On the Challenges to Develop Hybrid Faradaic-Capacitive Electrodes Incorporating a Sacrificial Salt for Lithium-ion Capacitors: The Case of Li3V1.95Ni0.05(PO4)3-AC-Li2C4O4

On he Challenges o De elop Hyb id Fa adaic-Capaci i e
Elec odes Inco po a ing a Sac i icial Sal o Li hium-ion
Capaci o s: The Case o Li3V1.95Ni0.05(PO4)3-AC-Li2C4O4
Miguel G anados-Mo eno,[a, b] Ma ia A naiz,[a] Emanuele Guccia di,[a]
Nahom Enkubah i As es,[a, b] Eide Goikolea,[b] and Jon Aju ia*[a]
The low capaci y o ac i a ed ca bon (AC) elec odes emains as
one o he majo limi ing ac o s o he de elopmen o high
ene gy densi y li hium-ion capaci o s (LICs). Hyb idiza ion o
capaci i e AC elec odes by inco po a ing a adaic ma e ials
in o he elec ode o mula ion could be pe o med o enhance
he capaci y o he o e all de ice. Howe e , his s a egy
equi es an accu a e elec ode design o maximize he pe o m-
ance. In his wo k, Li3V1.95Ni0.05(PO4)3(LVNP) was selec ed as
a adaic ma e ial due o i s compa ibili y wi h AC, showing high
capaci y, as ionic di usion, and ela i ely high conduc i i y.
Va ious o mula ions and mass loadings ha e been s udied o
analyze he impac o inco po a ing LVNP in o he posi i e
elec ode on he pe o mance o he hyb id elec ode. Mo e-
o e , o p ac ical LIC applica ions, a sac i icial sal -dili hium
squa a e, Li2C4O4- was included in he hyb id elec ode as a p e-
li hia ion addi i e, de eloping a e na y elec ode. The sac i icial
sal oxidized eleasing li hium ions, while he elec ochemical
pe o mance o he hyb id posi i e elec ode emained almos
unal e ed. Finally, a cycle li e es combined wi h a pos -mo em
analysis allows unde s anding he ailu e mechanisms o he
elec ode, sugges ing he need o u he imp o emen s o he
elec oly e and elec ode-elec oly e in e ace o de elop long
li e ime hyb id a adaic-capaci i e elec odes based on LVNP-AC
ac i e ma e ials.
1. In oduc ion
The high dependence o he mode n indus ial socie y on he
use o ossil uels is he main eason o he inc ease o CO2
emissions and he accele a ed clima e change. An ene gy
ansi ion owa ds a ne ze o emission socie y elying on
enewables is a p io i y challenge ha ci iliza ion is acing
oday.[1,2] The elec i ica ion o ou ene gy ecosys em needs
e icien and obus ene gy s o age sys ems (ESSs) capable o
assuming mul iple key oles wi hin he nex ew yea s. On he
one hand, ESSs will become a co ne s one in he expansion o
enewable ene gies, p o iding la ge-scale ene gy s o age and
s abili y o he elec ical g id. On he o he hand, ESSs will be
c i ical enable s o he po able use o ene gy, which is essen ial
o consume elec onics and elec ic ehicles.
Among he di e en ene gy s o age echnologies, li hium-
ion ba e ies (LIBs) a e he mos ex ended elec ochemical ESSs,
and hey a e cu en ly leading he e olu ion owa d elec ic
mobili y. LIBs a e he p e e ed sys ems due o hei high
ene gy densi y p o ided by a adaic eac ions in he elec ode.
Howe e , hei low powe densi y equi es non-cos -e ec i e
solu ions, such as he use o o e sized ba e ies o ex e nally
hyb idized LIBs and elec ochemical capaci o s, which equi e
complex ba e y managemen sys ems (BMSs). Unlike LIBs,
elec ochemical capaci o s a e cha ac e ized by hei high
powe densi y bu low ene gy densi y. In o de o b idge he
ene gy-powe gap be ween LIBs and elec ochemical capaci-
o s, a hyb id de ice combining a ba e y- ype elec ode and a
capaci o - ype elec ode was i s p oposed in 2001.[3] Tha
hyb id de ice, called li hium-ion capaci o (LIC), igge ed he
de elopmen o me al-ion capaci o (MIC) echnology. Cu -
en ly, LICs a e d awing he a en ion o bo h academia and
indus y, wi h an exponen ially inc easing numbe o yea ly
published a icles as well as new s a -up companies.[4]
Despi e he p og ess and he de elopmen deg ee achie ed
o da e, LICs p esen un esol ed challenges ha hinde hei
comme cial expansion and limi hei ma ke sha e. The mos
impo an one is o inc ease he ene gy o alues close o hose
o e ed by powe ba e ies.[5] In he ea ly yea s, he esea ch
ocused on he de elopmen and imp o emen o he a adaic
nega i e elec ode. Ini ially, in he i s LICs, nega i e elec odes
we e c a ed om g aphi e.[6,7] This ma e ial con inues o be
ex ensi ely employed in bo h comme cial and esea ch cells,
hanks o i s well-es ablished a ibu es such as high capaci y,
chemical s abili y, and mechanical obus ness. Howe e , i s
ising cos , he supply de ici o na u al g aphi e eeds ock, he
[a] M. G anados-Mo eno, M. A naiz0000-0001-8800-0643, E. Guccia di,
N. Enkubah i As es, J. Aju ia
Cen 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), Ala a Technology Pa k,
Albe Eins ein 48, 01510 Vi o ia-Gas eiz, Spain
E-mail: [email p o ec ed]
[email p o ec ed]
Homepage: 0000-0001-8800-0643
[b] M. G anados-Mo eno, N. Enkubah i As es, E. Goikolea
Depa men o O ganic and Ino ganic Chemis y, Facul y o Science and
Technology, Uni e si y o he Basque Coun y UPV/EHU, 48940 Leioa, Spain
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is an open access a icle unde he e ms o he C ea i e Commons A i-
bu ion License, which pe mi s use, dis ibu ion and ep oduc ion in any
medium, p o ided he o iginal wo k is p ope ly ci ed.
