Spatially Offset Raman Spectroscopy for Characterization of a Solid-State System

Ci a ion: Jaime-Ba que o, E.; Zhang,
Y.; D ewe , N.E.; López-A angu en,
P.; Zulue a, E.; Bekae , E. Spa ially
O se Raman Spec oscopy o
Cha ac e iza ion o a Solid-S a e
Sys em. Ba e ies 2023,9, 20.
h ps://doi.o g/10.3390/
ba e ies9010020
Academic Edi o : Chuang Yu
Recei ed: 26 Oc obe 2022
Re ised: 19 Decembe 2022
Accep ed: 24 Decembe 2022
Published: 27 Decembe 2022
Copy igh : © 2022 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
ba e ies
A icle
Spa ially O se Raman Spec oscopy o Cha ac e iza ion o a
Solid-S a e Sys em
Edu ne Jaime-Ba que o 1,2, Yan Zhang 1, Nicholas E. D ewe 1, Ped o López-A angu en 1, Ekai z Zulue a 2
and Emilie Bekae 1,*
1Cen e o Coope a i e Resea ch on Al e na i e Ene gies (CIC ene giGUNE), Basque Resea ch and
Technology Alliance (BRTA), Pa que Tecnológico de Ala a, Albe Eins ein 48, 01510 Vi o ia-Gas eiz, Spain
2Depa men o Sys ems Enginee ing and Au oma ion, School 10 o Enginee ing o Vi o ia-Gas eiz,
Uni e si y o The Basque Coun y (UPV/EHU), Nie es Cano 12, 01006 Vi o ia-Gas eiz, Spain
*Co espondence: ebekae @cicene gigune.com
Abs ac :
Solid-s a e ba e ies ep esen a p omising echnology in he ield o high-ene gy-densi y
and sa e s o age sys ems. Imp o ing he unde s anding o how de ec s o m wi hin hese cells
would g ea ly acili a e u u e de elopmen , which would be bes se ed by applying nondes uc i e
analy ical ools capable o cha ac e iza ion o he key componen s and hei changes du ing cycling
and/o aging. Spa ially o se Raman spec oscopy (SORS) ep esen s a po en ially use ul echnique,
bu cu en ly he e is a lack o knowledge ega ding i s use in his ield. To ill his gap, we p esen
an in es iga ion in o he use o simple de ocused mic o-SORS on sys ems cons uc ed using ypical
componen s ound wi hin solid-s a e cells. By analyzing he cons i uen s and he assembled sys em,
i was possible o ob ain dep h p o iling spec a and show ha spec a may be ob ained om laye s
which a e no mally obscu ed, demons a ing he echnique’s po en ial o nondes uc i e chemical
analysis o he subsu ace. In his way, he esul s p esen ed alida e he po en ial o mic o-SORS as a
echnique o de elop o suppo u u e solid-s a e ba e y de elopmen , as well as he nondes uc i e
ba e y analy ical ield.
Keywo ds:
solid-s a e ba e ies; nondes uc i e analy ical ools; spa ially o se Raman spec oscopy;
mic o-SORS
1. In oduc ion
The g owing demand o clean and e icien ene gy wi hin mode n socie y has gene -
a ed conside able in e es in ad ancing he ield o ene gy s o age o acili a e his ans-
o ma ion [
1
–
3
]. While con en ional li hium-ion ba e ies ha e been success ul [
4
–
6
] due
o hei high ene gy densi y and high powe densi y, he use o o ganic liquid elec oly es
ep esen s a conce n due o hei sa e y isk (swelling, gas elease, en ing, oxic gas emis-
sions, accumula ed gas igni ion, elec oly e leakage, high cell p essu e, cell case up u e o
explosion, o e en i e) [7,8].
An al e na i e o liquid elec oly es is o ins ead employ solid-s a e elec oly es, which
no only hold he po en ial o inc ease sa e y, due o g ea e he mal s abili y, bu also
o e a po en ial inc ease in ene gy densi y by enabling he use o li hium me al in place o
con en ional g aphi e anodes ( hus educing he weigh o he sys em) [
8
–
10
]. Howe e ,
signi ican challenges emain, p ima ily ela ing o p ocessing scalabili y and he mechani-
cal p ope ies o he de ice [
10
–
12
], which ypically in ol e a ious ypes o de ec s and
hei o ma ion (e.g., lack o con ac , c acks, Li dend i es, e c.). Unde s anding hese de ec s
is, he e o e, key o u u e solid-s a e elec oly e de elopmen [
13
–
15
]. Fu he mo e, a
signi ican bo leneck is he lack o unde s anding o he key in e acial cha ac e is ics (pa -
icula ly upon o ma ion, cycling, and aging), as he e is a lack o nondes uc i e echniques
capable o analysis wi hou damage ha may esul in al e a ion [16].
Ba e ies 2023,9, 20. h ps://doi.o g/10.3390/ba e ies9010020 h ps://www.mdpi.com/jou nal/ba e ies
Ba e ies 2023,9, 20 2 o 11
In his way, a as , nondes uc i e analy ical echnique, pa icula ly one able o p o ide
chemical in o ma ion, would be highly desi able as i would o e a lexible app oach o
de ec de ec ion including in si u and ex si u examina ion du ing elec ochemical cycling,
and hus con ibu e signi ican ly o u he es ablishing guidelines o u u e solid-s a e
ba e y de elopmen .
Raman spec oscopy has p e iously p o en o be a powe ul echnique o cha ac-
e izing many ba e y ma e ials (o en o e ing complemen a y da a o s uc u al cha -
ac e iza ion), and has been used no only on ma e ials and componen s bu also wi hin
cells o in si u and in ope ando measu emen s [
15
,
17
–
23
]. While con en ional Raman
spec oscopy is ela i ely s aigh o wa d in applica ion ( ypically equi ing no speci ic
sample p epa a ion), i ne e heless su e s om an inabili y o p obe samples wi h di use
sca e ing in dep h (whe e he pho on di ec ions a e mixed and di ec imaging canno be
easily used o disc imina e be ween he di e en laye s as he pho ons p opaga e in all
spa ial dimensions) [24–26].
