es iga ions ha analyze he eac ion
be ween me allic li hium and ni ogen gas claim ha he eac ion is no
56 ‖ 3. Li hium su ace in e ac ion wi h pu e a mosphe ic gases
spon aneous[131,132], hence con adic ing all he s udies ha con i m Li3N
o ma ion.
3.1.2 Wo k unc ion o moni o li hium su ace s abili y
In his chap e li hium su ace wo k unc ion () e olu ion is moni o ed in o de
o e alua e he s abili y o li hium su ace as a esul o ea men wi h di e en
gases. To de ine he wo k unc ion, we ha e o look a he di e en ene ge ic
le els o he su ace o a me al as illus a ed in Figu e 3.2. The Fe mi ene gy
le el e e s o he ene gy when he elec on occupa ion p obabili y equals o 0.5
in he elec onic ene gy. As he elec on dis ibu ion can be ep esen ed by a s ep
unc ion, i can be app oxima ely conside ed ha elec ons mainly ill he ene gy
le els below Fe mi ene gy le el a he ini e empe a u e, while le els abo e a e
unoccupied[147]. This e m is de ined in ela ion o he a e age elec os a ic
po en ial ene gy o an elec on o he conduc ion band, , deep inside he me al:
(−∞). becomes cons an again a a la ge enough dis ance om he su ace,
(+∞). Howe e , o de ine he acuum le el we also need o conside he dipole
laye de ined by he Gal ani po en ial () in which all elec os a ic in e ac ions,
no included in , a e included. The di e ence in he absence o excess elec ic
cha ge on he su ace is he su ace po en ial :
(
−
∞
)
−
(
+
∞
)
=
(3.1)
The chemical po en ial o he elec on is de ined by
=
−
(
+
∞
)
(3.2)
And consequen ly, we ob ain he wo k unc ion
=
−
+
(3.3)
whe e is he cha ge o an elec on. The wo e ms o equa ion (3.3) ep esen
he ollowing: one pa () desc ibes he elec ical wo k o he elec on o go
h ough he in insic su ace dipole laye and he o he (−) is equi alen o he
chemical po en ial. Acco ding o his de ini ion, he wo k unc ion in acuum
co esponds o he minimum wo k needed o ex ac one elec on om he
su ace o he acuum le el, being ee o excess elec ic cha ge.
3.1 In oduc ion ‖ 57
When he condi ion o absence o any excess su ace cha ge, i.e. equa ion (3.1) is
ul illed, he ela ion be ween he Fe mi ene gy and he wo k unc ion is
=
−
(3.4)
Then, he wo k unc ion in a me al is equi alen o he posi ion o he Fe mi le el
wi h espec o he acuum le el[148]. I depends on he su ace s uc u e and is
a ec ed by he ou e mos laye o he sample.
Figu e 3.2. Cha ac e is ic elec onic ene gies a he me al/ acuum con ac in he absence o excess
su ace cha ge. Symbols and e ms a e explained in he ex . Adap ed om[148].
In o de o co ela e he wo k unc ion o he li hium su ace wi h i s s abili y, in
a i s app oach we could conside he ela ionship be ween Fe mi ene gy and he
chemical po en ial o a sys em. When empe a u e is ze o Kel in, bo h e ms a e
equi alen [149]. In u n, open ci cui po en ial () o an elec ochemical cell is
de ined by he chemical po en ials o elec odes[19]:
=
(
−
)
/
(3.5)
Then, a ze o Kel in we should ha e he ollowing equi alence:
=
=
(3.6)
Howe e , conside ing ha ou li hium is a oom empe a u e, i he elec ic
po en ial di e ences o each in e ace in an elec ochemical sys em is conside ed,
oge he wi h he su ace po en ial o he elec oly e, he cell po en ial di e ence
58 ‖ 3. Li hium su ace in e ac ion wi h pu e a mosphe ic gases
can be exp essed as he di e ence o absolu e elec ode po en ials
() which is ela ed o he elec ode wo k unc ion
(
)
=
+
∆
(3.7)
as desc ibed by T asa i[150], whe e is he Fa aday cons an and ∆
is
he con ac (Vol a) po en ial o he elec ode-elec oly e sys em. Wi h his
de ini ion, we obse e ha wo k unc ion changes on he li hium su ace a e
indica i es o elec ode absolu e po en ial modi ica ions, which will a ec i s
s abili y agains he elec oly e.
Then, a a i s app oxima ion, an inc ease in he wo k unc ion will make he
su ace less ene ge ically a o able ans e an elec on, hus mo e s able agains
he elec oly e. In his line, a modi ied li hium me al anode wi h a highe wo k
unc ion han ba e me allic li hium will help o gain s abili y in he elec ode-
elec oly e in e ace.
3.2 Spec a measu ing condi ions and da a
analysis guidelines
Li hium su ace eac ions wi h O2, CO2 and N2 gases we e cha ac e ized wi h wo
su ace sensi i e echniques: XPS and UPS. The i s one is used o de e mine he
composi ion o he Li su ace. The second su ace cha ac e iza ion echnique is
used o de e mine wo k unc ion, and i also ga e in o ma ion abou he alence
band s uc u e. Bo h spec oscopies (XPS and UPS) we e ca ied ou in he
mul i echnique su ace analysis sys em a ailable a CIC Ene gigune (Figu e 2.7),
using ins umen s explained in sec ions 2.3.1.3 and 2.3.3.1.
XPS measu emen s we e eco ded wi h a non-monoch oma ic Mg Kα pho on
sou ce (hν = 1253.6 eV). The pass ene gy was se o 90 eV o su ey spec a
acquisi ion and 40 eV o he de ailed egions o each elemen . UPS spec a we e
aken wi h a He I emission lamp (hν = 21.2 eV), using a pass ene gy o 1 eV and
pola izing he sample -12 V.
3.2.1 XPS and UPS da a analysis guidelines
XPS spec a is analyzed wi h CasaXPS e sion 2.3.16de 52 (Casa So wa e L d,
Teighmou h, UK). The binding ene gy ze o is calib a ed in e e y spec um p io o
3.2 Spec a measu ing condi ions and da a analysis guidelines ‖ 59
i ing he pho oelec on lines o each elemen . A su ey spec um is eco ded
o e e y sample o ensu e he su ace is ee om any con aminan s. The binding
ene gy calib a ion, in he case o he O2 in e ac ion, is done using he me allic
li hium componen in he Li 1s egion and li hium oxide componen in he O 1s
egion. Fo he CO2 in e ac ion, he binding ene gy calib a ion is based on he
posi ion o me allic li hium componen in he Li 1s egion and li hium ca bona e
componen in he C 1s egion. Fo he las gas s udied, N2, he binding ene gy is
calib a ed wi h espec o he posi ion o me allic li hium in Li 1s egion and
posi ion o li hium oxide in he O 1s egion.
The peak backg ound is simula ed by a Shi ley unc ion. A Voig p o ile (30%-70%,
Lo en zian-Gaussian dis ibu ions) is used as peak lineshape o i all componen s
excep o me allic li hium. The lineshape o his las one is a pseudo-Voig
unc ion (LF(1.5,2,5,50)) which akes in o accoun he asymme ic ail in he highe
binding ene gy side o he me allic peak; caused by he small kine ic ene gy losses
o igina ed by he in e ac ion o he co e le el elec ons wi h he conduc ion band
o he me al. This shape is equi alen o he asymp o ic o m o heo e ical
Doniach-Sunjic asymme ic lineshape. Fi s wo pa ame e s o LF(1.5,2,5,50)
de ine he asymme y o he lineshape, hi d one is he Gaussian con ibu ion and
ou h he damping pa ame e o o ce he ail o educe owa ds he limi s o he
in eg a ion limi s[151].
The assignmen o he compounds has been done based on epo ed binding
ene gies (BE) in wo ks whe e he s udied sys em is simila o ou case[127,133,137,152].
Wi h hese e e ences, and conside ing a BE unce ain y o ± 0.1 eV, we a e able
o clea ly iden i y he ollowing compounds: Li0, Li2O, Li2O2, Li2CO3 and Li3N. The
maximum FWHM ( ull wid h a hal maximum) o hese compounds is a iable
depending on he elemen . Table 3.1 summa izes he BE and FWHM cons ains
used o i he da a. Any o he compound ha is no in he able will be discussed
in i s sec ion.
To quan i y he su ace composi ion, he concen a ion o each compound is
calcula ed om equa ion (2.7). The a ea o e e y i ed pho oelec on line is
co ec ed wi h he co esponding sensi i i y ac o () o each elemen and o bi al
based on Sco ield c oss sec ions oge he wi h a ansmission unc ion () speci ic
o his pho oelec on analyze . An exponen ial ac o is also used o co ec o
he di e en pho oelec on escape dep hs. This co ec ion is needed because all
co e le els a e measu ed using same pho on ene gy, so pho oelec ons emi ed
60 ‖ 3. Li hium su ace in e ac ion wi h pu e a mosphe ic gases
om each o hese le els will ha e a di e en inelas ic mean ee pa h (). Wi h
hese co ec ions, he a omic concen a ion () o each compound can be
ob ained om (2.7).
Table 3.1. Fi ing pa ame e s used o iden i y he compounds o med on he li hium me al su ace;
based on epo ed BE[127,133,137,152] and expe imen al e idence.
Compound Fi ing
cons ains (eV) Li 1s O 1s C 1s N 1s
Li0BE 54.90-55.10
FWHM 1-1.3
Li2O BE 56.30-56.50 531.10-531.30*
FWHM 1.6-1.8 1.4-1.6
Li2O2
BE 57.40-57.60 534.05-534.25
FWHM 1.8-2 1.8-2
Li2CO3
BE 57.90-58.10 534.60-534.80 292.60-292.80
FWHM 1.6-1.8 unde ined 1.5-1.7
Li3N BE 54.70-54.50 395.20-
395.40
FWHM 1.4-1.6 1.2-1.4
* he esidual amoun o oxide we ind a e cleaning he li hium has a smalle BE, a ound 530.8 eV,
as p e iously epo ed[133] and in ag eemen wi h suboxide o ma ion due o he ion assis ed
cleaning p ocess.