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low powe esponse and he li hium pla ing isk, p omp ed he
sea ch o al e na i e candida es. O he ca bonaceous ma e ials
ha e also been p oposed as nega i e elec ode, such as ha d
ca bon (HC),[8,9] so ca bon (SC)[10] and g aphene,[11] among
o he s. In gene al, hose al e na i e- o-g aphi e ca bons p esen
lowe capaci y, p o iding sligh ly lowe ene gy densi y, bu
o e ing be e a e capabili y, wha o e all, p o ides consid-
e ably highe powe densi y. Beyond ca bons, me al oxide
based elec odes such as Li4Ti5O12 (LTO)[12] o Fe2O3,[13] wi h as
kine ics ha e also been widely s udied. Howe e , hei high
po en ial pla eau, usually abo e 1 V s. Li+/Li, limi s he ou pu
ol age o he inal cell. Las , se e al me alloids and me als such
as Si o Sn[14–16] ha ope a e h ough alloying/dealloying
eac ions ha e caugh he in e es o esea che s owning o
hei e y high speci ic capaci y (ca. 994 mAhg1o
3579 mAhg1 o Sn and Si espec i ely[17,18]). Un o una ely,
hese ma e ials su e om la ge olume ic changes upon
li hia ion and deli hia ion p ocesses, p esen ing a poo solid
elec oly e in e phase (SEI) s abili y and mechanical ac u es,
and consequen ly, equi ing composi e elec odes[19,20] o nano-
s uc u ing[15,21] o imp o e he li e ime o he cells, among
o he s. In e me allic compounds ha e been de eloped as a
s a egy o inc ease he s abili y o hose me als, main aining
e y high capaci y ou pu as in he case o TiSb2
[22,23] o
Sn4P3,[24,26] al hough li e ime is s ill limi ed. Ye , despi e all he
esea ch on he sea ch o ad anced ma e ials, ca bon is he i s
op ion when conside ing ma ke a ailable p oduc s.[27]
Rega ding he posi i e elec ode, ac i a ed ca bon (AC) has
been he mos widely used ma e ial, owing o i s excellen
high- a e pe o mance as well as long cyclabili y. AC has been
widely s udied and imp o ed: om inexpensi e and eco-
iendly biowas e de i ed ACs o supe as and highly s able
nanos uc u ed ACs.[28,29] Func ionaliza ion is ano he s a egy
ha can be used o imp o e he conduc i i y and capaci y o
he AC elec odes.[30,32] Howe e , despi e he excep ional
ea u es o ACs, hei low capaci y o ca. 40–60 mAhg1, limi ed
by i s capaci i e na u e, is a majo d awback in e ms o ene gy
densi y. The o al o pa ial eplacemen o ACs by a adaic
ma e ials in he posi i e elec ode has been s udied as an
al e na i e o inc ease hei capaci y and he o e all ene gy
densi y o LICs.[33,36] Fas and well-known a adaic ma e ials such
as LiFePO4
[19,37] o LiNixMnyCo1-x-yO2
[38] ha e been combined wi h
AC o ab ica e composi e posi i e elec odes in LICs. Al hough
he hyb id a adaic-capaci i e ma e ials show 3–5 imes g ea e
capaci ies wi h espec o ACs, he capaci y e en ion a high
cu en densi ies is, in gene al, lowe and he cycle li e o
de ices is also nega i ely a ec ed.
Li2V1.95Ni0.05(PO)4(LVNP) was de eloped by Secchia oli
e al.,[39,40] as a high-capaci y as inse ion/deinse ion a adaic
ma e ial. The Ni doping inc eased he capaci y wi h espec o
Li2V2(PO)4(LVP), while main aining he high elec onic con-
duc i i y and ionic di usion. The excellen p ope ies and
sui able elec ochemical s abili y window (ESW) make i an
in e es ing choice as posi i e elec ode ma e ial o LICs. Fu he
in es iga ions we e ocused on he use o a bina y LVNP-AC
elec ode, aking ad an age o bo h he high capaci y o LVNP
and he high a e capabili y o he AC.[36] The composi e
elec ode success ully imp o es capaci y e en ion, inc easing
bo h he g a ime ic and olume ic capaci y a high cu en
densi y. The excellen p ope ies o LVNP pushed he de elop-
men o ull de ices cons i u ed by LTO-AC nega i e elec odes
and LVNP o LVNP-AC posi i e elec ode.[41] The de ices showed
high ene gy densi y, high powe , and long cycle li e, high-
ligh ing he iabili y o using LVNP-based posi i e elec odes
also in LICs.