Spa ially o se Raman spec oscopy (SORS), howe e , is a a ian o Raman spec-
oscopy which exploi s he ac ha mos ma e ials a e nei he comple ely anspa en
no opaque o ligh , bu ins ead end o sca e i , and hus i opens up he possibili y o
nondes uc i e dep h p o iling by de ec ing Raman signals om (and by his, enables he
chemical analysis o ) laye s benea h he o he wise obscu ing su aces [
27
,
28
]. I should be
no ed ha he deg ee o he dep h pene a ion o he Raman lase is dependen on a numbe
o a iables (such as na u e o ma e ial, lase wa eleng h, lase in ensi y) and, consequen ly,
applicabili y o SORS should be de e mined on a case-by-case basis wi h espec o he
sys em o be in es iga ed [
29
–
33
]. Fo SORS o be able o de ec hese subsu ace Raman
signals, he lase mus i s pene a e and con e he pho ons o he egion o in e es in o
Raman pho ons, which mus hen di use back o he su ace o be de ec ed. To achie e his,
wo condi ions mus be me : i s , he lase mus pene a e o each he laye s o in e es ,
and second, he pho ons mus be de ec able. I is impo an o conside ha he pa h o
subsu ace pho ons is longe (compa ed o he pa h o pho ons a he su ace), causing
mo e la e al sca e ing and a spa ial displacemen o spa ial o se be ween he illumina ed
and collec ed a eas on he su ace o he sample [
27
,
28
]. De ec ing signals conside ing his
spa ial displacemen enables he de ec ion o deepe pho ons and, hus, inne laye s o
he sample [28].
The e a e se e al a ia ions o SORS (such as poin -like SORS, ing collec ion SORS, ing-
illumina ion SORS o in e se SORS, de ocusing SORS, and TRS) [
27
], which ha e been success-
ully u ilized o a wide ange o a ious applica ions (e.g., in he
pha maceu ical [34,35]
, o en-
sics and secu i y [36], medical [37], ood science [38], and analy ical his o y [39–41] ields).
To he bes o ou knowledge, despi e he po en ial o p o ide in o ma ion abou
he chemical cha ac e is ics o he species wi hin elec ochemical cells, SORS has no
ye been applied in he ield o ene gy s o age. Consequen ly, his s udy p esen s an
in es iga ion in o his po en ial new applica ion o he nondes uc i e SORS echnique in
o de o de e mine sui abili y o , and challenges wi h espec o, collec ing dep h p o iling
in o ma ion.
2. Ma e ials and Me hods
2.1. X- ay Di ac ion
X- ay di ac ion (XRD) pa e ns o he as-p epa ed sample we e collec ed using a
B uke D8 Disco e (B uke , Bille ica, MA, USA) di ac ome e wi h
θ
/2
θ
B agg–B en ano
geome y, wi h monoch oma ic Cu adia ion: K
α1
= 1.54056 Å. All samples we e moun ed
inside an a mosphe e-p o ec i e PXRD sample holde wi h a Kap on ilm co e .
2.2. Raman Cha ac e iza ion
Raman spec a we e collec ed a oom empe a u e (ca. 23
◦
C) wi h a Raman mi-
c oscope (Renishaw In Via con ocal Raman, Reinshaw, Wo on-unde -Edge, UK), using a
Ba e ies 2023,9, 20 3 o 11
532 o 785 nm wa eleng h lase ocused h ough an in e ed mic oscope (Leica, We zla ,
Ge many), ia a 50×objec i e (Leica).
Sui able il e s we e used o minimize he lase powe a he su ace o <5 mW
µ
m
2
.
All samples we e measu ed inside a sealed collec ion cell, unde a gon a mosphe e, he
co e ing slides o which we e made om Raman inac i e ma e ials.
2.3. De ocused Mic o-SORS
This echnique was used o pe o m laye -by-laye dep h analysis. The exci a ion and
collec ion zones o Raman sca e ing we e enla ged by mo ing he mic oscope objec i e
ou o he ocusing posi ion on he op su ace o he samples. The mo ing dis ance is om
ca. 5 o 15
µ
m pe s ep, depending on he hickness o he laye being p obed, while Raman
signals om he sublaye unde in es iga ion cons an ly imp o e un il he bes signal
in ensi ies a e achie ed. A 532 nm wa eleng h lase was used in his wo k, wi h minimized
lase powe . Fluo escence and ambien ligh we e emo ed om he backg ound.
2.4. S ep-Like Sys em Assembly
To enable in es iga ion o he applicabili y o SORS o ypical solid-s a e ba e y
cons i uen s, a “s ep-like” sys em (see Figu e 1) was cons uc ed so ha each possible con-
igu a ion ( om comple ely exposed o co e ed by all p e ious laye s) could be measu ed.
Ba e ies2022,8,xFORPEERREVIEW3o 12

ch oma icCu adia ion:K
α1
=1.54056Å.Allsampleswe emoun edinsideana mos‐
phe e‐p o ec i ePXRDsampleholde wi haKap on ilmco e .
2.2.RamanCha ac e iza ion
Ramanspec awe ecollec eda  oom empe a u e(ca.23°C)wi haRamanmic o‐
scope(RenishawInViacon ocalRaman),usinga532o 785nmwa eleng hlase  o‐
cused h oughanin e edmic oscope(Leica), iaa50×objec i e(Leica).
Sui able il e swe eused ominimize helase powe a  hesu ace o<5mWμm
2
.
Allsampleswe emeasu edinsideasealedcollec ioncell,unde a gona mosphe e, he
co e ingslideso whichwe emade omRamaninac i ema e ials.
2.3.De ocusedMic o‐SORS
This echniquewasused ope o mlaye ‐by‐laye dep hanalysis.Theexci a ion
andcollec ionzoneso Ramansca e ingwe eenla gedbymo ing hemic oscopeob‐
jec i eou o  he ocusingposi ionon he opsu aceo  hesamples.Themo ingdis‐
anceis omca.5 o15μmpe s ep,dependingon he hicknesso  helaye being
p obed,whileRamansignals om hesublaye unde in es iga ioncons an lyimp o e
un il hebes signalin ensi iesa eachie ed.A532nmwa eleng hlase wasusedin his
wo k,wi hminimizedlase powe .Fluo escenceandambien ligh we e emo ed om
hebackg ound.
2.4.S ep‐LikeSys emAssembly
Toenablein es iga iono  heapplicabili yo SORS o ypicalsolid‐s a eba e y
cons i uen s,a“s ep‐like”sys em(seeFigu e1)wascons uc edso ha eachpossible
con igu a ion( omcomple elyexposed oco e edbyallp e iouslaye s)couldbe
measu ed.