The wo k unc ion is calcula ed om he minimum kine ic ene gy measu ed in he
pho oelec on spec um (seconda y elec on cu -o ), he maximum kine ic
ene gy measu ed o a pho oelec on emi ed om he Fe mi le el and he
pho on ene gy, as explained in chap e 2 sec ion 2.3.3 (Figu e 2.13). The
seconda y elec on cu -o is ob ained wi h a linea i ing o he low kine ic ene gy
side o he pho oelec on spec um, whils he Fe mi edge is ob ained by i ing a
s ep unc ion ha will de ine he ze o o he binding ene gy.
3.3 Li oil su ace cleaning
The s a ing poin o his s udy is a comme cial li hium oil (Rockwood Li hium,
Ba e y G ade), which was s o ed in an a gon illed Glo e Box (MBRAUN) whe e
O2 and H2O le els we e below 0.1 ppm. A e being moun ed in he pho oemission
sample holde s, he oils we e anspo ed o he UHV sys em wi h a speci ic
ans e ool (Figu e 2.8) ha p e en ed ai exposu e
3.3 Li oil su ace cleaning ‖ 61
This oil has a pu i y o 99.8%. E en so, he XPS spec a o he Li oil s o ed in he
glo e box, ep esen ed in Figu e 3.3, e eals a comple ely oxidized li hium su ace.
Binding ene gy o i s main Li 1s peak is a ound 57 eV, which can be assigned o a
mix u e o li hium oxide and li hium ca bona e acco ding o Table 3.1. To be able
o analyze he in e ac ion o me allic li hium and he selec ed gases, A ion
spu e ing a 5 keV was pe o med, a 4·10-7 mba o a leas 5 hou s. Wi h his
me hod, p e iously used in li e a u e[127,133,152,153] we go a su ace composed by
(93.6 ± 1.9)% o pu e me allic li hium, whe e he es o he su ace is li hium
oxide. The Li 1s pho oelec on peak o a cleaned oil (Figu e 3.3) e eals some
plasmon loss s uc u es ha co espond o su ace plasmons o me allic
li hium[154]. These ea u es can be used as an indica i e o clean me allic
li hium[135]. Ano he indica i e o ha ing a clean li hium su ace is he alue o he
wo k unc ion measu ed by UPS, which is 3.01 ± 0.08 eV, in ag eemen wi h
epo ed alues o me allic li hium su aces[155].
Figu e 3.3.Compa ison o Li 1s XSP spec a o a li hium oil s o ed in a gon a mosphe e and a e
cleaning he su ace in UHV by A + spu e ing. Whi he cleaning p ocedu e, i s binding ene gy o he
main pho oelec on peak shi s down o he binding ene gy o me allic li hium, and i also shows
plasmon loss s uc u es (highligh ed wi h an a ow), indica i e o me allic li hium[135].
62 ‖ 3. Li hium su ace in e ac ion wi h pu e a mosphe ic gases
3.4 O2, CO2 and N2 gases e ec s on clean li hium
su aces
We dosed O2 (P axai , 99.9%), CO2 (Labo gase, 99.995%) and N2 (P axai , 99.9%)
gases in h ee dose anges: 1-2-3-4-5-6-7-8-9-10 L as low dose ange, 1-10-100-
1000 L as medium dose ange, and highe doses up o he o de o 1·108 L.
Langmui (L) uni co esponds o a dose o 10-6 To o a gi en gas du ing one
second. E e y dosing sequence was deployed s a ing om a UHV cleaned li hium.
The speci ic pa ial p essu es we use in each dose a e de ailed in he analysis o
he in e ac ion wi h each gas.
3.4.1 Oxygen in e ac ion
In e ac ion o a clean li hium su ace wi h oxygen gas was s udied a he
condi ions summa ized in Table 3.2. Figu e 3.4 shows he e olu ion o XPS spec a,
analyzed wi h he pa ame e s om Table 3.1. The i s compound g owing on he
li hium me al su ace is li hium oxide, Li2O. The oxygen dose ha leads o a ull
co e age o Li su ace by li hium oxide has been es ima ed om he peak a ea
e olu ion o he Li2O componen in he Li 1s pho oelec on line. Acco ding o he
slope change measu ed in Figu e 3.5a, he ull su ace co e age dose is a ound 3
– 4 L o O2, which is in ag eemen wi h he disappea ance o plasmon loss s uc u e
and e olu ion o Li2O ene gy loss peaks assigned o su ace exci ons o Li2O[134,156],
ep esen ed in Figu e 3.5b.
Table 3.2.P essu es used o each s udied dose in he analysis o he in e ac ion o li hium me al
su ace wi h O2 gas.
Range Dose (L) P essu e ange (mba )
Low dose 1,2,3,4,5,6,7,8,9,10 10-8
Medium dose
1 10-8
10 10-7
100 10-6
1000 10-5
High dose 5000,1·10410-4
5·108101
Besides Li2O, he e is no o he compound e ol ing on he su ace un il we ge o
he high dose ange, when a new peak a highe binding ene gy o O 1s and Li 1s
XPS spec a appea s, as shown in Figu e 3.4 o 104L o 5·108 L O2 doses. Looking
3.4 O2, CO2 and N2 gases e ec s on me allic li hium su ace ‖ 63
o epo ed binding ene gies[133], ha peak can be assigned o Li2O2. In o de o
con i m he assignmen o his new compound, we compa ed he O 1s XPS and O
2p UPS spec a in Figu e 3.6. Li2O and Li2O2 posi ions ha e been iden i ied in O 2p
egion acco ding o li e a u e alues[127]. The inc ease o Li2O2 concen a ion wi h
he oxygen dose is con i med om bo h spec a.
Figu e 3.4. Fi ing o he XPS pho oelec on peaks om a li hium su ace exposed o oxygen gas a
selec ed low (1 L, 5 L, 10 L), medium (100 L, 1000 L) and high (1·104 L o 5·108 L) dose anges. The
compounds ha o m in he su ace a e shown by he decon olu ion o he 1s pho oelec on peaks
o oxygen (le panel) and li hium ( igh panel). In he spec a, he i ed cu e (black line) ollows
expe imen al da a (do s), and backg ound is ep esen ed by a dash line.
70 ‖ 3. Li hium su ace in e ac ion wi h pu e a mosphe ic gases
To alida e which is he mos adequa e i ing o ou da a we compa e he
esidual s anda d e o o bo h si ua ions (calcula ed by CasaXPS e sion
2.3.19PR1.0). I we look o he esidual s anda d de ia ion o he i ed spec a
wi h espec o da a, i ing wi h one o wo componen s will esul in e y simila
alues, be ween 0.85 and 1.35 (Figu e 3.10a). Howe e , when compa ing he
esul ing e o es ima es when using i ing A o i ing B me hods, he esidual
s anda d de ia ion o he compounds is no ably educed when wo componen s
a e used (Figu e 3.10b).
Figu e 3.10. Compa ison be ween he esidual s anda d de ia ion (RSD) o wo ypes o i ing o CO
species in C 1s XPS spec a, whe e a) ep esen s he RSD o he i ing spec a and b) ep esen s he
RSD o each compound.
The calcula ion o he s anda d de ia ion o he compounds is based on Mon e
Ca lo analysis whe e he e o es ima es a e an indica o o how s able a peak
model is wi h espec o noise. One o he ad an ages o using his e o analysis
is ha i highligh s when a quan i ica ion pa ame e is poo ly de e mined by he
combina ion o model and op imiza ion p ocedu e. To be able o apply i , we need
o ha e Poisson noise dis ibu ion in he spec a. To alida e i we ha e Poisson
noise dis ibu ion, a egion absen o co e-le el exci a ions can be analyzed by a
linea eg ession. I he s anda d de ia ion gi en by CasaXPS is a ound 1 hen we
3.4 O2, CO2 and N2 gases e ec s on me allic li hium su ace ‖ 71
assume a Poisson noise dis ibu ion, which in ou case is be ween 0.8 – 1.15,
hence in good ag eemen wi h a Poisson noise dis ibu ion[161].
Using his app oach, we can con i m ha a leas we ha e wo compounds ha
o m he CO species, which will be e e ed as (CO)a and (CO)b. Figu e 3.11 shows
he e olu ion o O 1s, C 1s and Li 1s spec a when exposing he clean li hium
su ace o CO2 gas, whe e C 1s spec a ha e been analyzed using i ing A (Table
3.4) and es o compounds ha e been iden i ied acco ding o he i ing
pa ame e s included in Table 3.1. In he igu e we obse e ha compounds
c ea ed on he su ace as a consequence o Li-CO2 in e ac ion a e Li2O, Li2CO3 and
CO species. I we look o he Li 1s peak e olu ion a he lowes doses, we de ec
ha Li2O is o med on he Li su ace a 1 L CO2, p io o he o ma ion o li hium
ca bona e. We also see ha , o low and medium doses, Li2O is g owing he mos
i compa ed wi h he es o compounds. Fo his eason, we calcula ed he CO2
dose needed o co e all he su ace me allic li hium by he sa u a ion o Li2O a ea
om O 1s spec a, ep esen ed in Figu e 3.12a. Acco ding o his, he needed dose
o each a monolaye co e age is 8 L, which also ag ees wi h he disappea ance o
plasmon loss ea u e ep esen ed in Figu e 3.12b. In his las igu e ene gy loss
peaks co esponding o Li2O a e no p esen , in con as wi h he esul s a e O2
dosing, sugges ing ha ca bon-based compounds a e g owing on op o Li2O and
p e en he de ec ion o li hium oxide ene gy loss ea u es.
O 1s spec a om Figu e 3.11 canno be used o iden i y he con ibu ions om
CO species and Li2CO3, which a e o e lapped abo e 534 eV. This co e le el, a e
o ming bo h CO species and li hium ca bona e (Figu e 3.11, O 1s spec a a e 5
L), p esen s wo main peaks. The one a he lowe binding ene gy (531.20 ± 0.1
eV) co esponds o Li2O as de ined in he li e a u e[127,133,137], and i s FWHM (1.4-
1.6 eV) does no inc ease in ag eemen wi h he obse ed FWHM beha io o O2-
ea ed su aces. This sugges s ha he e a e no compounds ela ed o ca bon-
based species in low binding ene gy side. The e o e, he CO species a e going o
be somewhe e in he high binding ene gy side, along wi h li hium ca bona e which
has a well-de ined binding ene gy a 534.70 ± 0.1 eV[133]. Howe e , as we canno
assign any exac binding ene gy o he CO species in his peak, we use a b oad
peak which con ains bo h li hium ca bona e and CO species con ibu ions.