Howe e , he de elopmen o eal li e LICs equi es a p e-
li hia ion s ep o: i) compensa e li hium losses du ing he i s
cycle i e e sibili y and SEI o ma ion; and ii) adequa e he cell
po en ial o maximize he ene gy densi y ou pu . Cu en ly,
indus ial p e-li hia ion s a egies a e based on he use o
me allic li hium.[4] Ne e heless, me allic li hium comp omises
he sa e y o he echnology and equi es an ine a mosphe e
en i onmen du ing he p ocessing o Li, which inc eases he
p oduc ion p ice. O ganic li hium sac i icial sal s a e a cheap,
sa e and ai s able al e na i e ha has been s udied o
ca bonaceous elec odes.[42,43] The iabili y o using non-con-
duc i e sac i icial sal s in elec odes con aining a adaic ma e i-
als has been e alua ed and alida ed, wi h mos o he
sac i icial sal being decomposed du ing he i s cycle. Thus,
dili hium squa a e (Li2C4O4)[44] was selec ed as a p e-li hia ion
addi i e and included in he o mula ion o he posi i e
elec ode, de eloping o he i s ime - o he bes o ou
knowledge-, a e na y hyb id capaci i e- a adaic elec ode
including a p e-li hia ion agen .
P e ious esul s disclose he po en ial o LVNP o he
de elopmen o high ene gy and high powe de ices. Howe e ,
om undamen al s udies owa ds p o o ypes, elec odes mus
be adap ed o mee di e en s anda ds and equi emen s.
Besides echnical aspec s such as g a ime ic o olume ic
capaci y, he p ice o scalabili y o he p ocesses in ol ed in he
ab ica ion also need o be conside ed. In his wo k, ou
objec i e is he de elopmen o a ealis ic p oo -o -concep ,
op imizing he elec ode o mula ion and including a sac i icial
sal as an e ec i e p e-li hia ion s a egy. Fi s , LVNP elec odes
a e op imized o unde s and i s pe o mance limi a ions.
Second, LVNP is inco po a ed on an AC elec ode o de elop a
hyb id LVNP-AC elec ode. Thi d, he sac i icial sal (Li2C4O4, -Li)
is inco po a ed, de eloping a e na y LVNP-AC-Li elec ode.
Finally, a p oo -o concep LIC is demons a ed acing he LVNP-
AC-Li posi i e elec ode wi h a nega i e HC elec ode. This wo k
aims o b idge undamen al science and echnology, cla i ying
he c i ical s eps o he op imiza ion and adap a ion p ocess,
om undamen al s udies owa ds he s a ing poin o
p o o ypes de elopmen . The cons ain s o p o o ype cells
push elec odes owa ds hei limi s, e ealing blind spo s ha
migh be igno ed a lab-scale, bu a e undamen al o hei
sui abili y in eal li e applica ions.
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Expe imen al
Syn hesis o Li3V1.95Ni0.05(PO4)3and Li2C4O4
Li3V1.95Ni0.05(PO4)3(LVNP) was syn hesized by ball-milling and
ca bon- he mal educ ion me hods. Fi s , s oichiome ic amoun s o
li hium ca bona e (>99.0%, Sigma-Ald ich), ammonium me a ana-
da e (99.99%, Sigma-Ald ich), ammonium dihyd ogen phospha e
(99.999%, Sigma-Ald ich) and nickel(II) ni a e hexahyd a e
(99.999%, Sigma-Ald ich) we e mixed wi h 40 ml o e hanol.
Subsequen ly, poly(ac ylic acid) (Sigma-Ald ich) and D-(+)-glucose
(>99.5%, Sigma-Ald ich) we e in oduced as ca bon sou ces in a
4:1 mass a io, espec i ely. The mix u e was ans e ed o an
aga e ja wi h aga e balls o 1 cm diame e in a 20:1 ball o sample
mass a io. The mix u e was ball milled in a Pul e isse e 5 plane a y
milling a 350 pm o 8 h. Then, he mix u e was ans e ed o a
o a y e apo a o a 60°C o 30 min o emo e he sol en . The
ob ained powde was manually g inded in an aga e mo a o 1 h
o educe he pa icle size and ensu e he close con ac be ween
he p ecu so s. A e wa ds, he p ecu so s we e he mally ea ed in
wo s eps: he i s one a 350°C o 5 h, and he second one a
800°C o 8 h, bo h s eps we e pe o med unde dynamic A
a mosphe e. Be ween he i s and he second annealing, he
powde was manually g inded o 30 minu es.[40]
Dili hium squa a e (Li2C4O4) was syn hesized om 3,4-dihyd oxy-3-
cyclobu ene-1,2-dione (99.0%, Me ck) and li hium ca bona e (>
99.0%, Me ck). Wi hin a 1:1 mola s oichiome ic ela ion, each
p ecu so is dissol ed in he equi ed deionized wa e . Once bo h
a e well-dissol ed, Li2CO3solu ion is added slowly in o one o
squa ic acid and s i ed o e nigh . A e wa ds, he mix u e was
ans e ed o a o a y e apo a o a 50°C o emo e he wa e . The
Li2C4O4was eco e ed and d ied a 120°C unde acuum o e nigh
be o e being used.[44]
Physicochemical Cha ac e iza ion
The mo phology and composi ion o he samples was cha ac e ized
by scanning elec on mic oscopy (SEM) and ene gy dispe si e X- ay
spec oscopy (EDX), espec i ely, using a Quan a200 FEI (3 kV,
30 kV) mic oscope. Bo h E e ha -Tho nley (ETD) and backsca e ed
elec on (BSED) de ec o s we e used. Fo a be e unde s anding o
he elec ode mo phology, op iew and c oss sec ion SEM images
we e eco ded. C oss sec ion samples we e p epa ed using a
Hi achi 4000 Plus ion milling which u ilizes an A +ion beam (0–
6 kV accele a ion ol age) milling me hod. Tex u al p ope ies o
ac i e ma e ials we e de e mined by ni ogen adso p ion/deso p-
ion iso he ms. Iso he ms we e egis e ed a 196 °C using an ASAP
2460 ins umen om Mic ome i ics. The samples we e ou gassed
a 250°C o 12 h unde acuum p io o he analysis. Speci ic
su ace a eas (SSA) we e calcula ed om he B unaue –Emme –
Telle (BET) equa ion using he Rouque ol p ocedu e o he
monolaye capaci y.[45] Po e size dis ibu ions (PSD) we e calcula ed
using SAIEUS so wa e, applying he 2D-NLDFT model o he
adso p ion b anches da a.[46]
The elemen al composi ion o LVNP-based elec odes was de e -
mined by induc i ely coupled plasma op ical emission spec oscopy
(ICP-EOS) using a 5800 Agilen ICP-OES. The ollowing ope a ing
condi ions we e selec ed: 1.20 kW o RF powe , 12.0 L min1o
plasma-gas low and 1.00 L min1o auxilia y low. Solu ions we e
in oduced in o he plasma o ch using a concen ic glass nebulize
and a double pass sp ay chambe a a low a e o 0.70 L min1. The
wa eleng hs employed in he ICP-OES analysis a e 214.914 nm o
phospho ous, 231.604 nm o nickel and 309.310 nm o anadium
de e mina ion. Di e en wa eleng hs o each single elemen we e
also es ed o ensu e no spec al in e e ences a e a ec ing he
esul s.