Figu e1.Thelaye scons i u ing hes ep‐likesys em,and hesubsequen assemblyin hismode
wi hlaye  hicknessshown.
Thep epa a iono  he ou usedcomponen swasca iedou as ollows,wi hall
p ocedu esunde akeninana gon‐ illedglo ebox(<0.1ppmO
2
,<0.1ppmH
2
O).The
ca holy ewasp epa edbymixingpolye hyleneoxide(PEO,SigmaAld ich(Da ms ad ,
Ge many),Mw=5×10
6
)andLiTFSI(li hiumbis( i luo ome hanesul onyl)imide,Sol i‐
onic(Toulouse,F ance))ina20:1mola  a ioo e nigh a 200 pminace oni ile.The
ca hodewasp epa ed,asp e iouslydesc ibed,bymixingLTO(li hium i ana e,Sigma
Ald ich)nanopowde wi h hep ep epa edca holy esuch ha  he inal a iowasLTO:
ca holy e72:28,usinganIKAULTRA‐TURRAX
®
dispe se [42].Thiswas hencas a a
hicknesso 400μmusingaKcon olcoa e (byRKP in Coa Ins umen s).Thesolid
elec oly ewasp epa edbymixingPEO(Mw=5×10
6
)andLiTFSIin hemola  a io
20:1o e nigh a 200 pminace oni ile.Subsequen ly,i wascas  oa hicknesso 1500
μmandd ieda 50ºC.Finally,i washo ‐p essed(3T,100°C) oob aina hicknesso 
ca.30μm.TheLi
2
CO
3
a i icialSEIp epa a ionconsis edo mixingPEOandLi
2
CO
3

(li hiumca bona e,SigmaAld ich)in hemola  a ioo 20:1o e nigh a 200 pminace‐
oni ile.I was hencas a 1500μm hicknessandd iedo e nigh a 50°C.The hick‐
nesswasadjus edbyho p ess o50μm.Finally, hecomponen swe eassembledin he
s ep‐likecon igu a iononacleanedli hiumme alcoun e elec ode.
Figu e 1.
The laye s cons i u ing he s ep-like sys em, and he subsequen assembly in his mode
wi h laye hickness shown.
The p epa a ion o he ou used componen s was ca ied ou as ollows, wi h all p oce-
du es unde aken in an a gon- illed glo ebox (<0.1 ppm O
2
, <0.1 ppm H
2
O). The ca holy e
was p epa ed by mixing polye hylene oxide (PEO, Sigma Ald ich (Da ms ad , Ge many),
Mw = 5
×
10
6
) and LiTFSI (li hium bis( i luo ome hanesul onyl)imide, Sol ionic (Toulouse,
F ance)) in a 20:1 mola a io o e nigh a 200 pm in ace oni ile. The ca hode was p epa ed,
as p e iously desc ibed, by mixing LTO (li hium i ana e, Sigma Ald ich) nanopowde
wi h he p ep epa ed ca holy e such ha he inal a io was LTO: ca holy e 72:28, using an
IKA ULTRA-TURRAX
®
(IKA, Wilming on, NC, USA) dispe se [
42
]. This was hen cas
a a hickness o 400
µ
m using a K con ol coa e (by RK P in Coa Ins umen s, Roys on,
UK). The solid elec oly e was p epa ed by mixing PEO (Mw = 5
×
10
6
) and LiTFSI in
he mola a io 20:1 o e nigh a 200 pm in ace oni ile. Subsequen ly, i was cas o a
hickness o 1500
µ
m and d ied a 50
◦
C. Finally, i was ho -p essed (3 T, 100
◦
C) o ob ain a
hickness o ca. 30
µ
m. The Li
2
CO
3
a i icial SEI p epa a ion consis ed o mixing PEO and
Li
2
CO
3
(li hium ca bona e, Sigma Ald ich) in he mola a io o 20:1 o e nigh a
200 pm
in ace oni ile. I was hen cas a 1500
µ
m hickness and d ied o e nigh a 50
◦
C. The
hickness was adjus ed by ho p ess o 50
µ
m. Finally, he componen s we e assembled in
he s ep-like con igu a ion on a cleaned li hium me al coun e elec ode.
2.5. Elec ode–Ape u e Sys em Assembly
To in es iga e he abili y o SORS o analyze a ypical solid-s a e cell con igu a ion,
an LTO ca hode was p epa ed as desc ibed o he s ep-like sys em, wi h a small ape u e
made o enable he Raman lase o pass h ough.
This was in eg a ed wi h he solid elec oly e and cleaned li hium me al coun e
elec ode, and con ac ed unde an Al cu en collec o mesh in a con igu a ion ypical
Ba e ies 2023,9, 20 4 o 11
o Raman expe imen s whe e a con en ional cu en collec o would block he signal
(see Figu e 2).
Ba e ies2022,8,xFORPEERREVIEW4o 12

2.5.Elec ode–Ape u eSys emAssembly
Toin es iga e heabili yo SORS oanalyzea ypicalsolid‐s a ecellcon igu a ion,
anLTOca hodewasp epa edasdesc ibed o  hes ep‐likesys em,wi hasmallape ‐
u emade oenable heRamanlase  opass h ough.
Thiswasin eg a edwi h hesolidelec oly eandcleanedli hiumme alcoun e 
elec ode,andcon ac edunde anAlcu en collec o meshinacon igu a ion ypical
o Ramanexpe imen swhe eacon en ionalcu en collec o wouldblock hesignal
(seeFigu e2).

Figu e2.Thelaye scons i u ing heelec ode–ape u esys em,and hesubsequen assemblyin
hismodewi hlaye  hicknessshown.
Thiswas henmoun edinsideanai ‐sensi i eholde whileinsideana gon‐ illed
glo ebox(<0.1ppmO
2
,<0.1ppmH
2
O).
3.Resul s
3.1.Cons i uen Cha ac e iza ion
Ino de  oexamine hecons i uen componen s,analyseswe eca iedou onLTO,
PEO,andli hiumme alsamples.TheX‐ aydi ac ion(XRD)pa e no  he ecei edLTO
powde (seeFigu e3A) e ealedi couldbeassigned opu eLi
4
Ti
5
O
12
( hough heb ead h
o  hepeaksindica edadeg eeo lacko c ys allini y),while heXRDpa e no  hePEO
(seeFigu e3B) e ealed hecha ac e is ic wopeaksa ca.19°and24°,asexpec ed.