72 ‖ 3. Li hium su ace in e ac ion wi h pu e a mosphe ic gases
Figu e 3.11. Fi ing o he XPS pho oelec on peaks o a li hium su ace expose o ca bon dioxide gas
a selec ed low (1 L, 5 L, 10 L), medium (100 L, 1000 L) and high (1·104 L, 8·108 L) dose anges. The
compounds ha o m he su ace a e shown by he decon olu ion o he peaks o oxygen, ca bon
and li hium. In he spec as, expe imen al da a (do s) ollows he i ed cu e (black line) and
backg ound is ep esen ed by a dashed line.
As i can be obse ed in C 1s spec a e olu ion om Figu e 3.11, binding ene gy
o CH/CC p esen s a lowe alue (0.5 eV lowe ) a he highes dose. This
compound, ela ed o ad en i ious ca bon, is widely used as a e e ence o
calib a e he spec a in XPS. Howe e , a ecen pape obse es ha he binding
ene gy o he CH/CC ela ed o ad en i ious ca bon can a y as much as 1.44 eV,
and hey ind a co ela ion be ween he changes in he sample wo k unc ion and
3.4 O2, CO2 and N2 gases e ec s on me allic li hium su ace ‖ 73
he biding ene gy o CH/CC[159]. This obse a ion could explain he a ia ion we
obse e in he binding ene gy o CH/CC.
The e olu ion o he no malized composi ion and wo k unc ion a ia ion in he
Li-CO2 sys em is shown in Figu e 3.13. Fo he no malized su ace composi ion, we
conside ed he Li0 om Li 1s, Li2O om O 1s, and CH/CC, Li2CO3 and (CO)a and
(CO)b species om C 1s egions. In his case, he e olu ion o he wo k unc ion is
only ep esen ed o he low dose ange and medium dose ange, because some
echnical p oblems p e en ed o measu e he wo k unc ion a he high dose
ange.
I he eac ion o li hium me al su ace wi h O2 gas and CO2 gas is compa ed, i is
obse ed ha he oxida ion p ocess is slowe in he case o he CO2 gas, whe e
e en a e he highes CO2 dose is applied, me allic li hium can s ill be de ec ed on
he su ace (Figu e 3.11 and Figu e 3.13). Then, o e laye hickness should be
below 10 nm o allow Li 1s pho oelec ons om subsu ace Li0 o escape and o
be de ec ed. A possible explana ion o he slowe kine ics o he oxida ion
eac ion is ha he e is a laye slowing down he li hium oxida ion, p obably
Li2CO3, he p edominan one a he highes dose.
Figu e 3.12. a) Li2O peak a ea (blue poin s) measu ed om O 1s XPS spec a as a unc ion o gas
dose, whe e he in e sec ion, slope change, o bo h linea i s (da k g ey) co esponds o he CO2
dose needed o ully co e he li hium me al su ace which is a ound 8 L. 95% con idence bands o
he i ing a e shown in ligh g ey. b) he disappea ance o me allic plasmon loss s uc u e om Li 1s
XPS spec a a ound 63 eV ag ees wi h he measu ed slope change in he le panel igu e.
74 ‖ 3. Li hium su ace in e ac ion wi h pu e a mosphe ic gases
Figu e 3.13. Composi ional and wo k unc ion e olu ion o a clean me allic li hium oil exposed o
ca bon dioxide gas o a) low dose ange b) medium dose ange and c) high dose ange. In all he
cases, he su ace has a la ge amoun o me allic li hium. He e, as happens wi h he O2 gas, he wo k
unc ion dec eases because o he eac ion o he su ace.
3.4 O2, CO2 and N2 gases e ec s on me allic li hium su ace ‖ 75
The wo k unc ion e olu ion o low dose and medium dose o CO2 ollows a
dec easing end (Figu e 3.13a and b) likewise i happens when dosing wi h O2.
This is also in ag eemen wi h he DFT geome y op imiza ion and molecula
dynamics calcula ions pe o med by Koch e al.[140] up o one monolaye co e age.
As ea lie men ioned, o he low and medium dose anges he compound ha is
g owing mos on he su ace is li hium oxide. Then, i is mo e han plausible ha
he wo k unc ion is going o be domina ed by Li2O. Figu e 3.14 shows ha , in ac ,
e olu ion in he Li-CO2 sys em adjus s also o he same co ela ion p e iously
ob ained o he oxida ion o li hium wi h O2 om equa ion (3.8). The de ia ions
o he exponen ial decay o O2 and CO2 dosed li hium su aces can be
explained by he e ec ha ca bon-based compounds ha e on i .
Figu e 3.14.Co ela ion o he and he li hium oxide no malized su ace pe cen age. The
exponen ail i co esponds o he li hium dosed by O2 gas (blue do s) al eady shown in Figu e 3.8.
I we add o his plo he da a om he Li-CO2 sys em in he low and medium dose anges (o ange
do s), we obse e hey ollow he same exponen ial endency, sugges ing ha in his ange he wo k
unc ion is manly de e mined by he ammoun o li hium oxide on he su ace.
3.4.3 Ni ogen in e ac ion
The e olu ion o he li hium me al su ace a e N2 exposu e, acco ding o N2 doses
o Table 3.1Table 3.5 is ep esen ed in Figu e 3.15, whe e spec a ha e been
analyzed acco ding o i ing pa ame e s shown in Table 3.1. The no malized
76 ‖ 3. Li hium su ace in e ac ion wi h pu e a mosphe ic gases
su ace composi ion and wo k unc ion e olu ion a e shown in Figu e 3.16. Fo
he quan i ica ion, we conside he Li0 om Li 1s, Li2O and Li2O2 om O 1s and
ni ogen-based compounds om N 1s. As occu ed o he Li-CO2 sys em, we we e
no able o measu e he e olu ion o he wo k unc ion a high dose ange.
Table 3.5. P essu es used o each s udied dose in he analysis o he in e ac ion o me allic li hium
wi h N2 gas.
Range Dose (L) P essu e ange (mba )
Low dose 1,2,3,4,5,6,7,8,9,10 10-8
Medium dose
1 10-8
10 10-7
100 10-6
1000 10-5
High dose 1·10410-4
1·108101
Fo bo h low and medium dose anges we do no de ec any in e ac ion be ween
me allic li hium and ni ogen gas. The su ace composi ion, and consequen ly he
wo k unc ion (Figu e 3.16), emain almos cons an h oughou exposu e o N2
doses be ween 1 and 1000 L. When going up o highe doses, we see some
ni ogen-based compounds a 1·104 L N2 gas (Figu e 3.15). Howe e , his su ace
is s ill domina ed by me allic li hium, he o al amoun o ni ogen-based
compounds is less han 1.2% (Figu e 3.16c). Fu he mo e, Li3N is jus he 0.28% o
he su ace. Because o his low amoun o Li3N, we could no i a componen o
accoun o i in Li 1s spec a. We name he o he ni ogen-based compounds
shown in Figu e 3.15 a 1·104 L as N1 (binding ene gy o 397.3 eV) and N2 (binding
ene gy o 399.9 eV). Looking o li e a u e and compa ing epo ed binding
ene gies wi h ou s, we can disca d ha any o hese wo compounds is LiN3[162] o
LiNO3[163]. Bo h o hem could be ela ed o ca bon-based compounds. N1 binding
ene gy co esponds o a poly(aniline)[164,165], and N2 could be py olic-N[166] o
ca bon ni ide[167]. I his would be he case, we should see he co esponding
con ibu ion in he C 1s spec a. Howe e , he esolu ion o he C 1s spec a we
ha e is no enough o de e mine whe he his is he case o no . The ela i e
sensi i e ac o o C 1s in ou sys em is 1, smalle han ha o N 1s (1.77), and we
al eady obse e a small amoun o N compounds (Figu e 3.16c, each N1 and N2
con ibu ions a e less han 0.8 % o no malized su ace composi ion). I is ai o
men ion ha possible eac ion pa hways o p oduce poly(aniline) o py olic-N
jus om ni ogen gas is a he unlikely. Also, epo ed ca bon ni ide[167] was
p oduced using a magne on spu e ing, hen adding mo e ene gy o he sys em
3.4 O2, CO2 and N2 gases e ec s on me allic li hium su ace ‖ 77
han ha we ha e jus wi h ni ogen gas a oom empe a u e. In o de o con i m
he assignmen o hese peaks we would need u he s udies wi h e e ence
ma e ials, o his eason we keep naming hem as N1 and N2.
Figu e 3.15. Fi ing o he XPS pho oelec on peaks o a li hium su ace exposed o ni ogen gas a
selec ed low (1 L, 5 L, 10 L), medium (100 L, 1000 L) and high (1·104 L, 1·108 L) dose anges. The
compounds ha o m he su ace a e shown by he decon olu ion o he peaks o oxygen, ni ogen
and li hium 1s. The i ed cu e (black line) ollows expe imen al da a (do s) and backg ound is
ep esen ed by a dashed line.
78 ‖ 3. Li hium su ace in e ac ion wi h pu e a mosphe ic gases
When ea ing he su ace a he highes dose (1·108 L N2), he su ace is
comple ely oxidized, bu wha we obse e is li hium pe oxide and none o he
expec ed ni ogen-based compounds. This esul a high doses is compa ible wi h
he aces o oxygen impu i ies p esen in he ni ogen gas line. We also obse e
he e ha ni ogen-based compounds o med a e a dose o 1·104 L N2 a e no
s ong enough o passi a e he li hium su ace and p e en i s oxida ion, and ha
li hium su ace is much mo e likely o eac wi h oxygen han wi h ni ogen gas.
As men ioned ea lie , we could no measu e he wo k unc ion o his high doses
because o some echnical p oblems in he UPS sys em. Bu we could assume ha
he wo k unc ion o he dose o 1·104 L is going o be e y simila o clean li hium
su ace, and he las one is going o ha e a smalle wo k unc ion expec ed om
he in e ac ion o li hium and oxygen (sec ion 3.4.1).