Elec ochemical Cha ac e iza ion
Elec odes we e p epa ed by dispe sing he LVNP and/o AC (No i )
ac i e ma e ials, Supe C65 ca bon as a conduc i e ma e ial (Ime ys,
C-NERGY) and poly inylidene luo ide (PVdF, Sole ) as a binde in N-
me hyl-2-py olidinone (NMP, Me ck). Dili hium squa a e (Li2C4O4)
sac i icial sal was included in he elec ode o mula ion as he p e-
li hia ion addi i e. The dispe sion was igo ously s i ed, and he
NMP-based slu ies coa ed on o aluminum oil. Lamina es we e
d ied a 80°C o 12 h unde acuum. Elec ode discs o 12 mm
we e u he punched ou o he lamina es and d ied a 120 °C
o e nigh unde acuum p io o cell assembly. Di e en elec ode
o mula ions we e used: i) LVNP:C65:PVdF wi h a mass a io o
80:10:10, 90:5: 5 and 90: 7:3; ii) LVNP:AC:C65:PVdF wi h a mass
a io o 45: 45 :5:5; and, iii) LVNP:AC:Li2C4O4:C65:PVdF wi h a mass
a io o 27.5: 27.5 :35:5:5. Elec ode o mula ions a e summa ized
in Table 1.
Following he same p ocedu e, HC ca bon elec odes ha e been
ab ica ed using 90 % o HC (Ku a ay), 5 % Supe C65 (Ime ys, C-
NERGY) and 5 % PVdF (Sole ). In he case o HC elec odes, he
NMP-based slu ies we e coa ed in cuppe oil.
Elec ochemical pe o mance o LVNP-based elec odes was e al-
ua ed by using 3-elec ode Swagelok cells in hal -cell con igu a ion.
Elec odes o ca. 2.5 mg cm2mass loading we e used unless
o he wise s a ed. O e sized sel -s anding AC elec odes o >25 mg
cm2mass loading and me allic li hium discs o 10 mm in diame e
we e used as coun e and e e ence elec ode, espec i ely. Wha -
man D- ype glass ibe discs o 13 mm in diame e we e selec ed as
sepa a o s. 1 M LiPF6in EC : DMC (e hylene ca bona e : dime hyl
ca bona e, 50: 50 ol.) (99%, Sigma-Ald ich) was used as elec oly e.
Gal anos a ic cha ge-discha ge (GCD) and cyclic ol amme y (CV)
measu emen s be ween 3–4.3 V s. Li+/Li we e pe o med in a
VMP3 gene a o om Biologic. In GCD, du ing bo h cha ge and
discha ge he same cu en densi y is applied wi h a 5 s hold s ep
bu no cons an cu en cons an ol age (CCCV) s ep[47,48] is
included. Dili hium squa a e sac i icial sal is decomposed by
cycling he elec ode a CLi2C4O4/10 o 10 cycles, being CLi2C4O4 =
425 mAhg1. Bo h he applied cu en densi y (A g1) and he
calcula ed speci ic capaci y (mA h g1) a e p esen ed pe mass o
ac i e ma e ial in he elec odes unless ano he me ic is indica ed.
Table 1. Summa y o he di e en elec ode o mula ions used in his
wo k.
Sample LVNP
(%)
C65
(%)
PVdF
(%)
AC
(%)
Li2C4O4
(%)
LVNP-
80:10:10
80 10 10 – –
LVNP-90:5:5 90 5 5 – –
LVNP-90:7:3 90 7 3 – –
LVNP-AC 45 5 5 45 –
LVNP-AC-Li 27.5 5 5 27.5 35
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2. Resul s
2.1. Elec ode Fo mula ion and Mass Loading
Elec odes a e usually composed o h ee main componen s:
ac i e ma e ial, conduc i e addi i e and binde . The a io
be ween hose h ee componen s de e mines many o he
o e all p ope ies o he elec ode and impac s he inal de ice
pe o mance. The conduc i e addi i e in e connec s ac i e
ma e ial pa icles, inc easing he o e all elec ode conduc i i y
and i also helps o homogeneously dis ibu e he binde
h ough he elec ode. A las , he binde ensu es he mechan-
ical p ope ies o he elec ode, hus, an insu icien amoun o
i could lead o elec ode ac u es and delamina ion, bu an
excess will a ec conduc i i y and speci ic capaci y since
binde s a e usually insula ing non-elec ochemically ac i e
polyme ic ma e ials. The ac i e ma e ial is he main playe ,
s o ing cha ge by a adaic, pseudocapaci i e and/o capaci i e
mechanisms. The e o e, i is o u mos impo ance o p ope ly
design he elec ode o mula ion o allow a high mass loading
o he ac i e ma e ial wi hou penalizing he elec ochemical
pe o mance and mechanical p ope ies.
