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ReseaRch aR icle
Hie a chical Nanocellulose-Based Gel Polyme Elec oly es
o S able Na Elec odeposi ion in Sodium Ion Ba e ies
Nee u Mi al, Sean Tien, E lan z Lizundia,* and Ma kus Niede be ge *
N. Mi al, S. Tien, M. Niede be ge
Labo a o y o Mul i unc ional Ma e ials
Depa men o Ma e ials
ETH Zü ich
Vladimi -P elog-Weg 5, Zu ich 8093, Swi ze land
E-mail: [email p o ec ed]
E. Lizundia
Li e Cycle Thinking G oup
Depa men o G aphic Design and Enginee ing P ojec s
Facul y o Enginee ing in Bilbao
Uni e si y o he Basque Coun y (UPV/EHU)
Bilbao 48013, Spain
E-mail: [email p o ec ed]
E. Lizundia
BCMa e ials
Basque Cen e o Ma e ials
Applica ions and Nanos uc u es
UPV/EHU Science Pa k
Leioa 48940, Spain
The ORCID iden i ica ion numbe (s) o he au ho (s) o his a icle
can be ound unde h ps://doi.o g/10.1002/smll.202107183.
DOI: 10.1002/smll.202107183
1. In oduc ion
Recha geable ba e ies a e one o he co -
ne s ones o he g een ene gy ansi ion
and a e des ined o play a c ucial ole in
he deca boniza ion and elec i ica ion
o he wo ldwide economies. The imple-
men a ion o e icien , sus ainable, and
sa e ba e ies will acili a e he shi in o
ci cula economy models as hey ha e he
po en ial o educe he deple ion o na u al
esou ces and he en i onmen al pollu ion
caused by ossil uels.[1] Sodium ion ba -
e ies (NIBs) ha e een e ed he sus ain-
able ene gy s o age landscape as iable
al e na i es o he ubiqui ous li hium ion
ba e ies (LIBs). Acco ding o li e cycle
assessmen , NIBs show gene ally lowe
impac s in global wa ming, ossil deple-
ion po en ial, eshwa e eu ophica ion,
and human oxici y po en ial when com-
pa ed o LIBs.[2] Sodium is na u ally abun-
dan (0.0017% s 2.36% in Ea h’s c us ).[3]
I is ma kedly cheape han li hium
(17 $ kg–1 o Li, 0.15 $ kg–1 o Na),[4] and
NIBs p esen no ably lowe supply isks.
Fu he mo e, he s abili y o NIBs in he ully discha ged s a e
signi ican ly imp o es he sa e y o s o ed ba e ies, which is o
pa icula in e es o la ge-sized ba e ies.[5] A he same ime,
he lowe heo e ical capaci y o 1165 mAh g–1 o Na compa ed
o 3829 mAh g–1 o Li ansla es in o lowe speci ic ene gy den-
si ies o 90–120 Wh kg–1, limi ing he implemen a ion o NIBs
o la ge-scale s a iona y applica ions.[6,7] As sa e y du ing use
is o u mos impo ance in g id scale ene gy s o age applica-
ions,[8] de eloping al e na i es o con en ional ma e ials used
in NIBs is o p ime ele ance. Addi ionally, u he ad ances
a e equi ed o boos he ene gy densi y o NIBs close o
250 Wh kg–1, a alue ha is es ablished in The Eu opean S a-
egic Ene gy Technology Plan Ac ion 7 o 2030 o make hem
compe i i e wi h LIBs.