Figu e3.(A)XRDpa e no  heLTOsample(black),wi haLi
4
Ti
5
O
12
 e e ence( ed,JCPDSca d
no.49‐0207);(B)XRDpa e no  hePEO;(C)Ramanspec a akeno acleanedli hiumme alsu ‐
aceusinga532nm(g een)anda785nm( ed)wa eleng hlase .
Ramanspec oscopicanalysiso  hecleanedli hiumme alwasunde akeninsidean
ai ‐sensi i eholde usinga785nmanda532nmwa eleng hRamanlase (seeFigu e3C).
F om his,i couldbeseen ha li hiumhyd oxidepeakswe ep esen on heme alsu ‐
acea ca.287,297,and328cm
−1
(whichwe en a i elya ibu e oLi–Os e ching
modes),sugges ing ha  esidualspeciesmays illbede ec ed o li hiumme alp epa ed
o useasacoun e elec ode.Signi ican ly,i canalsobeseen ha  hesebandswe e
mo ein ensein hespec aob ainedusing he532nm han he785nmwa eleng hlase .
Thisobse a ionmaybea ibu ed o heene gyle elo pho onsp oducedbylasinga
ma e ialbeingin e selyp opo ional o hewa eleng ho  helase ,leading o helowe 
wa eleng ho 532nmp oducing hemo eene ge icpho ons(and husleading oabe ‐
e signal‐ o‐noise a io o  he esul ingspec um).Consequen ly,all u u eRaman
Figu e 2.
The laye s cons i u ing he elec ode–ape u e sys em, and he subsequen assembly in his
mode wi h laye hickness shown.
This was hen moun ed inside an ai -sensi i e holde while inside an a gon- illed
glo ebox (<0.1 ppm O2, <0.1 ppm H2O).
3. Resul s
3.1. Cons i uen Cha ac e iza ion
In o de o examine he cons i uen componen s, analyses we e ca ied ou on LTO,
PEO, and li hium me al samples. The X- ay di ac ion (XRD) pa e n o he ecei ed LTO
powde (see Figu e 3A) e ealed i could be assigned o pu e Li
4
Ti
5
O
12
( hough he b ead h
o he peaks indica ed a deg ee o lack o c ys allini y), while he XRD pa e n o he PEO
(see Figu e 3B) e ealed he cha ac e is ic wo peaks a ca. 19◦and 24◦, as expec ed.
Ba e ies2022,8,xFORPEERREVIEW4o 12
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2.5.Elec ode–Ape u eSys emAssembly
Toin es iga e heabili yo SORS oanalyzea ypicalsolid‐s a ecellcon igu a ion,
anLTOca hodewasp epa edasdesc ibed o  hes ep‐likesys em,wi hasmallape ‐
u emade oenable heRamanlase  opass h ough.
Thiswasin eg a edwi h hesolidelec oly eandcleanedli hiumme alcoun e 
elec ode,andcon ac edunde anAlcu en collec o meshinacon igu a ion ypical
o Ramanexpe imen swhe eacon en ionalcu en collec o wouldblock hesignal
(seeFigu e2).

Figu e2.Thelaye scons i u ing heelec ode–ape u esys em,and hesubsequen assemblyin
hismodewi hlaye  hicknessshown.
Thiswas henmoun edinsideanai ‐sensi i eholde whileinsideana gon‐ illed
glo ebox(<0.1ppmO
2
,<0.1ppmH
2
O).
3.Resul s
3.1.Cons i uen Cha ac e iza ion
Ino de  oexamine hecons i uen componen s,analyseswe eca iedou onLTO,
PEO,andli hiumme alsamples.TheX‐ aydi ac ion(XRD)pa e no  he ecei edLTO
powde (seeFigu e3A) e ealedi couldbeassigned opu eLi
4
Ti
5
O
12
( hough heb ead h
o  hepeaksindica edadeg eeo lacko c ys allini y),while heXRDpa e no  hePEO
(seeFigu e3B) e ealed hecha ac e is ic wopeaksa ca.19°and24°,asexpec ed.

Figu e3.(A)XRDpa e no  heLTOsample(black),wi haLi
4
Ti
5
O
12
 e e ence( ed,JCPDSca d
no.49‐0207);(B)XRDpa e no  hePEO;(C)Ramanspec a akeno acleanedli hiumme alsu ‐
aceusinga532nm(g een)anda785nm( ed)wa eleng hlase .
Ramanspec oscopicanalysiso  hecleanedli hiumme alwasunde akeninsidean
ai ‐sensi i eholde usinga785nmanda532nmwa eleng hRamanlase (seeFigu e3C).
F om his,i couldbeseen ha li hiumhyd oxidepeakswe ep esen on heme alsu ‐
acea ca.287,297,and328cm
−1
(whichwe en a i elya ibu e oLi–Os e ching
modes),sugges ing ha  esidualspeciesmays illbede ec ed o li hiumme alp epa ed
o useasacoun e elec ode.Signi ican ly,i canalsobeseen ha  hesebandswe e
mo ein ensein hespec aob ainedusing he532nm han he785nmwa eleng hlase .
Thisobse a ionmaybea ibu ed o heene gyle elo pho onsp oducedbylasinga
ma e ialbeingin e selyp opo ional o hewa eleng ho  helase ,leading o helowe 
wa eleng ho 532nmp oducing hemo eene ge icpho ons(and husleading oabe ‐
e signal‐ o‐noise a io o  he esul ingspec um).Consequen ly,all u u eRaman
Figu e 3.
(
A
) XRD pa e n o he LTO sample (black), wi h a Li
4
Ti
5
O
12
e e ence ( ed, JCPDS ca d no.
49-0207); (
B
) XRD pa e n o he PEO; (
C
) Raman spec a aken o a cleaned li hium me al su ace
using a 532 nm (g een) and a 785 nm ( ed) wa eleng h lase .
Raman spec oscopic analysis o he cleaned li hium me al was unde aken inside an
ai -sensi i e holde using a 785 nm and a 532 nm wa eleng h Raman lase (see Figu e 3C).