In o de o analyze he e ec o li hium ni ide o ma ion on he elec onic
s uc u e o li hium, a di e en app oach based on he wo k done by Ishi ama e
al.[132] was used o ob ain Li3N: eac i e ion implan a ion, using an ion sou ce ha
gene a es a N+ beam wi h an ene gy o 0.5 keV a a p essu e o 4·10-6 mba o 5
minu es. This me hod allowed o p epa e a su ace mainly composed by li hium
me al and li hium ni ide, as de e mined by XPS analysis o Li 1s, O 1s and N 1s
shown in Figu e 3.17. The no malized su ace composi ion calcula ed om Figu e
3.17 spec a esul s on a su ace composed by 68.4% Li0, 19.8 Li3N, 8.1% Li2O and
3.7% being small amoun s o impu i ies.
The o ma ion o Li3N leads o a change on he alue as de e mined by UPS,
om 3.01 eV o Li0 o 2.49 eV in he Li3N-con aninig su ace. Conside ing ha he
inal su ace also con ains 8.1% o Li2O, one could hink ha he dec ease is
due o li hium oxide o ma ion. Howe e , acco ding o he co ela ion ob ained
o O2 and CO2 dosing in equa ion (3.8), such amoun o su ace Li2O should esul
in = 2.97 eV a abo e he 2.49 eV measu ed. So, li hium ni ide o ma ion also
educes he wo k unc ion o he li hium me al su ace.
3.4 O2, CO2 and N2 gases e ec s on me allic li hium su ace ‖ 79
Figu e 3.16. Composi ional and wo k unc ion e olu ion o a clean me allic li hium oil exposed o
ni ogen gas o a) low dose ange b) medium dose ange and c) high dose ange. The e is no eac ion
be ween me allic li hium and ni ogen gas o low and medium dose anges. Fo high dose ange, a
small amoun o ni ogen based compunds is de ec ed, ep esem ed by he inse . We could no
collec he wo k unc ion o he high dose ange.
As men ioned in he in oduc ion, he e a e se e al s udies whe e Li3N is ob ained
jus by di ec eac ion be ween li hium and ni ogen gas[139,144–146]. All hese
s udies use a mosphe ic p essu es, conside ing how sensi i e is li hium o bo h
86 ‖ 4. S udy o Li ca bona e e olu ion on Li me al su ace
con ibu ion o he gaussian unc ion and w is a damping pa ame e o o ce he
ail o educe owa ds he limi s o he in eg a ion limi s[151]. This Li0 asymme ic
lineshape is, o pho on ene gies o 510 eV, 600 eV and 750 eV, LF(1,2,20,100). Fo
pho on ene gy o 280 eV is LF(1,5,20,100). O he species ha a e no in he able
a e explained la e in hei co esponding sec ion.
Th ee o he s udied pho on ene gies (280 eV, 510 eV and 750 eV) a e chosen o
be able o measu e he di e en co e le els o he su ace a same kine ic ene gy.
In his way, he only da a needed o calcula e he a omic concen a ion o he
su ace elemen s is he c oss sec ion o he elemen s a each speci ic pho on
ene gy and he lux o elec ons, acco ding o equa ion (2.7). In his chap e ,
o e laye a enua ion me hod (sec ion 2.3.1.2) is used o calcula e hickness o
su ace laye s, using da a om Li 1s co e le el a di e en pho on ene gies.
Physical pa ame e s needed o calcula e bo h he a omic concen a ions and
hicknesses calcula ions a e summa ized in Table 4.2, Table 4.2 and Table 4.4.
Table 4.2. Physical pa ame e s used o quan i y su ace a omic concen a ion and o calcula e
hickness o su ace laye s la e in he chap e .
Pho on ene gy
(eV) Pho on FluxICo e le el Kine ic ene gy
(eV)
C oss sec ionII
(Mba n)
280 0.231 Li 1s
~
222 0.1103
510 0.705 C 1s
Li 1s
~
222
~
452
0.2563
0.0199
750 0.47 O 1s
Li1s
~
222
~
542
0.2931
0.0063
Ino malized alue o pho on lux (pho ons/s mA m), expe imen al pa ame e measu ed in
beamline 9.3.2 o Ad anced Ligh Sou ce.
II om da abase[175]
Table 4.3. A omic densi y o su ace compounds used o calcula e hickness o su ace laye s la e
in he chap e .
Compounds A omic densi y o Li (10-22 a om/cm3)
Li04.6
Li
2
O 8.1
Li
2
CO
3
3.4
Li
2
C
2
O
4
2.5
4.3 Li oil su ace cleaning ‖ 87
Table 4.4. Inelas ic mean ee pa h o elec ons om Li 1s spec a used o calcula e hickness o he
o e laye s. Da a ob ained om he so wa e QUASES-IMFP calcula ion by TPP2m o mula.
PE
(eV)
Elec ons
o igina ed in
Kine ic
ene gy
(eV)
h ough
Li0 laye
(Å)
h ough
Li2O laye
(Å)
h ough
Li2CO3 laye
(Å)
h ough
Li2C2O4
laye (Å)
280
Li0225.0 10.68
Li
2
O 223.6 8.67
Li
2
CO
3
222.0 9.15
Li
2
C
2
O
4
222.2 9.50
510
Li0455.0 18.2
Li
2
O 453.6 13.78
Li
2
CO
3
452.0 14.41
Li
2
C
2
O
4
452.2 14.87
600
Li0545.0 20.96
Li
2
O 543.6 15.69
Li
2
CO
3
542.0 16.38
Li
2
C
2
O
4
542.2 16.90
750
Li0695.0 25.43
Li
2
O 693.6 18.79
Li
2
CO
3
692.0 19.59
Li
2
C
2
O
4
692.2 19.67
4.3 Li oil su ace cleaning
Li hium oil used in his s udy is a comme cial oil om Al a Aesa (99.9% pu i y,
me al basis, 1.5 mm hick). This oil has been cha ac e ized in wo si ua ions: a e
sc aping i in A a mosphe e and a e sc aping i in UHV condi ions.
4.3.1 Cha ac e iza ion o Li oil su ace cleaned in A a mosphe e
Comme cial li hium oil was s o ed in an a gon a mosphe e glo e box, whe e H2O
and O2 gas le els we e below 0.1 ppm. Su ace was sc aped using a UHV cleaned
blade in he glo e box, a s anda d p ocedu e in ba e y communi y be o e using
he li hium as an anode. Sample was hen ans e ed om he glo e box o he
load lock o APXPS ins umen , p e en ing su ace exposu e o a mosphe ic ai .
Figu e 4.1 ep esen s he APXPS spec a o he Li oil measu ed wi h a high pho on
ene gy (835 eV) in o de o b oaden ange o measu ed binding ene gies.
De ec ed su ace elemen s o his oil a e oxygen, ca bon and li hium.
88 ‖ 4. S udy o Li ca bona e e olu ion on Li me al su ace
Figu e 4.1. APXPS su ey spec a collec ed in UHV o a li hium oil sc aped in a gon a mosphe e glo e
box. De ec ed su ace elemen s a e li hium, oxygen and ca bon.
Figu e 4.2 shows he in-dep h dis ibu ion o su ace compounds. Acco ding o i ,
he e is no clea e idence o ha ing me allic li hium on he su ace e en a he
highes pho on ene gy, 750 eV. This pho on ene gy has an es ima ed p obing
dep h o 8 nm, which co esponds o 3 imes he inelas ic mean ee pa h () o
elec ons. Indeed, conside ing binding ene gies o Table 4.1, we can easily iden i y
li hium ca bona e and li hium oxide. Ad en i ious ca bon con amina ion is also
de ec ed on he su ace (CH/CC). A deepe analysis o he na u e and binding
ene gy o i can be ound la e in his chap e . I is also wo h men ioning ha
species such as LiOH and Li2O2 could also be p esen on he su ace. Howe e , aim
o his sec ion is o ha e a gene al iew o he e ec i eness o cleaning he li hium
in a gon a mosphe e condi ion mo e han conduc ing a de ailed analysis o
su ace composi ion, so we a e only going o conside he dominan compounds
o su ace.
When compa ing O 1s and Li 1s spec a a di e en pho on ene gies in Figu e 4.2,
he mo e su ace sensi i e (smalle pho on ene gies), he mo e ca bona e he e is
on he su ace, as signal o ca bona e inc eases while signal o Li2O dec eases.
Then, in a comme cial li hium oil cleaned in a gon a mosphe e, li hium-based
compounds in he i s 8 nm o he su ace a e Li2O and Li2CO3, whe e he
ca bona e lies on op o he oxide.
4.3 Li oil su ace cleaning ‖ 89
Figu e 4.2. APXPS spec a showing dep h p o iling o a comme cial li hium oil which has been
sc aped in a gon a mosphe e glo e box.
Looking o he amoun o ca bona e his Li oil su ace p esen s, one can hink he
comme cial oil has al eady been exposed o CO2 gas. In o de o check i , 400
mTo CO2 gas we e added o he su ace. Figu e 4.3 shows ha su ace is no
changing a e ea ing i wi h he gas. Howe e , in he p e ious chap e we lea n
ha e en a low dose o 10-8 mba (7.5·10-5 mTo ) o CO2 gas is modi ying a clean
me allic li hium su ace. This di e ence sugges s ha , as was specula ed, li hium
su ace s o ed in an ine gas has al eady been exposed o an a mosphe e ha
con ains CO2 gas, which could be happening in he glo e box. Al hough glo e boxes
ypically ha e senso s o bo h O2 and H2O, hey nei he moni o no con ol o
po en ial CO2 gas con amina ion.
90 ‖ 4. S udy o Li ca bona e e olu ion on Li me al su ace
Figu e 4.3. E olu ion o C 1s APXPS spec a o a comme cial li hium oil measu ed a a pho on ene gy
o 600 eV while dosing 400 mTo o CO2 gas.
4.3.2 Cha ac e iza ion o Li oil su ace cleaned in UHV
As in p e ious chap e , in his one Li oil also needs o be cleaned in UHV
condi ions in o de o be able o s udy he in e ac ion o Li0 wi h CO2 gas. The way
chosen o clean he su ace o li hium oil is di e en om p e ious chap e . He e,
ins ead o using a gon ion bomba dmen , Li su ace was cleaned by sc aping i in
UHV condi ions. Fo ha , a wobble s ick ha has bo h linea and 22o angle
mo ions wi h a blade a he edge was assembled on a CF po o he load lock o
APXPS UHV sys em. Wi h his ool li hium samples we e sc aped in UHV
condi ions, wi h a base p essu e in he ange o 1·10-8 To . Figu e 4.4 shows he
di e ence be ween sc aped and non-sc aped su aces.