LVNP elec odes epo ed in li e a u e we e mos ly ab i-
ca ed con aining 80% o ac i e ma e ial, 10% o conduc i e
addi i e and 10% o PVdF binde (named as LVNP-80:10:10).
Thus, some op imiza ion is necessa y o de elop a inal de ice.
Wi h he objec i e o inc easing he con en o he ac i e
ma e ial in he elec ode, a new o mula ion wi h 90% o ac i e
ma e ial, 5% o C65 and 5% o PVdF (LVNP-90:5:5) was
de eloped.
Gal anos a ic cha ge-discha ge (GCD) cu es o LVNP-
80:10:10, LVNP-90:5:5 elec odes a e shown in Figu e 1. I is
wo h men ioning ha no cons an cu en cons an ol age
(CCCV)[47,48] s ep is included nei he du ing cha ge no dis-
cha ge, since LICs a e in ended o ope a e unde cons an
cu en only. Du ing cha ge and discha ge, LVNP unde goes
h ee Li+inse ion/deinse ion eac ions as s a ed in Equa-
ion (1) o (3).[39] The eac ions co espond o he h ee pla eaus
obse ed in Figu e 1a. a 3.6, 3.7 and 4.11 V s. Li+/Li. As
expec ed, educing he amoun o C65 has a de imen al e ec
on he elec ode pe o mance due o i s highe esis i i y,
especially a high cu en densi y, as p o ed when compa ing
LVNP-80:10:10 and LVNP-90 :5:5. Unde he same applied
cu en densi y, he discha ge ime o LVNP-90:5: 5 is educed,
accoun ing o less capaci y, and he o e po en ial almos
ipled he one o he e e ence LVNP-80:10: 10, as epo ed in
Table 2. The o e po en ial is de ined as he addi ional po en ial
beyond he he modynamic equi emen s needed o d i e a
eac ion.[49] Low conduc i i y a adaic ma e ials end o show
high o e po en ial, which p omo es ac i e ma e ial deg ada ion
and leads o low ene gy e iciency i inco po a ed in a ull cell.
This is a key poin o conside since low ene gy e iciency
means ha ene gy losses a e ans o med in o hea , equi ing
mo e complex and expensi e hea managemen sys ems.
Li3V1:95Ni0:05ðPO4Þ3$Li2:5V1:95Ni0:05ðPO4Þ3
þ0:5Liþa 3:6V s:Liþ=Li (1)
Li2:5V1:95Ni0:05ðPO4Þ3$Li2V1:95Ni0:05ðPO4Þ3þ
0:5Liþa 3:7V s:Liþ=Li (2)
Li2V1:95Ni0:05ðPO4Þ3$Li1V1:95Ni0:05ðPO4Þ3þ
1Liþa 4:11 V s:Liþ=Li (3)
In iew o his esul , an ad anced o mula ion was
de eloped in o de o main ain he p omising pe o mance
shown by LVNP when o mula ed wi h high con en o
conduc ing agen and binde . To his aim, he amoun o binde
was educed while he amoun o C65 was inc eased o each a
o mula ion consis ing o LVNP-90:7: 3. GCD o LVNP-90:7:3
elec odes, obse ed in Figu e 1, shows in e media e beha io
Figu e 1. Cha ge/discha ge cu es o LVNP-80:10:10 ( ed), LVNP-90:5: 5 (black) and, LVNP-90:7 : 3 (blue) elec odes a (a) 0.5 A g1, (b) 1 A g1and (c) 3 A g1.
Table 2. O e po en ial o LVNP-80:10:10, LVNP-90 :5:5 and LVNP-90:7: 3
elec odes a 1 A g1and 3 A g1cu en densi ies calcula ed by he
po en ial di e ence exis ing be ween he uppe pla eau du ing cha ge and
discha ge.
Fo mula ion (LVNP:C65:PVdF) O e po en ial
1 A g13 A g1
80:10:10 0.041 V 0.081 V
90:5:5 0.11 V 0.285 V
90:7:3 0.05 V 0.114 V
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be ween LVNP-80:10:10 and LVNP-90 :5:5, wi h ela i ely good
capaci y e en ion a high cu en densi y, and simila o e -
po en ial o LVNP-80:10:10.