F om his, i could be seen ha li hium hyd oxide peaks we e p esen on he me al su ace
a ca. 287, 297, and 328 cm
−1
(which we en a i ely a ibu e o Li–O s e ching modes),
sugges ing ha esidual species may s ill be de ec ed o li hium me al p epa ed o use as
a coun e elec ode. Signi ican ly, i can also be seen ha hese bands we e mo e in ense in
he spec a ob ained using he 532 nm han he 785 nm wa eleng h lase . This obse a ion
may be a ibu ed o he ene gy le el o pho ons p oduced by lasing a ma e ial being
in e sely p opo ional o he wa eleng h o he lase , leading o he lowe wa eleng h o
532 nm p oducing he mo e ene ge ic pho ons (and hus leading o a be e signal- o-noise
a io o he esul ing spec um). Consequen ly, all u u e Raman measu emen s we e
pe o med using he 532 nm lase in o de o maximize he signals o in e es wi hin he
measu ed da a.
3.2. In es iga ion in o De ocused Mic o-SORS Applicabili y Using a S ep-Like Sys em
To de e mine i mic o-SORS may be sui able o analyzing ypical solid-s a e cells,
i was necessa y o de e mine he abili y o he Raman echnique o ex ac da a and
in o ma ion om egions o in e es . Consequen ly, i was necessa y o in es iga e which
Ba e ies 2023,9, 20 5 o 11
laye s o ypical solid-s a e sys em componen s he Raman migh success ully analyze
h ough which o he laye s.
This was ca ied ou using he s ep-like sys em, so as o enable each possible con igu-
a ion ( om comple ely exposed o co e ed by all p e ious laye s) o be in es iga ed.
3.2.1. Raman Spec a o he S ep-Like Sys em Cons i uen s
Ini ially, he Raman spec a o each o he indi idual componen s used we e collec ed
sepa a ely (see Figu e 4), o enable iden i ica ion o he cha ac e is ic signal peaks.
Ba e ies2022,8,xFORPEERREVIEW5o 12
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measu emen swe epe o medusing he532nmlase ino de  omaximize hesignals
o in e es wi hin hemeasu edda a.
3.2.In es iga ionin oDe ocusedMic o‐SORSApplicabili yUsingaS ep‐likeSys em
Tode e minei mic o‐SORSmaybesui able o analyzing ypicalsolid‐s a ecells,i 
wasnecessa y ode e mine heabili yo  heRaman echnique oex ac da aandin‐
o ma ion om egionso in e es .Consequen ly,i wasnecessa y oin es iga ewhich
laye so  ypicalsolid‐s a esys emcomponen s heRamanmigh success ullyanalyze
h oughwhicho he laye s.
Thiswasca iedou using hes ep‐likesys em,soas oenableeachpossiblecon igu a‐
ion( omcomple elyexposed oco e edbyallp e iouslaye s) obein es iga ed.
3.2.1.Ramanspec ao  hes ep‐likesys emcons i uen s
Ini ially, heRamanspec a o eacho  heindi idualcomponen susedwe ecol‐
lec edsepa a ely(seeFigu e4), oenableiden i ica iono  hecha ac e is icsignalpeaks.

Figu e4.Spec ao eachsys emcomponen (elec ode,black;solidelec oly e,pu ple;a i icial
SEI,ligh g een;li hium,o ange),compa edwi h hesignalso  hei cons i uen s(elec odecon‐
s i uen s:LTO, ed;PEO,da kblue;LiTFSI,cyan;solidelec oly econs i uen s:PEO,da kblue;
LiTFSI,cyan;a i icialSEIcons i uen s:Li2CO3,da kg een;PEO,da kblue;li hiumcons i uen s:
LiOH,pink;Li2CO3,da kg een).
To acili a eeasie iden i ica ion, hemos in enseobse edbandswe eiden i ied
o eacho  hesecomponen s(seeTable1)and hei modesassigned.
Table1.Assigna ions o mos signi ican obse edsignals
Componen RamanBand
Posi ion(cm−1)Assigna ionRe .
Li2CO31090C–Obondsymme ics e ching[43–46]
LiOH
287
297
328
Li–Os e ching[47–50]
LiTFSI745S–N–Ss e ching[51–53]
PEO845
862Hyd oxylendg oups[54]
Figu e 4.
Spec a o each sys em componen (elec ode, black; solid elec oly e, pu ple; a i icial SEI,
ligh g een; li hium, o ange), compa ed wi h he signals o hei cons i uen s (elec ode cons i uen s:
LTO, ed; PEO, da k blue; LiTFSI, cyan; solid elec oly e cons i uen s: PEO, da k blue; LiTFSI, cyan;
a i icial SEI cons i uen s: Li
2
CO
3
, da k g een; PEO, da k blue; li hium cons i uen s: LiOH, pink;
Li2CO3, da k g een).
To acili a e easie iden i ica ion, he mos in ense obse ed bands we e iden i ied o
each o hese componen s (see Table 1) and hei modes assigned.
Table 1. Assigna ions o mos signi ican obse ed signals.
Componen Raman Band Posi ion (cm−1) Assigna ion Re .
Li2CO31090 C–O bond symme ic s e ching [43–46]
LiOH 287
297
328 Li–O s e ching [47–50]
LiTFSI 745 S–N–S s e ching [51–53]
PEO 845
862 Hyd oxyl end g oups [54]
PEO 1241
Diso de ed o amo phous po ions o he polyme chain
[54]
LTO 233 Bending ib a ion o O–Ti–O [55–58]
LTO 400–420 S e ching–bending ib a ions o Li–O bonds in LiO4[55–58]
LTO 663 Vib a ion o Ti–O bond in TiO6oc ahed a [55–59]

Ba e ies 2023,9, 20 6 o 11
F om his, i can be seen ha he cleaned li hium me al oil indica ed he exis ence
o a esidual su ace (li hium me al i sel has no signal, only con ibu ing o luo escence)
p edominan ly belonging o li hium ca bona e (peak a ca. 1090 cm
−1
, which ep esen s he
symme ic s e ching o he ca bona e anion) and li hium hyd oxide (peaks a ca.
287, 297
,
and 328 cm−1).