4.3 Li oil su ace cleaning ‖ 91
Figu e 4.4. The e is a clea di e ence in colo and shine o a Li oil coming om a gon a mosphe e
be ween he UHV sc aped and non-UHV sc aped sides.
Figu e 4.5a shows he APXPS spec a o he UHV sc aped side o he oil, whe e
p esence o Li0 in he su ace is clea . The h ee co e le els om Figu e 4.5a ha e
he same kine ic ene gy, so he a eas o ha pho oelec on peaks can be used o
quan i y he concen a ion o he compounds om he su ace, using equa ion
(2.7) and Table 4.2 pa ame e s. This quan i ica ion is shown in Figu e 4.5b. In
Figu e 4.5a, Li0 and Li2O ha e been iden i ied wi h he cons ains om Table 4.1.
The p esence o Li0 is also co obo a ed by he plasmon loss s uc u es
ep esen a i e o me allic li hium[135] indica ed in Li 1s spec a. Abou LiOH, i has
been iden i ied wi h he posi ion o he highes binding ene gy peak o O 1s, which
co esponds o ha o LiOH acco ding o li e a u e[133]. Bo h Li2O and LiOH a e
i ed using jus one peak in Li 1s spec a, called Li+. Ca bon con amina ion has
con ibu ion om alipha ic ca bon C-H/C-C and a highe binding ene gy ca bon
ha can be co ela ed o C-O bond[176], bo h ypical om ad en i ious ca bon[159].
A small amoun o g aphi ic ca bon (C=C) appea s also a lowe binding ene gies
han ad en i ious ca bon[177]. Acco ding o he su ace compounds quan i ica ion
ep esen ed in Figu e 4.5b, i 1 oxygen a om is assigned o each C-O species om
C con amina ion, C-O based oxygen only ep esen 1.1% o o al oxygen a oms.
Due o his, we neglec i s con ibu ion in O 1s spec a.
92 ‖ 4. S udy o Li ca bona e e olu ion on Li me al su ace
Binding ene gy o alipha ic ca bon (a ound 288 eV, Figu e 4.5a) is highe han ha
om p e ious chap e (285 eV, Figu e 3.11). A s udy ha ocuses on he co ec
assignmen o binding ene gies in li hium oil men ions he exis ence o wo ypes
o CH/CC: one om he bulk a ound 285 eV, and ano he one a 3 eV highe han
ha one, which comes om he su ace[178]. Then, CH/CC measu ed in his chap e
will co espond o su ace CH/CC, which ag ees wi h he highe su ace sensi i i y
pho on ene gies o his chap e . To con i m ha he p oposed binding ene gy
based on Li0 posi ion is easible, we measu ed he Fe mi edge egion o his same
sample by APXPS. Figu e 4.6 shows ha he Fe mi edge lies a 0 eV, as expec ed
o me allic samples. Then, we can assume calib a ion based on Li0 is adequa e.
Figu e 4.5. a) APXPS spec a o Li 1s, O 1s, and C 1s co e le els o a UHV sc aped li hium oil measu ed
a same kine ic ene gy, which allows o use he a eas o he pho oelec on peak o quan i y he
su ace composi ion. In he spec a, he i ed cu e (black line) ollows expe imen al da a (do s),
and backg ound is ep esen ed by a dashed line. b) Fi s 3 nm su ace composi ion o a UHV cleaned
comme cial li hium oil.
4.3 Li oil su ace cleaning ‖ 93
Figu e 4.6. APXPS spec a o e mi edge egion on a UHV sc aped li hium oil measu ed a a pho on
ene gy o 280 eV. The posi ion o he e mi edge is a 0 eV, as should be o a me allic sample.
Acco ding o he su ace compounds quan i ica ion ep esen ed in Figu e 4.5b,
su ace is domina ed by Li0 and Li2O. This quan i ica ion co esponds o he i s 3
nm o he su ace, since p obing dep h can be es ima ed as 3 imes inelas ic mean
ee pa h () o elec ons. In o de o ob ain he in-dep h dis ibu ion o Li species
in he su ace, we measu ed Li 1s spec um a se e al pho on ene gies. Figu e 4.7
shows ha when su ace is measu ed a he highes pho on ene gies, oxidized
li hium in ensi y dec eases compa e o ha o Li0, meaning oxidized laye is on op
o Li0 subs a e. In his same igu e, p obing dep h o each pho on ene gy is also
indica ed.
To ha e an es ima ion o he hickness o oxidized o e laye , SESSA (NIST
Da abase o he Simula ion o Elec on Spec a o Su ace Analysis) so wa e was
used. Conside ing a pho on ene gy o 600 eV and ins umen se ings o he
spec ome e om beamline 9.3.2 a he Ad ance Ligh Sou ce synch o on,
se e al Li 1s spec a we e simula ed o he ollowing sys em: Li2O laye on op o
a Li0 subs a e, whe e he a iable is he hickness o Li2O o e laye . Wi h hese
spec a a co ela ion was ob ained be ween he in ensi ies o Li0 and Li2O
measu ed a a pho on ene gy o 600 eV, om Li 1s co e le el. This co ela ion is
illus a ed in Figu e 4.8 and ep esen ed in he ollowing equa ion:
=
59
.
9
−
13
.
1
l
n
+
(4.5)
94 ‖ 4. S udy o Li ca bona e e olu ion on Li me al su ace
whe e is he hickness o Li2O o e laye (Å) and a e he in ensi ies o Li2O and
Li0 peaks om Li 1s measu ed a a pho on ene gy o 600 eV. Wi h his equa ion
we can di ec ly ob ain he hickness o he o e laye , using he in ensi ies o Li 1s
Li0 and Li2O measu ed a 600 eV.
Figu e 4.7. Li 1s APXPS spec a collec ed in UHV a di e en pho on ene gies o illus a e he dep h
p o iling o a clean Li su ace. In he spec a, he i ed cu e (black line) ollows expe imen al da a
(do s), and backg ound is ep esen ed by a dashed line. The app oxima ed p obing dep h (3 imes
he inelas ic mean ee pa h) o each measu ed pho on ene gy and ha o Mg sou ce a e indica ed
in he igu e. The mean oxide laye hickness p esen on UHV cleaned li hium su aces is also
speci ied in he igh side o he igu e.
The oxide laye on op o Li 1s o ou UHV cleaned li hium oil is o med by bo h
Li2O and LiOH, as shown in Figu e 4.5. I we wan o use he abo e-men ioned
equa ion o calcula e he hickness o he o e laye ha is a enua ing Li0 in ensi y
in Li 1s co e le el, we a e assuming ha all Li+ is ela ed o Li2O. Howe e ,
acco ding o Figu e 4.5b, Li2O accoun s o he 60% o oxidized li hium.
Fu he mo e, i we compa e he o elec ons coming om Li0 measu ed a a
pho on ene gy o 600 eV, o elec ons h ough Li2O laye is 15.72 Å, and h ough
4.4 Li2CO3 e olu ionon Li me al su ace ‖ 95
LiOH laye is 16.28 Å. This simila makes he a enua ion ha bo h compounds
p oduce on Li me al su ace elec ons compa able. Then, we conside he use o
equa ion (4.5) o calcula e he hickness o he oxidized laye on op o li hium
me al is adequa e. The calcula ed a e age hickness o oxidized laye o he
se e al clean su aces s udied on his chap e is 1.6 ± 0.9 nm, which is indica ed in
Figu e 4.7.
Figu e 4.8. Rela ion be ween he Li0 and Li2O compound in ensi ies o Li 1s and he hickness o Li2O
o e laye , calcula ed by SESSA so wa e. Simula ion conside s a pho on ene gy o 600 eV and
ins umen se ings o beam line 9.3.2 om he Ad anced Ligh Sou ce.
4.4 Li2CO3 e olu ion on Li me al su ace
Fo he s udy o Li2CO3 g ow h on Li me al, h ee CO2 gas (5.0 esea ch pu i y om
P axai ) p essu es we e conside ed: 0.1 mTo , 10 mTo and 400 mTo . Gas was
dosed o a ound one hou a each p essu e. E e y gas p essu e dose s a ed wi h
a UHV cleaned Li oil.
4.4.1 E olu ion o ca bon-based compounds
In o de o s udy he e olu ion o ca bon while ea ing Li wi h CO2 gas, i s o all,
he compounds p esen on C 1s spec um need o be de ined. In his spec um,
binding ene gy o li hium ca bona e is clea ly de ined a 292.70 eV (Table 4.1).
Binding ene gy o CH/CC om ad en i ious ca bon has also been al eady ound o
102 ‖ 4. S udy o Li ca bona e e olu ion on Li me al su ace
Figu e 4.14. APXPS C 1s spec a o a li hium oil a e 10 mTo CO2 gas ea men o abou one
hou , decon olu ed wi h wo ypes o i ings. Measu emen was done a a pho on ene gy o 600 eV
in UHV condi ion. All ca bon-based compounds ha e been iden i ied. In he spec um, he i ed
cu e (black line) ollows expe imen al da a (do s), and backg ound is ep esen ed by a dashed line.
4.4.2 Li2CO3 g ow h kine ics
When analyzing in mo e de ail he e olu ion o Li2CO3 ep esen ed inFigu e 4.15,
we can obse e ha Li2CO3 g ow h is linea -pa abolic, esembling Deal G oo e
oxide hick g ow h model[189]. In his ype o g ow h wo egimes a e dis inguished:
a eac ion limi ed egime and a di usion limi ed egime, whe e he laye o med
in he ini ial egion is esponsible o he di usion limi a ion. The linea egime is
de ined by he ollowing g ow h a e:
−
=
(4.6)
And he pa abolic egime is:
−
=
(4.7)
whe e is he hickness o he laye a ime , he ini ial hickness o he laye ,
and and a e he linea and pa abolic eac ion a e cons an espec i ely.
Bo h eac ion a e cons an s a each p essu e a e calcula ed based on he Li2CO3
a ea o no malized C 1s spec a measu ed a a pho on ene gy o 600 eV, which is
ela ed o he hickness o he laye . Figu e 4.16 shows ha , in he linea egime,
4.4 Li2CO3 e olu ionon Li me al su ace ‖ 103
p essu e plays a no iceable ole in he eac ion a e. Howe e , in he case o
pa abolic egime, he eac ion a e a 10 mTo CO2 and 400 mT CO2 is e y simila .