Figu e 2a epo s he g a ime ic capaci y o he h ee
di e en o mula ions pe mass o LVNP. The op imiza ion o
he elec ode o mula ion is no i ial as can be obse ed in
Figu e 2; LVNP-80 :10:10 displays highe capaci y han LVNP-
90:5:5 and LVNP-90:7 :3 and has an excellen a e capabili y. In
Figu e 2b, he g a ime ic capaci y is epo ed pe mass o
elec ode, conside ing he weigh o he o al elec ode coa ing
(coa ), including he conduc ing agen and he binde and
excluding he cu en collec o . The me ic epo ed in Fig-
u e 2b s anda dizes he capaci y o he h ee elec odes,
enabling an accu a e compa ison be ween he h ee o mula-
ions. In ha case, LVNP-90:7: 3 clea ly ou pe o ms LVNP-
80:10:10 due o he highe a io o ac i e ma e ial in he o al
mass, which inc eases he speci ic capaci y o he cell. Thus,
when epo ing wi h espec o me ely LVNP, capaci y is
o e es ima ed as he epo ed me ic neglec s he weigh o
he C65 and PVdF. Figu e 2c epo s he olume ic capaci y o
he elec ode coa ing in mAh cmcoa 3. In olume ic e ms, he
low densi y o C65 and PVdF compa ed wi h LVNP se e ely
a ec s he capaci y o he elec ode, hus, highe amoun o
LVNP enhances he elec ochemical pe o mance.
O e all, LVNP-90:7:3 elec odes ha e shown he bes
pe o mance, wi h high-capaci y alues o ca. 82 mAh gAM1, 79
mAh gcoa 1and 31 mA cmelec ode3, good capaci y e en ion and
low o e po en ial o 0.05–0.114 V a 1–3 A g1, eaching an
op imum balance be ween he ac i e ma e ial, he conduc i e
addi i e and he binde .
Nex , he impac o he mass loading o he elec odes was
s udied. I is expec ed ha highe loading elec odes will su e
om inc eased in e nal esis ance and slowe ion di usion
h ough he bulk elec ode. Despi e he good pe o mance o
LVNP-90:7:3, a o mula ion con aining only 3% o PVdF did no
show he equi ed mechanical s abili y o he ab ica ion o
high mass loading elec odes, which collapsed du ing he
cu ing s ep. Then, LVNP-90:5: 5 elec odes (i.e., wi h highe
amoun o binde , 5%) we e de eloped ins ead. Mass loadings
o 2.5, 4.5, 6 and 9 mg cm2we e ab ica ed o e alua e he
in luence o he mass loading in he elec ochemical pe o m-
ance. GCD cu es o LVNP-90 :5:5 elec odes o 2.5, 4.5 and
6 mg cm2a e shown in Figu e 3. The e ec o he mass loading
is almos negligible a low cu en densi y, while a high cu en
densi y, he la ge di usion imes and esis ance, inc eased he
o e po en ial o he elec odes as shown in Table S1. When
using 6 mg cm2elec odes, he second Li+inse ion eac ion
occu ing in he hi d pla eau could no ake place below 4.3 V
s Li+/Li, which is conside ed o be he po en ial limi o a oid
elec oly e deg ada ion.[50] In hal -cell con igu a ion, he ca-
paci y o 6 and 9 mg cm2elec odes d ama ically d ops as
obse ed in Figu e S1. Thus, e en i LVNP conduc i i y and ionic
di usion a e ema kably high compa ed wi h o he a adaic
ma e ials,[39,51] i is no enough o ope a e high mass loading
elec odes. Inc easing he mass loading o LVNP-based elec o-
des would equi e he addi ion o a cons an ol age s ep when
cha ging he ma e ial.[47] This is a c i ical poin since LICs aim o
ope a e in cons an cu en cha ge-discha ge mode only.
Ins ead, as desc ibed in he nex sec ion, he ac i e ma e ial will
be modi ied o enhance he conduc i i y.
2.2. LVNP-AC Composi e Elec ode
Finally, once LVNP bounda ies a e well de ined and unde s ood,
i s hyb idiza ion wi h an AC o de elop a hyb id a adaic-
capaci i e elec ode as a high ene gy posi i e elec ode o LICs
is en isaged. As i s app oach, he composi e elec ode is
designed by combining bo h componen s in a 1:1 mass a io
wi h he o e all elec ode o mula ion being 45% LVNP, 45%
AC, 5% C65 and 5% PVdF ( om now on called LVNP-AC), and
an a e age mass loading o 2.5 mgAM cm2. The ex u al
Figu e 2. (a) Speci ic capaci y epo ed espec o he ac i e ma e ial (AM)
weigh ; (b) speci ic capaci y epo ed espec o he elec ode coa ing
weigh ; and (c) olume ic capaci y o he elec ode coa ing o LVNP-
80:10:10 ( ed), LVNP-90:5:5 (black), and LVNP-90:7:3 (blue) elec odes a
di e en cu en densi ies.
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p ope ies o LVNP powde and LVNP-AC mix u e we e
de e mined om ni ogen adso p ion/deso p ion iso he ms and
included in Figu e 4a. As expec ed, he non-po ous LVNP poses
a e y low BET speci ic su ace a ea (SSA)[40] o 29 m2g1, while
he addi ion o AC inc eases he BET SSA o he composi e up
o 586 m2g1. Po e size dis ibu ions (PSD) o bo h samples a e
Figu e 3. Cha ge/discha ge cu es o 2.5 (black), 4.5 (g een) and 6 ( ed) mg cm2elec odes a (a) 0.5 A g1, (b) 1 A g1and (c) 3 A g1.
Figu e 4. (a) Ni ogen adso p ion/deso p ion iso he ms and (b) po e size dis ibu ion o LVNP and LVNP-AC ac i e ma e ials.
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depic ed in Figu e 4b. The LVNP-AC mix u e is composed o
mic opo es (<2 nm) and small mesopo es (2–50 nm), which a e
a ibu ed o he AC. Top iew and c oss sec ion SEM images o
LVNP-AC elec odes a e p esen ed in Figu e 5. The homoge-
neous dis ibu ion o LVNP and AC in he elec ode ensu es he
o e all elec onic conduc i i y o he elec ode owing o he
close con ac o low conduc i e LVNP pa icles wi h he highly
conduc i e AC. Mo eo e , LVNP pa icles o ca. 1μm a e
sandwiched in be ween AC pa icles, wi h an a e age size o
5μm, con ibu ing o dec ease he in e nal esis i i y.