While he p esence o his signal enables de ec ion o he li hium su ace i sel , i
was impo an o be able o clea ly de e mine i he in e ace be ween he li hium and
he elec oly e could also be de ec ed. Fo his eason, an addi ional “a i icial SEI” laye
consis ing o Li
2
CO
3
was included in he s ep-like sys em o bo h ep esen a species
commonly ound wi hin SEIs and o p o ide a s ong signal (peaks obse able a
1090 cm−1)
o acili a e de ec ion. F om he compa ison o he spec a o he solid elec oly e wi h
i s cons i uen s, i can be seen ha he e a e peaks co esponding o LiTFSI (a 745 cm
−1
)
and PEO (a 1241 cm
−1
). Addi ionally, examina ion o he LTO ca hode spec a e eals
signi ican signals om LiTFSI and PEO ( esul ing om he ca holy e componen ), as
well as hose co esponding o he LTO (Li=O and Ti=O ib a ions a 400–420 cm
−1
and
663 cm−1, espec i ely).
3.2.2. De ocused Mic o-SORS o Assembled S ep-Like Sys em
The sys em was assembled as desc ibed in he expe imen al sec ion, wi h a “s ep-like”
con igu a ion. While no ully ep esen a i e o he design o a con en ional cell, his
con igu a ion was selec ed as i enabled he Raman lase o be ocused on each o he
laye s o he s ep-like sys em be o e mo ing he beam la e ally h ough co e ing laye s.
In his way, i was possible o collec in o ma ion h ough a ange o laye con igu a ions,
hus demons a ing he deg ee o Raman lase pene a ion (and, hus, applicabili y o his
mic o-SORS app oach). Raman spec a we e aken om each con igu a ion and plo ed
(see Figu e 5).
Ba e ies2022,8,xFORPEERREVIEW7o 12
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
Figu e5.Ramanspec aob ained om hes ep‐likesys embymic o‐SORS,wi h hei posi ionsdesig‐
na ednume ically.Fo compa ison, heob ainedspec ao  helaye cons i uen s(asshowninFigu e4)
a eusedasa e e ence oindica e hemainpeaks,whichcanbeobse edby*.Examina iono  heob‐
se ablepeaksincompa ison o helaye dep ha which heywe emeasu eddemons a es hedeg ee
owhich hede ocusedmic o‐SORSwasable oob ainspec oscopicda a.
Toaidanalysis, abula ed alues o  heobse edpeaksand hees ima edcons i uen 
con ibu ionsa egi enin hesuppo ingin o ma ion(seeTablesS1–S10).F om hisda‐
a, he ollowingkeyobse a ionscouldbemade:
 Thesignal om heli hiumsu ace(measu emen 7)couldbeobse ed h ough he
li hiumca bona ea i icialSEI(measu emen 8),and h oughbo h hePEO/LiTFSI
memb aneanda i icialSEI(measu emen 9).Howe e , hissignalcouldno be
obse ed h ough heLTOca hode(measu emen 10).
 Thea i icialSEIsignal(measu emen 4)couldbeobse ed h ough hePEO/LiTFSI
memb ane(measu emen 5),bu no  heLTOca hode(measu emen 6).
 ThePEO/LiTFSImemb ane(measu emen 2)i sel couldno beobse ed h ough
heLTOca hode(measu emen 3),as heassocia ed isiblePEOandLiTFSIpeaks
a e om heca holy e,andnosigni ican addi ionalcon ibu ion om hemem‐
b anecouldbede ec ed.
Tosumma ize,whilecon igu a ionso  heli hiumsu ace,a i icialSEI,and
PEO/LiTFSImemb aneenabled hislaye ‐by‐laye de ec ionapp oach, heLTOca hode
signalo e whelmed hesignalso  heo he laye sand husp e en edmeasu emen .
Thisdemons a es ha , o mic o‐SORS obeusedina ypicalsolid‐s a ecell,i willei‐
he benecessa y o indaway oenhance hesignalso  hedesi edlaye s,o  oengi‐
nee  hesys eminsuchaway ha  he ea e egionswhe e he eisanabsenceo  heob‐
scu inglaye s.
3.3.In es iga ionin oMic o‐SORSApplicabili yUsinganElec ode–Ape u eSys em
Onesimpleapp oach oo e coming heissueshighligh edby hes ep‐likecon igu‐
a ionwouldbe ouseacon en ionalsolid‐s a esys em( houghwi hamesh‐likecu ‐
en collec o  h oughwhich heRamanlase maypass),and oinduceasmallape ‐
Figu e 5.
Raman spec a ob ained om he s ep-like sys em by mic o-SORS, wi h hei posi ions
designa ed nume ically. Fo compa ison, he ob ained spec a o he laye cons i uen s (as shown in
Figu e 4) a e used as a e e ence o indica e he main peaks, which can be obse ed by *. Examina ion
o he obse able peaks in compa ison o he laye dep h a which hey we e measu ed demons a es
he deg ee o which he de ocused mic o-SORS was able o ob ain spec oscopic da a.
Ba e ies 2023,9, 20 7 o 11
To aid analysis, abula ed alues o he obse ed peaks and he es ima ed cons i uen
con ibu ions a e gi en in he suppo ing in o ma ion (see Tables S1–S10). F om his da a,
he ollowing key obse a ions could be made:
•
The signal om he li hium su ace (measu emen 7) could be obse ed h ough he
li hium ca bona e a i icial SEI (measu emen 8), and h ough bo h he PEO/LiTFSI
memb ane and a i icial SEI (measu emen 9). Howe e , his signal could no be
obse ed h ough he LTO ca hode (measu emen 10).
•
The a i icial SEI signal (measu emen 4) could be obse ed h ough he PEO/LiTFSI
memb ane (measu emen 5), bu no he LTO ca hode (measu emen 6).
•
The PEO/LiTFSI memb ane (measu emen 2) i sel could no be obse ed h ough
he LTO ca hode (measu emen 3), as he associa ed isible PEO and LiTFSI peaks
a e om he ca holy e, and no signi ican addi ional con ibu ion om he memb ane
could be de ec ed.
To summa ize, while con igu a ions o he li hium su ace, a i icial SEI, and PEO/LiTFSI
memb ane enabled his laye -by-laye de ec ion app oach, he LTO ca hode signal o e -
whelmed he signals o he o he laye s and hus p e en ed measu emen . This demon-
s a es ha , o mic o-SORS o be used in a ypical solid-s a e cell, i will ei he be necessa y
o ind a way o enhance he signals o he desi ed laye s, o o enginee he sys em in such
a way ha he e a e egions whe e he e is an absence o he obscu ing laye s.