This indica es ha di usion h ouhg he laye o Li2CO3 o med a he linea
egimes o hese p essu es is limi ing he eac ion mo e han he incoming CO2
molecules.
Figu e 4.15. In he Li2CO3 a ea e olu ion ( om C 1s APXPS spec a measu ed a 600 eV) o a li hium
oil dosed by h ee p essu es o CO2 gas, wo egimes wi h di e en eac ion a es can be
dis inguished.
Figu e 4.16. Reac ion a e cons an o Li2CO3 g ow h on Li me al in linea and pa abolic egime a
h ee p essu es. Reac ion a es ha e been calcula ed om he a eas o Li2CO3 in C 1s APXPS spec a,
measu ed a a pho on ene gy o 600 eV.
104 ‖ 4. S udy o Li ca bona e e olu ion on Li me al su ace
4.4.3 Dep h p o iling o li hium-based compounds
An impo an ad an age o pe o ming APXPS measu emen s in a synch o on
adia ion acili y is he abili y o pe o m non-des uc i e dep h p o iling
expe imen s. To ob ain he in o ma ion o compounds dis ibu ion along he
su ace dep h, same su ace is measu ed using se e al pho on ene gies. Figu e
4.17 shows he inal su ace o Li oil ea ed a h ee CO2 gas p essu es, measu ed
a h ee di e en pho on ene gies.
Figu e 4.17. APXPS Li 1s spec a showing dep h p o iling o li hium oil ea ed wi h h ee p essu es
o CO2 gas, measu ed a UHV condi ion. In he spec a, he i ed cu e (black line) ollows
expe imen al da a (do s), and backg ound is ep esen ed by a dashed line.
4.4 Li2CO3 e olu ionon Li me al su ace ‖ 105
Acco ding o Table 4.1, Li0, Li2O and Li2CO3 compounds a e easily iden i ied in
li hium 1s spec a. Bu , as jus concluded analyzing C 1s spec a e olu ion, Li2C2O4
is also p esen on he su ace. We couldn´ ind any XPS binding ene gy e e ence
o Li2C2O4 in Li 1s spec a. Howe e , epo ed binding ene gies o ROCO2Li and
Li2C2O4 in C 1s co e le el spec a a e e y simila [180,185]. Then, we conside ha
bo h Li 1s om Li2C2O4 and ROCO2Li will also ha e simila binding ene gies. Tha
o Li 1s in ROCO2Li is 0.2 eV lowe han Li2CO3[180]. Because o he small di e ence
be ween bo h binding ene gies, we i he spec a coupling con ibu ions o Li2CO3
and Li2C2O4 in one peak. Binding ene gy o his peak lies be ween 58.1-57.7 eV.
FWHM cons ains will be 0.2 eV highe han ha o Li2CO o Li 1s om Table 4.1
o accoun o he wo ypes o Li wi h sligh ly di e en binding ene gies.
Looking o Figu e 4.17, he highe he pho on ene gy (highe p obing dep h), he
highe he in ensi y o Li2O. This beha io is he same o he h ee p essu es,
sugges ing ha he laye ed su ace s uc u e is: Li0 on he bo om, an in e media e
Li2O laye , and a opmos su ace laye wi h bo h Li2CO3 and Li2C2O4. In o de o
check whe he ac ually elec ons om Li2O laye a e being a enua ed by an
o e laye , we analyze he a enua ion o Li2O pho oelec on in ensi y o he same
sample measu ed a di e en pho on ene gies. Acco ding o equa ion (2.8), i an
o e laye is co e ing a subs a e, in ensi y should obey an exponen ial decay
when subs a e elec ons ha e lowe inelas ic mean ee pa h, which happens a
di e en pho on ene gies. To be able o compa e he in ensi ies measu ed a
di e en pho on ene gies, acco ding o equa ion (2.7), in ensi y has o be
co ec ed by he pho on lux (Table 4.2), c oss sec ion (Table 4.2) and he inelas ic
mean ee pa h, (Table 4.4). Figu e 4.18 shows Li2O pho oelec on in ensi y is
a enua ed ollowing an exponen ial decay a all h ee p essu es, con i ming Li2O
is co e ed by an o e laye .
The i ing equa ions o each p essu e a e he ollowings:
0
.
1
→
=
3
.
5
·
1
0
.
/
(4.8)
0
→
=
4
.
9
·
10
.
/
(4.9)
400
→
=
4
.
0
·
10
.
/
(4.10)
106 ‖ 4. S udy o Li ca bona e e olu ion on Li me al su ace
whe e I indica es he co ec ed in ensi y o Li2O pho oelec on peak o m Li 1s
co e le el, is he inelas ic mean ee pa h o Li2O elec ons passing h ough he
o e laye o Li2CO3 and Li2C2O4, and 11.1, 16.8 and 20.6 pa ame e s a e he
o e laye hickness (Å) o Li oil dosed o abou one hou a 0.1, 10 and 400 m o
CO2, espec i ely.
Figu e 4.18. E olu ion o he co ec ed in ensi ies o Li 1s Li2O pho oelec on peak spec a measu ed
a h ee pho on ene gies: 280 eV, 510 eV and 750 eV. Elec ons ha e a di e en inelas ic mean ee
pa h () in each pho on ene gy.
Conside ing his sequence and he in ensi ies o he compounds om Figu e 4.17,
hicknesses o su ace laye s a e calcula ed using equa ions (2.11) and (2.12) and
he pa ame e s om Table 4.3 and Table 4.4. Fo he ca bonaceous laye
a enua ing Li2O in ensi y, a omic densi y and inelas ic mean ee pa h a e
calcula ed accoun ing each con ibu ion o Li2CO3 and Li2C2O4. This con ibu ion is
gi en by he a ea a ios o hem in C 1s spec a o dosed su aces, measu ed a a
pho on ene gy o 600 eV in UHV condi ion. Figu e 4.19 shows laye ed ske ches o
inal li hium su ace, whe e con ibu ion o each li hium ca bonaceous
compounds is also indica ed. Thickness alues a e a e age hickness calcula ed by
each pho on ene gy, and he de ia ion be ween he pho on ene gies is indica ed
by e o ba s. Ob ained Li2CO3/Li2C2O4 laye hicknesses ep esen ed in Figu e 4.19
a e in good ag eemen wi h ha ones om equa ions (4.8), (4.9) and (4.10).
4.4 Li2CO3 e olu ionon Li me al su ace ‖ 107
In Figu e 4.19, we obse e he highe he gas p essu e he la ge he Li2CO3 and
Li2C2O4 hickness. Li2O also e ol es conside ing ini ial clean Li su ace has an
oxidized laye o 16 ± 9 Å hickness, acco ding o Figu e 4.7. Li2O as a consequence
o his in e ac ion was al eady obse ed in p e ious chap e (sec ion 3.4.2).
Howe e , Li2O laye hickness is almos he same o he h ee p essu es, so he
gas p essu e is no playing a ole in he g ow h o Li2O.
Figu e 4.19. Laye ed ske ches o li hium su aces a e exposing hem o CO2 gas a h ee p essu es
o abou an hou . Es ima ed hicknesses o he o e laye s and composi ion o each laye a e
indica ed in he igu e o he i s 100 Å o he su ace.
I is wo h men ioning ha in he laye ed model we a e no conside ing CH/CC, C-
O and C=O con ibu ions. These compounds will p oduce an ex a a enua ion in
Li 1s elec ons. Bu we assume his a enua ion o be he same o all Li 1s
compounds, so he a ios be ween Li 1s compounds should no be a ec ed by
hem.
4.4.4 Insigh s in o he eac ion mechanism
So a , ou analysis e eals ha ca bona e, oxala e, CH/CC and Li2O e ol e on he
su ace upon Li in e ac ing wi h CO2 gas. Fu he mo e, we obse e ha Li2O lies
be ween Li0 and ca bonaceous compounds. In o de o explain his su ace
e olu ion, and based on p oposed eac ion mechanisms[137,187], we sugges a
possible eac ion pa hway, ep esen ed in Figu e 4.20.
Acco ding o his mechanism, CO2 eac ion o Li0 si es leads o Li2O and CO.
Depending on he a ailabili y, his CO could be adso bed in bo h Li0 and Li2O si es.
In he i s case (pa hway A in Figu e 4.20), he eac ion pa hway ollowed o o m
ca bona e will be he one pos ula ed by Zhuang e al.137, whe e mo e Li2O will be
c ea ed on he su ace, and he oxide will hen eac wi h CO2 o o m ca bona e.
In he second case (pa hway B in Figu e 4.20), oxala e will be o med, which will
108 ‖ 4. S udy o Li ca bona e e olu ion on Li me al su ace
ac as an in e media e o c ea e ca bona e and mo e CO, as claimed ea lie 186. This
second pa hway is a sel - ecycling p ocess due o he con inuous CO e olu ion. O
he wo possible eac ion pa hways, we hink B is a o ed. Ou easoning elies on
he small C inc ease, compa ed o ha o ca bona e, ha occu s on he su ace,
in e ed om Figu e 4.12 when we compa e Li2CO3 and CH/CC e olu ions. Ano he
conclusion ha we can d aw om he eac ion mechanism is ha pa hway B
equi es me allic li hium o be accessible on he su ace o c ea e he equi ed CO
o he o ma ion o oxala e. In o he wo ds, i he su ace we e comple ely
oxidized, no oxala e would e ol e, and ca bona e would domina e he su ace.
Figu e 4.20. Li hium ca bona e g ow h eac ion mechanism.
4.4.5 O2 gas e ec on Li2CO3 g ow h
To u he explo e he sys em, CO2 gas was codosed wi h O2 gas (5.0 esea ch
pu i y om P axai ). S a ing om a UHV cleaned Li oil, we i s added 0.1 mTo
CO2 gas and a e 10 minu es, O2 gas was also added o he sys em o one hou .
When doing so, Li2CO3 g ow h is p omo ed, changing he g ow h a e o he
eac ion i i is compa ed wi h a pu e CO2 educ ion, as shown in Figu e 4.21.