In o de o conduc he elec ochemical e alua ion o LVNP-
AC, LVNP and AC elec odes we e also e alua ed and used as
e e ences (all o hem wi h a 90% AM,5% C65 and 5% PVdF
o mula ion). Thus, GCD cu es o LVNP-90:5 :5, LVNP-AC and
AC elec odes cha ac e ized be ween 3–4.3 V s. Li+/Li a e
illus a ed in Figu es 6a, 6b and 6c. The AC displays he ypical
linea shape indica i e o a capaci i e cha ge s o age mecha-
nism, while LVNP -desc ibed be o ehand- shows h ee well-
de ined pla eaus. The hyb id LVNP-AC elec ode clea ly shows a
combina ion o bo h mechanisms: a capaci i e linea p o ile
o e he whole po en ial window wi h slope changes a 3.6, 3.7
and 4.1 V s. Li+/Li. As he cu en densi y inc eases, he
capaci i e mechanism o LVNP-AC becomes mo e and mo e
dominan , and a 3 A g1 he e a e less in lec ion poin s in he
iangula -shaped cu e. O e all, LVNP elec ode shows he
highes capaci y ou pu wi hin all he applied cu en densi y
ange, as shown in Figu e 6d. Howe e , he o e po en ial o he
LVNP elec ode a 3 A g1is e y high, which could lead o
sho e cycle li e and dec ease he ene gy e iciency when ull
cells a e assembled. On he con a y, LVNP-AC exhibi s double
he capaci y o he AC and he symme ic cha ge-discha ge
p o ile wi h low in e nal esis i i y, wha an icipa es high ene gy
e iciency. The same end is obse ed in he ol ammog ams o
LVNP, LVNP-AC and AC elec odes depic ed in Figu e S2
showing he displacemen o he edox peaks o LVNP wi h
espec o LVNP-AC.
2.3. Inco po a ing Dili hium Squa a e in he LVNP-AC
Elec ode as P e-Li hia ion Agen
LICs equi e a p e-li hia ion s ep o compensa e o he Li losses
du ing he i s cycle, co esponding mainly o he SEI o ma ion
in he nega i e elec ode. In addi ion, he p e-li hi ia ion s ep
adjus s he cell ol age, lowe ing he po en ial o he nega i e
elec ode o maximize he capaci y and ene gy densi y o he
ull cell.[52] To his aim, dili hium squa a e (Li2C4O4) is used, since
i has al eady been alida ed as p e-li hia ion agen combined
Figu e 5. (a, b) Top iew SEM and (c, d) c oss-sec ion SEM images o LVNP-AC elec ode by using ETD (a, c) and BSED (b, d) de ec o s.
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wi h AC in he posi i e elec ode. Howe e , Li2C4O4is needed in
a high amoun , be ween 10–40% o he elec ode weigh o
p e-li hia ion, depending on LIC chemis y and cell mass
balance. Such an amoun o addi i e migh nega i ely impac
he o e all pe o mance o he elec ode when i inco po a es a
low conduc i i y a adaic componen . When Li2C4O4sac i icial
sal decomposes, i lea es some holes in he elec ode ha
could de ach pa icles om each o he , inc easing he in e nal
in e pa icle esis i i y o he elec ode. When i is inco po a ed
on ca bonaceous elec odes, his ea u e is a mino issue due o
he high conduc i i y o ACs. Ne e heless, as p esen ed in
Sec ion 3.1 and 3.2, an inc ease in he in e nal esis i i y could
lead o poo a e pe o mance, de imen al o he use o hose
elec odes in high powe de ices.
A posi i e elec ode wi h o mula ion 27.5% LVNP, 27.5%
AC, 35% Li2C4O4, 5% C65 and 5% PVdF was de eloped
(he ea e called LVNP-AC-Li) and i s elec ochemical esponse
was e alua ed. Dili hium squa a e decomposes du ing he i s
cycles as shown in Figu e S3. The capaci y deli e ed du ing he
i s cycle co esponds p edominan ly o he Li2C4O4decom-
posi ion (CLi2C4O4 =425 mAhg1), due o mos o he Li2C4O4is
success ully decomposed du ing he i s cycle. Fu he mo e,
he Li2C4O4decomposi ion has been con i med by XRD as
illus a ed in Figu e S4, whe e he signals ela ed o Li2C4O4
disappea a e he i s GCD cycle.