3.3. In es iga ion in o Mic o-SORS Applicabili y Using an Elec ode–Ape u e Sys em
One simple app oach o o e coming he issues highligh ed by he s ep-like con igu a-
ion would be o use a con en ional solid-s a e sys em ( hough wi h a mesh-like cu en
collec o h ough which he Raman lase may pass), and o induce a small ape u e(s) in
he LTO elec ode o enable measu emen o he laye s benea h. To alida e his speci ic
app oach, an elec ode–ape u e sys em was assembled, and Raman measu emen s we e
aken om he LTO elec ode and om he solid elec oly e and li hium me al (see Figu e 6).
In o de o in es iga e he abili y o his con igu a ion o enable key spec a o be
collec ed om a eas o in e es , spec a we e collec ed om he LTO elec ode su ace
(see Figu e 6A)
, he solid elec oly e h ough he ape u e (see Figu e 6B), and he li hium
me al elec ode h ough he ape u e and he solid elec oly e (see Figu e 6C), and plo ed
along wi h compa ison spec a o hei ele an cons i uen s. To be e see he li hium me al
signal in he spec um aken o he li hium elec ode h ough he solid elec oly e (li hium
elec ode, see Figu e 6C), a ma hema ical sub ac ion o he elec oly e con ibu ion was
pe o med ( he spec um o he solid elec oly e was no malized wi h espec o ha aken
o he li hium elec ode based on he in ensi ies o nono e lapping peaks, hen sub ac ed)
and he ex ac ed spec um (ex ac ed li hium) was plo ed along wi h he spec um aken
o he ba e li hium me al i sel o compa ison (see Figu e 6D). F om his da a, he ollowing
key obse a ions could be made:
•
As in con en ional Raman cells used o in si u measu emen s, i was possible o
measu e he LTO ca hode h ough he mesh cu en collec o .
•
Addi ionally, he solid elec oly e could be easily de ec ed h ough he elec ode ape -
u e, implying ha his app oach would be app op ia e o de ec ing he elec oly e
and subsequen laye s wi hin a con en ional con igu a ion.
•
Finally, a signal a ca. 1850 cm
−1
obse ed o he ba e li hium me al could also be
de ec ed in he li hium me al spec um ob ained h ough he solid elec oly e ( his
band may be a ibu ed o he s e ching mode o a C
≡
C g oup wi hin esidual
su ace Li
2
C
2
) [
60
]. This demons a es ha i is possible o use he elec ode–ape u e
con igu a ion o nondes uc i ely in es iga e laye s benea h he solid elec oly e.
To summa ize, by in oducing a small ape u e in o he LTO elec ode, i was possible
o ob ain Raman da a om all o he laye s. This s ongly sugges s ha a simila app oach
migh be applicable when ca ying ou in si u o ex si u Raman o u u e solid-s a e sys ems.
I is belie ed ha by in oducing se e al ape u es o sui able sizes, a sui able dis ances,
Ba e ies 2023,9, 20 8 o 11
i will be possible o unde ake spec oscopic dep h p o iling o a eas o in e es wi hin a
wide ange o solid-s a e sys ems.
Ba e ies2022,8,xFORPEERREVIEW8o 12

u e(s)in heLTOelec ode oenablemeasu emen o  helaye sbenea h.To alida e
hisspeci icapp oach,anelec ode–ape u esys emwasassembled,andRamanmeas‐
u emen swe e aken om heLTOelec odeand om hesolidelec oly eandli hium
me al(seeFigu e6).

Figu e6.Ramanspec aob ained om(A) heLTOelec ode(black)andi scons i uen s(elec‐
odecons i uen s:LTO, ed;PEO,da kblue;LiTFSI,cyan);(B) hesolidelec oly e(pu ple)and
i scons i uen s(solidelec oly econs i uen s:PEO,da kblue;LiTFSI,cyan);(C) heli hiumme al
su ace h ough hesolidelec oly e(li hiumelec ode,b own), hesolidelec oly e(pu ple),and
ba eli hiumme alsu ace(li hiumme al,o ange);(D)acompa isono  heba eli hiumme alsu ‐
ace(li hiumme al,o ange)and heex ac edli hiumcon ibu ion(ex ac edli hium,da k ed).
Ino de  oin es iga e heabili yo  hiscon igu a ion oenablekeyspec a obe
collec ed oma easo in e es ,spec awe ecollec ed om heLTOelec odesu ace
(seeFigu e6A), hesolidelec oly e h ough heape u e(seeFigu e6B),and heli hi‐
umme alelec ode h ough heape u eand hesolidelec oly e(seeFigu e6C),and
plo edalongwi hcompa isonspec ao  hei  ele an cons i uen s.Tobe e see he
li hiumme alsignalin hespec um akeno  heli hiumelec ode h ough hesolid
elec oly e(li hiumelec ode,seeFigu e6C),ama hema icalsub ac iono  heelec o‐
ly econ ibu ionwaspe o med( hespec umo  hesolidelec oly ewasno malized
wi h espec  o ha  akeno  heli hiumelec odebasedon hein ensi ieso nono e ‐
lappingpeaks, hensub ac ed)and heex ac edspec um(ex ac edli hium)wasplo ‐
edalongwi h hespec um akeno  heba eli hiumme ali sel  o compa ison(see
Figu e6D).F om hisda a, he ollowingkeyobse a ionscouldbemade:
 Asincon en ionalRamancellsused o insi umeasu emen s,i waspossible o
measu e heLTOca hode h ough hemeshcu en collec o .
 Addi ionally, hesolidelec oly ecouldbeeasilyde ec ed h ough heelec ode
ape u e,implying ha  hisapp oachwouldbeapp op ia e o de ec ing heelec‐
oly eandsubsequen laye swi hinacon en ionalcon igu a ion.
 Finally,asignala ca.1850cm−1obse ed o  heba eli hiumme alcouldalsobe
de ec edin heli hiumme alspec umob ained h ough hesolidelec oly e( his
Figu e 6. Raman spec a ob ained om (A) he LTO elec ode (black) and i s cons i uen s (elec ode
cons i uen s: LTO, ed; PEO, da k blue; LiTFSI, cyan); (
B
) he solid elec oly e (pu ple) and i s
cons i uen s (solid elec oly e cons i uen s: PEO, da k blue; LiTFSI, cyan); (
C
) he li hium me al
su ace h ough he solid elec oly e (li hium elec ode, b own), he solid elec oly e (pu ple), and
ba e li hium me al su ace (li hium me al, o ange); (
D
) a compa ison o he ba e li hium me al su ace
(li hium me al, o ange) and he ex ac ed li hium con ibu ion (ex ac ed li hium, da k ed).