To u he s udy he e ec o he oxygen, we epea ed he codose expe imen bu
in e sing he o de o adding he gases. In his second codose, i s we added O2
gas, and hen CO2 gas. I we compa e he inal C 1s spec a and he e olu ion o
Li2C2O4 o bo h co doses (Figu e 4.22), when O2 gas is added i s , no Li2C2O4 is
e ol ed, and he inal su ace is pu e ca bona e wi hou e en ad en i ious ca bon.
In Figu e 4.22 we also clea ly see how he addi ion o O2 gas is changing he
mechanism o o m Li2CO3 in he i s co-dose s udied, whe e Li2C2O4 s ops
e ol ing.
4.4 Li2CO3 e olu ionon Li me al su ace ‖ 109
Figu e 4.21. E olu ion o aw a ea o Li2CO3 om C 1s APXPS spec a, measu ed a a pho on ene gy
o 600 eV and a wo di e en dose condi ions.
Figu e 4.22. E olu ion o aw a eas o Li2C2O4 om C 1s spec a measu ed a a pho on ene gy o 600
eV, a wo CO2 and O2 gas codoses expe imen s. In he i s codose CO2 gas is added i s (o ange)
and in he second co dose O2 gas is added i s (g een). Final C 1s spec a a e also indica ed in he
igh side o he igu e o each co dose. In he spec a, he i ed cu e (black line) ollows
expe imen al da a (do s), and backg ound is ep esen ed by a dashed line.
110 ‖ 4. S udy o Li ca bona e e olu ion on Li me al su ace
This beha io indica es ha he eac ion mechanism o c ea e li hium ca bona e
is bypassed when he a mosphe e is comp ised o bo h CO2 and O2; his has been
schema ically ep esen ed in Figu e 4.23. Due o he in e ac ion o oxygen wi h
li hium, li hium oxide is c ea ed on he su ace, and he e is no me allic li hium
accessible o eac wi h CO2, hus no oxala e e ol es on he su ace, u he
suppo ing he p oposed eac ion mechanism o Figu e 4.20.
Figu e 4.23. E ec o O2 gas in li hium ca bona e g ow h eac ion mechanism.
Ano he consequence o adding he gases in a di e en o de is he a e o li hium
oxida ion, as can be in e ed om Figu e 4.24 by he as disappea ance o su ace
Li when adding i s O2.
I is wo h men ioning ha oxala e con ibu ion has no been aken in o accoun
conside ing i s small con ibu ion compa e o ha o Li2CO3 (Figu e 4.22). The
as e oxida ion o he su ace wi h O2 ag ees wi h he conclusion ob ained in
p e ious chap e : O2 gas oxida ion a e is highe han ha o CO2 gas (sec ion 3.4).
This beha io implies ha , i li hium is p e ea ed wi h CO2 and hen O2 gas is
added, oxida ion is slowe and mo e me allic li hium will be a ailable nea he
su ace egion, as shows Figu e 4.24.
We calcula e he e olu ion o he hickness o Li2O and Li2CO3 o he i s co dose,
he one s a ed wi h CO2 gas. Fo ha , Li 1s 600 eV unc ion o ime spec a
compounds in ensi ies, equa ions (2.11) and (2.12) and pa ame e s om Table 4.3
and Table 4.4 a e used. Resul s a e summa ized in Figu e 4.25. The e, we obse e
ha , when CO2 gas con ac s Li0 su ace, bo h Li2CO3 and Li2O a e o med wi h a
high eac ion a e. When adding O2 gas o he sys em (g een egion in Figu e 4.25),
Li2O keeps cons an , meaning all O2 is used o o m Li2CO3, u he a i ming
bypassed eac ion ep esen ed in Figu e 4.23.
4.4 Li2CO3 e olu ionon Li me al su ace ‖ 111
Figu e 4.24. APXPS Li 1s spec a e olu ion while codosing a li hium su ace wi h CO2 and O2 gases,
changing he o de o adding he gases. Spec a is measu ed a a pho on ene gy o 600 eV. In he
spec a, he i ed cu e (black line) ollows expe imen al da a (do s), and backg ound is ep esen ed
by a dashed line.
118 ‖ 5. Li hin ilm g ow h
The p ocess o g owing a hin ilm by deposi ion has six s eps: i s he a i ing
a oms ha e o adso b on he su ace, hen hey di use some dis ance, a e ha
a eac ion o he adso bed species wi h each o he and wi h he su ace occu s o
o m he bonds o he ilm. The ou h s ep is he nuclea ion, he ini ial
agg ega ion o he ilm ma e ial, and hen he s uc u e de elops. Finally,
di usional in e ac ions occu wi h he bulk o he ilm and wi h he subs a e[193].
Following, we a e going o summa ize he possible s uc u al de elopmen
mo phologies, which will be use ul o compa e wi h he mo phologies ob ained
expe imen ally la e in he chap e .
5.1.1 S uc u e De elopmen o a hin ilm
The e a e h ee basic s uc u al zones ha depends on he a io be ween he
subs a e empe a u e (Ts) and he mel ing poin o he ilm (Tm), all o hem
illus a ed in Figu e 5.1. Z1 occu s when Ts/Tm is so low ha su ace di usion is
negligible. Columns o Z1 ha e poo o none c ys allini y and a e sepa a ed by
oids. In Z2, when Ts/Tm is highe han Z1, su ace di usion is signi ican and he
s uc u e consis o columns ha ing igh g ain bounda ies be ween hem.
C ys alline columns a e less de ec ed han Z1 and a e o en ace ed a su ace. In
Z3, due o he highe empe a u e o he subs a e compa ed o p e ious zones,
we can conside bulk annealing o he ilm is aking place du ing deposi ion. This
is cha ac e ized by mo e iso opic o equiaxed c ys alli e shape. The e is an ex a
zone be ween Z1 and Z2 called he ansi ional zone (ZT), which con ains simila
columns o hose o Z1 bu oids and domes a e absen , and is usually associa ed
wi h ene gy enhanced p ocesses as spu e deposi ion. Some imes, anomalous
s uc u e o ms occu , in pa icula he whiske s, illus a ed also in Figu e 5.1.
Figu e 5.1 a) Cha ac e is ic c oss sec ion o he h ee basic s uc u e zones when de eloping a hin
ilm. Ra io o subs a e empe a u e o ilm mel ing op ion inc eases om le o igh . b) Whiske s
anomalous s uc u e o ma ion. Adap ed om[193].
5.2 Expe imen al p ocedu e ‖ 119
5.2 Expe imen al p ocedu e
Figu e 5.2 shows he UHV chambe sys em designed o e apo a e Li. Load lock o
he sys em is compa ible wi h he ai sensi i e po able ans e a m om Figu e
2.8, hus ai exposu e o he samples is p e en ed. Base p essu e o he sys em is
low 10-8 – high 10-9 mba .
Figu e 5.2. a) F on iew and b) side iew o he UHV chambe sys em whe e Li e apo a ions we e
pe o med.
Li sou ces used in his s udy a e comme cially a ailable ch oma e- ee me al apo
sou ces (al asou ces, om al a ec[97]). They con ain an in e me allic compound in
a gon a mosphe e inside a small s ainless-s eel ube, sealed wi h indium, as
shown in Figu e 5.3. Capaci y o he sou ces is 190 mg and diame e o he ube 5
mm.
Li hium me al is he mally e apo a ed om he in e me allic compound when
passing a cu en h ough he con ac ing laps. To do ha , he con ac ing laps
we e welded o wo conduc i e ods connec ed o he eed h ough o Figu e 5.2b.
Fundamen s o he mal e apo a ion a e explained in chap e 2 sec ion 2.1.1
sec ion.
In he ac i a ion o he sou ces, indium seal is emo ed in UHV condi ions.
Acco ding o he in o ma ion p o ided by he supplie , indium mel s om 1.5 A o
4 A, which causes he elease o he a gon, inc easing he p essu e o he chambe .
Expe imen ally, we obse ed his elease a an in ensi y o 5.3 A. Besides, in o de
120 ‖ 5. Li hin ilm g ow h
o emo e all he In om he sou ce, we kep i a a highe in ensi y o abou 3
hou s. Once he sou ce was ac i a ed, we ne e exposed i o ai a mosphe e.
Figu e 5.3. Con igu a ion o a ypical alkaline sou ce om al a ec. Adap ed om[97].
S eps ollowed o pe o m an e apo a ion we e:
1) Subs a e cleaning s ep. This s ep was pe o med in an ul asonic ba h, wi h
he ollowing sequence: i s ace one, hen e hanol and inally wa e , 10
minu es in each one. A e ha , subs a e was d ied in an o en a 80 °C
o e nigh .
2) Subs a e loading s ep. Subs a e was load in he load lock and le in acuum
a leas 12 hou s. Du ing his s ep, a i anium sublima ion pump was un o
help eco e he base p essu e o he chambe .
3) Remo al o impu i ies s ep. In his s ep, explained in sec ion 2.1.1, ola ile
impu i ies we e emo ed. Fo ha , sample was isola ed om he Li sou ce
closing he co esponding ga e al e, and in ensi y was inc eased up o 3 A
and kep i o hal an hou .
4) Li sou ce s abili y s ep. In ensi y passing h ough he sou ce was inc eased up
o he e apo a ing alue, which will be highe han 5 A o ou comme cial
sou ces acco ding o supplie ’s manual[97]. We kep ha in ensi y un il
p essu e was s able. P essu e should be in he ange o 10-7–10-8 mba .
5) E apo a ion s ep. Subs a e was mo ed o he on o he li hium sou ce,
opening he co esponding ga e al e. Sample was kep he e he desi ed
e apo a ion ime.
5.3 S udy o li hium hin ilm deposi ion ‖ 121
5.3 S udy o li hium hin ilm deposi ion
5.3.1 Li sou ce deposi ion a e calcula ion a 8 A
The deposi ion a e o he Li hin ilm is de ined by he in ensi y applied o he Li
sou ce, so each in ensi y will ha e a speci ic deposi ion a e. Aim o his sec ion is
o de e mine he deposi ion a e a an in ensi y o 8 A.
Thickness o hin ilms we e es ima ed by scanning elec on mic oscope (SEM),
using he ins umen de ailed in sec ion 2.3.4.1, and measu ing seconda y
elec ons a 30 kV. E apo a ed samples we e cu in he glo e box using a diamond
sc ibe and ans e ed o he SEM wi h he ai sensi i e ans e a m om he SEM
sys em (Figu e 2.14). The e, measu ing a c oss sec ion, hickness o he deposi ed
laye was ob ained, and he op iew images p o ided in o ma ion abou he
mo phology o he li hium.