A e Li2C4O4decomposi ion, he elec ode o mula ion is
42.31% LVNP, 42.31% AC, 7.69% C65 and 7.69% PVdF, which
has been used o he capaci y calcula ions. The mass loading
o he elec odes was 2.5 mgAM cm2, conside ing ha he ac i e
ma e ial is he mix u e o LVNP and AC. A e 10 ini ial cycles a
CLi2C4O4/10 be ween 3–4.3 V s Li+/Li, a a e capabili y es was
pe o med o he elec odes. GCD cu es o LVNP-AC and LVNP-
AC-Li a 0.5, 1 and 3 A g1a e illus a ed in Figu es 7a, 7b and
7c. Bo h elec odes show simila cha ge-discha ge p o iles,
despi e he inco po a ion o he sac i icial sal . Howe e , a he
highes applied cu en densi y, i.e. a discha ge ime o only
60 s, he o e po en ial inc eases up o 0.12 V when he Li2C4O4
is added o he elec ode, as obse ed in Figu e 7c. As abo e-
men ioned, he addi ion o Li2C4O4migh ha e induced a
de imen al e ec on he elec onic conduc i i y o he
elec ode due o he loss o in e pa icle con ac be ween LVNP
pa icles o wi h AC pa icles. Fu he mo e, capaci y a di e en
cu en densi ies is illus a ed in Figu e 7d. The addi ion o
Li2C4O4 esul s in a highe capaci y dec ease and sligh ly highe
o e po en ial a high cu en densi y, while inc eases he
capaci y and main ains negligible o e po en ial a low cu en
densi y. The capaci y inc ease caused by Li2C4O4sac i icial sal
has al eady been obse ed in ca bonaceous elec odes and is
now unde s udy.[44]
GCD expe imen s we e accompanied by cyclic ol amme y
(CV) o be e unde s and he dominan mechanisms. As
Figu e 6. Cha ge/discha ge cu es o LVNP (black), LVNP-AC ( ed) and AC (blue) elec odes a (a) 0.5 A g1, (b) 1 A g1and (c) 3 A g1. (d) Speci ic capaci y pe
g am o ac i e ma e ial o he h ee composi ions a di e en cu en densi ies.
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obse ed in Figu e S4, simila o LVNP-AC, LVNP-AC-Li elec o-
des showed mixed capaci i e and a adaic mechanisms, wi h
quad a ic shape in e up ed by a adaic peaks a 3.6, 3.7 and
4.1 V s. Li+/Li. LVNP-AC-Li shows wide CV in he 3.5–4.1 V s.
Li+/Li po en ial window, leading o highe capaci ies. Thus, i
can be concluded ha a hyb id posi i e elec ode con aining a
LVNP-AC composi e imp o es he capaci y o he adi ional AC
elec ode. In addi ion, Li2C4O4was success ully inco po a ed
in o he composi e elec ode o mula ion, which will be o
u mos impo ance when assembling a LIC cell wi hou a
sac i icial Li elec ode.
As a inal alida ion es , cyclabili y is shown in Figu e 8. The
li e ime o LVNP-AC and LVNP-AC-Li elec odes is e alua ed
be ween 3–4.3 V s. Li+/Li a a cu en densi y o 1 A g1(i.e.
discha ge ime o ca. 1–2 minu es). As shown in Figu e 8a, i
esul s in an unexpec ed sho li e ime o only 550 cycles o
bo h LVNP-AC and LVNP-AC-Li elec odes be o e hey each
20% loss o ini ial capaci ance. GCD cu es o LVNP-AC-Li
p esen ed in Figu e 8b show p og essi e educ ion o 3.6, 3.7,
and 4.1 V s. Li+/Li inse ion/deinse ion semi-pla eaus upon
cycling, which comple ely disappea a e 800 cycles.
The as capaci y decay obse ed is p ima ily caused by he
dec ease o he elec ochemical ac i i y o LVNP. In o de o
shed some ligh in o his unexpec ed pe o mance as well as
he deg ada ion mechanism behind he ailu e o he LVNP-AC-
Li elec odes, pos -mo em analysis o he elec odes was
pe o med. SEM and EDX cha ac e iza ion o he p is ine and
cycled elec odes is p esen ed in Figu e 8c and 8d and Table S2.
SEM images o p is ine and cycled LVNP-AC-Li elec odes show
a mo phology change a e cycling ha could be ela ed wi h a
phase ansi ion ha migh impac he elec ochemical ac i i y
o LVNP. Addi ionally, EDX measu emen s we e also pe o med:
in he case o p is ine elec odes only a he su ace while in he
case o pos -mo em elec odes bo h he su ace and he
in e nal pa o he elec ode -a e sc a ching he su ace- we e
analyzed. The anadium and phospho us con en o he
elec odes ob ained by EDX is epo ed in Table S2. P is ine
elec odes show simila anadium and phospho us amoun s,
bu he con en s change a e cycling. The su ace o he
elec odes is en iched in anadium while he in e nal pa is
anadium poo , sugges ing a mig a ion o anadium o he
su ace o he elec ode. The elemen al con en o a LVNP-AC-Li
elec ode cycled o 1000 cycles a 1 A g1has also been
cha ac e ized by induc i ely coupled plasma op ical emission
spec oscopy (ICP-OES). In o de o con i m and quan i y he
amoun o anadium dissol ed du ing he ageing es , a p is ine
LVNP-AC-Li elec ode was also cha ac e ized. The amoun o P,
Ni and V in he samples is shown in Table S3. LVNP-AC-Li
p is ine sample shows a simila con en o P and V, and a small
con en o Ni, as expec ed om he s oichiome y o he
ma e ial. In he case o LVNP-AC-Li cycled, he P and Ni con en
emains almos unal e ed wi h espec o LVNP-AC-Li p is ine. A
small di e ence in he amoun o Ni is obse ed, ne e heless i
migh be caused by expe imen al unce ain y since i only
Figu e 7. Cha ge/discha ge cu es o LVNP-AC ( ed) and LVNP-AC-Li (da k yellow) elec odes a (a) 0.5 A g1, (b) 1 A g1and (c) 3 A g1. (d) Speci ic capaci y
pe g am o ac i e ma e ial o LVNP-AC and LVNP-AC-Li elec odes a di e en cu en densi ies.
Wiley VCH Mi woch, 25.09.2024
2419 / 364655 [S. 37/41] 1
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