4. Discussion
Gi en ha u u e solid-s a e ba e y de elopmen has a s ong dependence on a
de ailed unde s anding o he physicochemical changes occu ing du ing aging and cycling,
he de elopmen o no el app oaches o nondes uc i e analysis capable o p o iding such
addi ional in o ma ion p o ides a powe ul ool o enhance insigh , and subsequen ly
pe o mance, pa icula ly du ing ope a ional li e. To help u he de elopmen o Raman
spec oscopy o solid-s a e ba e y cha ac e iza ion, we sys ema ically applied de ocused
spa ially o se Raman spec oscopy o sys ems composed o ypical solid-s a e ba e y
componen s o he i s ime.
The ac ha i was possible o collec spec a h ough an a i icial SEI and a PEO/LiTFSI
memb ane alida es he iabili y o he echnique, and ha i is, in p inciple, possible
o ca y ou nondes uc i e Raman dep h p o iling o an assembled solid-s a e sys em.
Signi ican ly, i was no possible, using he s ep-like con igu a ion, o ob ain a signal h ough
he LTO ca hode laye due o he s eng h o i s signal. This ep esen s a challenge, and so
i is impo an o no e ha , when conside ing app oaches o de ec ing spec a obscu ed by
he ca hode’s signal, he sca e ing in ensi ies o he componen s will need o be accoun ed
o ( ypically his depends on a a ie y o ac o s, including he concen a ion and he
Raman c oss-sec ion, and has a weak dependence on he wa eleng h o he exci ing lase ).
Thus, u u e wo k should likely ocus on de e mining op imal cell designs. Subsequen ly,
conside a ion could hen be gi en o ex ending his echnique o solid-s a e sys ems which
ha e unde gone elec ochemical cycling ( o enable ex si u and/o in si u nondes uc i e
dep h p o iling o he changes unde gone du ing cycling), hen o enhancing signals u he
Ba e ies 2023,9, 20 9 o 11
s ill, so as o acili a e mo e sensi i e de ec ion ( o example, enhancemen o he Raman
signal o speci ic laye s and/o in e aces/in e phases, in o de o p obe de ec s and hei
o ma ion wi hin he solid-s a e ba e ies).
In o de o p o ide a po en ial cell design capable o enabling mic o-SORS, we consid-
e ed he use o an ape u e in he elec ode h ough which Raman measu emen s migh be
aken. Ou p elimina y esul s show ha his app oach wo ked well, enabling da a o be
aken om he elec ode and om he laye s benea h he elec ode in a con igu a ion ypi-
cal o solid-s a e sys ems. While he use o he ape u e migh in oduce some deg ee o
complexi y when unde aking u u e Raman expe imen s, he possibili y o p obing laye s
and in e aces hi he o unmeasu able by nondes uc i e echniques o e s a g ea deal o
p omise and he an alizing possibili y o en iching he solid-s a e ba e y esea ch ield.
In summa y, he esul s p esen ed he e demons a e, o he i s ime, he po en ial
o spa ially o se Raman spec oscopy o he cha ac e iza ion o ypical solid-s a e cells,
pa icula ly wi h espec o nondes uc i e dep h p o iling analysis, allowing in es iga ion
o componen s no ypically obse able wi hou cell disassembly and pos mo em s udies.
In his way, his s udy p esen s a new al e na i e o exis ing nondes uc i e cha ac e iza ion
me hodologies, as well as a po en ial signi ican ex ension o he al eady widesp ead use o
Raman spec oscopy wi hin his ield, which would o e a new pe spec i e on iden i ica ion
o solid-s a e cell cons i uen s (and he changes hey may unde go du ing cycling and aging)
in o de o suppo u u e esea ch and de elopmen .
Supplemen a y Ma e ials:
The ollowing suppo ing in o ma ion can be downloaded a : h ps://
www.mdpi.com/a icle/10.3390/ba e ies9010020/s1, Figu e S1: Schema ic o he posi ions designed
nume ically o he mic o- SORS; Tables S1–S10: Componen con ibu ion o Spec a 1–10.
Au ho Con ibu ions:
Concep ualiza ion, N.E.D. and E.B.; me hodology, E.J.-B., Y.Z., N.E.D., P.L.-A.
and E.B.; alida ion, E.J.-B. and Y.Z.; o mal analysis, E.J.-B. and Y.Z.; in es iga ion, E.J.-B. and Y.Z.;
esou ces, Y.Z. and P.L.-A.; da a cu a ion, E.J.-B.; w i ing—o iginal d a p epa a ion, E.J.-B. and
N.E.D.; w i ing— e iew and edi ing, E.J.-B., Y.Z., N.E.D., P.L.-A., E.Z. and E.B.; isualiza ion, E.J.-B.;
supe ision, E.Z. and E.B.; p ojec adminis a ion, E.B.; unding acquisi ion, E.B. All au ho s ha e
ead and ag eed o he published e sion o he manusc ip .
Funding: This esea ch ecei ed no ex e nal unding.
Da a A ailabili y S a emen : Da a a e con ained wi hin he a icle.
Acknowledgmen s:
Edu ne Ba que o hanks CIC ene giGUNE o unding he Ph.D. ellowship. In
addi ion, he au ho s g a e ully acknowledge he suppo o Nu ia Gomez, A iana Pesce, Uxue Gon-
zalez, and Ande O ue ega ding he ab ica ion o he solid-s a e sys em cons i uen s
and assembly
.
Con lic s o In e es :
The au ho s decla e no con lic o in e es . The unde s had no ole in he design
o he s udy; in he collec ion, analyses, o in e p e a ion o da a; in he w i ing o he manusc ip ; o
in he decision o publish he esul s.
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2
Ca hode Ma e ial ia
S uc u e Design. J. Ene gy Chem. 2021,55, 114–123. [C ossRe ]
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Goodenough, J.B.; Pa k, K.-S. The Li-Ion Recha geable Ba e y: A Pe spec i e. J. Am. Chem. Soc.
2013
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