In o de o measu e he hickness om a c oss sec ion image, a subs a e which
will be easy o cu and will p oduce a sha p edge is needed, as silicon wa e is,
common subs a e o hin ilm deposi ions. We hen s a ed he s udy wi h a
silicon monoc ys alline wa e (<100>, Bo addi i e, ρ > 1 Ω·cm, Vi ginia
semiconduc o s). When cha ac e izing he e apo a ion o li hium in Si wa e some
singula i ies we e ound, which led us o s udy he e apo a ion p ocess in se e al
laye sequences, all o hem indica ed in Table 5.1. Following we a e going o
explain he conclusions ob ained in each sequence.
Table 5.1. Sequences used o s udy he e apo a ion and g ow h mo phology o li hium. Li hium
laye s a e e apo a ed a he cu en and imes speci ied in he able and Ti is spu e ed using a
magne on spu e ing.
Sequence Subs a e Laye 1 Laye 2 Laye 3
A Si wa e ILi
8 A, 12 h
B Si wa e ILi
8 A, 6 h
C Si wa e ILi Ti
8 A, 23 h 340 nm
D S ainless s eelII Li Ti
8 A, 23 h 340 nm
E Si wa e ITi Li Ti
255 nm 8 A, 24 hou s 680 nm
I Si wa e <100>, Bo addi i e, ρ > 1 Ω·cm
II S ainless s eel om he sample holde
122 ‖ 5. Li hin ilm g ow h
In o de o make su e ha he sou ce is e apo a ing li hium, i s e apo a ion
(sequence A om Table 5.1) was cha ac e ized using X- ay pho oelec on
spec oscopy ins umen explained in sec ion 2.3.1.3. Figu e 5.4 shows ha , a e
an e apo a ion o 8 A o 12 hou s in a Si wa e , he su ace is co e ed by a laye
ha con ains mainly li hium and oxygen.
Figu e 5.4. XPS su ey spec a measu ed by Mg sou ce o a silicon wa e a e li hium e apo a ion,
ollowing sequence A om Table 5.2. The no malized su ace a omic concen a ion is indica ed also
in he igu e. The inse ep esen s Li 1s egion measu ed a highe esolu ion wi h same pho on
sou ce.
Apa om he expec ed ad en i ious ca bon impu i y, he e is also a small
amoun o luo ine and sul u on he su ace. These elemen s a e a c oss
con amina ion because o some impu i y we had a ha ime in he XPS
ins umen , and bo h ep esen s a ound 5.5% o he no malized a omic
concen a ion om he su ace. Fo he calcula ion o his concen a ion we
conside ed he a eas o he peak o he elemen s om he su ey o Figu e 5.4.
A eas we e co ec ed wi h he co esponding ela i e sensi i e ac o o each
elemen based on Sco ield c oss sec ions and a ansmission unc ion co ec ion
5.3 S udy o li hium hin ilm deposi ion ‖ 123
speci ic o his equipmen . An exponen ial ac o was also used o co ec he
di e ence in he inelas ic mean ee pa h o elec ons. Wi h his measu emen , i
was con i med ha sou ce e apo a ed jus Li. Figu e 5.5 shows he c oss sec ion
and op iew o his sample, whe e he expec ed hin ilm is no obse ed. Indeed,
he su ace is ull o mic os uc u ed ea u es, as la ge as 3 mic ome e s.
Figu e 5.5. Ai sensi i e SEM a) c oss sec ion and b) op iew o a li hium e apo a ion on a
monoc ys alline Si wa e ollowing sequence A om Table 5.1.
Looking o he wei d Li deposi ion ob ained on Si, he s a e o Si wa e used in his
s udy was checked. Figu e 5.6a shows ha he cu made o measu e c oss sec ion
using a diamond sc ibe was no esponsible o he mic os uc u ed ea u es. In
Figu e 5.6b i can be obse ed ha ini ial Si wa e su ace was la , so Li
mo phology was no induced by an inhomogeneous subs a e.
Figu e 5.6. SEM images o he a) c oss sec ion and b) op iew o he monoc ys alline silicon wa e
used o e apo a e li hium.
Then, o ha e a be e unde s anding o how he li hium was g owing on he
silicon wa e , a new Li e apo a ion was pe o med, sho en he e apo a ion ime
124 ‖ 5. Li hin ilm g ow h
(sequence B om Table 5.1). Su ace o sequence B, ep esen ed in Figu e 5.7,
shows he same non-uni o m su ace go in p e ious e apo a ion. In addi ion,
when ying o ge images o he li hium su ace s uc u es a highe magni ica ion
han he ones om Figu e 5.5, Li was deg aded. This deg ada ion, a consequence
o he in e ac ion o Li me al wi h SEM elec on gun, ook place when measu ing
wi h magni ica ions ha show a scale equal o smalle o 2 m (magni ica ion o
x50000). Figu e 5.7b clea ly shows how he a ea whe e SEM images a e aken is
comple ely di e en om he es o he su ace. Figu e 5.7c also con i ms his
beha io , showing he disappea ance o a li hium ea u e om he su ace jus by
ying o ocus he a ea wi h SEM mic oscope. Time be ween each image is he
one needed o sa e he pic u e, a ound 5 seconds.
Figu e 5.7. Ai sensi i e SEM a) c oss sec ion and b) op iew o a li hium e apo a ion on a
monoc ys alline Si wa e ollowing sequence B om Table 5.1. c) E olu ion o a su ace Li ea u e
om he c oss sec ion exposed o SEM elec on gun. Time be ween each pic u e is a ound 5 seconds,
he one needed o sa e he images.
To a oid he p oblem o li hium deg ada ion, in he ollowing deposi ion
(sequence C om Table 5.1) e apo a ed Li was co e ed wi h a Ti laye , deposi ed
by magne on spu e ing. De ails abou he echnique and equipmen can be
ound in chap e 2 sec ion 2.1.2. Op imal spu e ing condi ions and deposi ion
5.3 S udy o li hium hin ilm deposi ion ‖ 125
a es o Ti we e ob ained by o he esea che s o he Ad anced In e ace Analysis
g oup om CIC Ene gigune. The same ai sensi i e ans e a m used o mo e
samples om Li e apo a ion UHV chambe sys em is compa ible wi h he
spu e ing sys em, p e en ing sample ai exposu e.
Li e apo a ion ime in he sequence C was highe (24 hou s) han p e ious cases
in o de o ha e be e sense o he s uc u al de elopmen o he li hium. When
measu ing he c oss sec ion and op iew o sequence C, ep esen ed in Figu e
5.8, we clea ly obse e ha li hium was no deposi ing as a uni o m laye .
Figu e 5.8. Ai sensi i e SEM a) c oss sec ion, b) op iew and c) il ed iew o a li hium e apo a ion
ollowing sequence C om Table 5.1, whe e a Ti laye is added o he li hium e apo a ed on Si wa e .
This ype o s uc u al de elopmen does no co espond o any o he basic zones
explained in Figu e 5.1a. In ac , his is an anomalous ype g ow h, simila o he
whiske s o Figu e 5.1b, ha some imes happens when ying o ob ain a hin
ilm[194]. Abou he e ec o co e ing Li wi h Ti o p e en i s deg ada ion, i was
been e ec i e. In his sample we we e able o go o high magni ica ions wi hou
damaging he su ace, which can be seen i Figu e 5.7b and Figu e 5.8b a e
compa ed
126 ‖ 5. Li hin ilm g ow h
Because o he con igu a ion o he e apo a ion sys em, we we e also e apo a ing
li hium and i anium on he holde , no jus on he Si subs a e. This holde is made
o s ainless s eel, so we ha e a new deposi ion sequence he e, he one called
sequence D in Table 5.1.Figu e 5.9 shows he compa isons o he su ace
mo phology o li hium deposi ed on he Si and Li deposi ed on he s ainless-s eel
(SS) holde . The e, i can be concluded ha whiske ype g ow h is ela ed o he
na u e o he subs a e. Fo his eason, in nex e apo a ion a Ti laye was added
be ween he Li and Si wa e , a me al conside ed a good bonding ma e ial[193]. This
deposi ion, called sequence E, is summa ized in Table 5.1.
Figu e 5.9. Ai sensi i e op iew SEM o a) sequence C and b) sequence D om Table 5.1, whe e he
di e ence be ween he sequences elies on he subs a e, Si wa e and s ainless s eel (SS)
espec i ely.
Figu e 5.10 shows he su ace mo phology o se e al s eps om sequence E. The
mo phology o he deposi ed Ti (Figu e 5.10a, b) is e y simila o ha one om
li e a u e o simila deposi ion condi ions[195]. Abou he mo phology o he
e apo a ed li hium in o Ti su ace (Figu e 5.10c), i is mo e simila o he
e apo a ed li hium in o s ainless s eel (Figu e 5.9b) han o he e apo a ed
li hium in o Si wa e (Figu e 5.9a). He e, acco ding o Figu e 5.10e, we also
obse e ha he e apo a ion o Ti on op o li hium is no changing he
mo phology o li hium.
When analyzing he mo phology o he e apo a ed Li om Figu e 5.10b, we can
obse e some oids on he su ace. This kind o mo phology co esponds o a Z1
s uc u al de elopmen , explained in he in oduc ion sec ion and schema ically
ep esen ed in Figu e 5.1a. In his ype o de elopmen su ace di usion is
neglec ed.
5.3 S udy o li hium hin ilm deposi ion ‖ 127
In he c oss sec ion images o sequence E, ep esen ed in Figu e 5.11, he laye o
li hium and he wo Ti laye s can easily be iden i ied. Wi h his in o ma ion, he
deposi ion a e o Li sou ce a 8 A was es ima ed o be in he ange o 120 – 400
nm/h.
Figu e 5.10. Ai sensi i e op iew SEM images and pic u es o he su aces made while ollowing
sequence E: a) spu e ed Ti on Si wa e , b) same as a) wi h highe magni ica ion, c) e apo a ed
li hium on Ti, d) pho o o e apo a ed li hium on Ti, e) spu e ed Ti on he e apo a ed Li and ) pho o
o spu e ed Ti on he e apo a ed Li.