Al hough conside ed as an ine o passi e componen
wi hin a ba e y,[9] he sepa a o –elec oly e pai is a c i ical com-
ponen de ining bo h he elec ochemical pe o mance and ba -
e y sa e y.[10,11] The ole o he sepa a o is o p e en in e nal
sho -ci cui be ween he elec odes while ensu ing an e icien
ion ans e ac oss his elec onically insula ing physical ba ie
du ing cha ge/discha ge.[12] Con en ional NIBs use a pe o-
leum-based polyole in o a glass ibe sepa a o soaked in o
an o ganic elec oly e such as e hylene ca bona e o p opylene
ca bona e con aining dissol ed sodium sal s (NaClO4, NaPF6,
Sodium ion ba e ies (NIBs) based on ea h-abundan ma e ials o e e i-
cien , sa e, and en i onmen ally sus ainable solu ions o a deca bonized
socie y. Howe e , o compe e wi h ma u e ene gy s o age echnologies such
as li hium ion ba e ies, u he p og ess is needed, pa icula ly ega ding he
ene gy densi y and ope a ional li e ime. Conside ing hese aspec s as well as
a ci cula economy pe spec i e, he au ho s use biodeg adable cellulose nan-
opa icles o he p epa a ion o a gel polyme elec oly e ha o e s a high
liquid elec oly e up ake o 2985%, an ionic conduc i i y o 2.32 mS cm−1,
and a Na+ ans e ence numbe o 0.637. A balanced a io o mechanically
igid cellulose nanoc ys als and lexible cellulose nano ibe s esul s in a
mesopo ous hie a chical s uc u e ha ensu es close con ac wi h me allic
Na. This a chi ec u e o e s s able Na pla ing/s ipping a cu en densi ies
up o ±500µA cm−2, ou pe o ming con en ional ossil-based NIBs con-
aining sepa a o –liquid elec oly es. Pai ed wi h an en i onmen ally sus ain-
able and economically a ac i e Na2Fe2(SO4)3 ca hode, he ba e y eaches an
ene gy densi y o 240 Wh kg−1, deli e ing 69.7 mAh g−1 a e 50 cycles a a a e
o 1C. In compa ison, Celga d in liquid elec oly e deli e s only 0.6 mAh g−1
a C/4. Such gel polyme elec oly es may open up new oppo uni ies o
sus ainable ene gy s o age sys ems beyond li hium ion ba e ies.
© 2022 The Au ho s. Small published by Wiley-VCH GmbH. This
is an open access a icle unde he e ms o he C ea i e Commons
A ibu ion-NonComme cial 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 and is no used o comme cial pu poses.
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e c.).[13] Howe e , his con igu a ion su e s om in e io elec-
ochemical pe o mance and sa e y issues due o he ela i ely
low po osi y ha hinde s e icien ion ans e , poo elec oly e
we abili y o he sepa a o ha ypically mani es s i sel in high
in e nal esis ance, p oblems wi h elec oly e leakage and den-
d i ic punc u e ha can cause a sho -ci cui in he ba e y.[14]
Replacing he con en ional sepa a o -liquid elec oly e pai by
a gel-like elec oly e ep esen s a s ep o wa d owa d sa e ba -
e ies.[15] In his ega d, polyme s wi h pola unc ional g oups
play a p edominan ole, gi en hei po en ial o dissol e alkali
me al sal s.[16] Due o hei imp o ed mechanical p ope ies
compa ed o adi ional mic opo ous sepa a o s, gel polyme
elec oly es (GPEs) a e pa icula ly ad an ageous in sup-
p essing he o ma ion o high su ace a ea me al deposi s (also
known as dend i es).[15,17] The mechanically con o mable na u e
o GPEs o e s enhanced compa ibili y wi h elec odes, ou pe -
o ming hei liquid elec oly e coun e pa s in e ms o in e -
acial compa ibili y wi h elec odes and ope a ing li e ime.[18,19]
Addi ionally, as he elec oly e emains immobilized wi hin a
c osslinked polyme ma ix, dissolu ion o ac i e ma e ial is
limi ed, and undesi able sa e y issues associa ed wi h elec o-
ly e leakage a e a oided.[19] The mos ep esen a i e example
o GPEs is polye hylene oxide [(CH2CH2O)n], whe e he e he
oxygen g oups can dissol e sal s, p o iding ion ans e pa h-
ways.[20] Poly inylidene luo ide and i s copolyme s, polyac y-
loni ile, and poly(me hyl me hac yla e) a e o he p e e en
polyme s o ab ica e gel elec oly es.[20] Howe e , he achie ed
low ionic conduc i i ies a oom empe a u e esul in o elec o-
chemical pe o mances ha lag a behind hose equi ed o
hei p ac ical implemen a ion, while he poo in e ace com-
pa ibili y and educed elec ochemical s abili y be ween elec o-
ly es and he elec odes can esul in a con inuous pe o mance
de e io a ion.[14] In addi ion, he en i onmen al impac s a ising
om non- enewable and non-deg adable sepa a o ma e ials
and GPEs a e a sou ce o conce n ega ding sus ainabili y and
end o li e sa e y as hey con ibu e o was e s eams ending in
land ills and oceans. Conside ing hese issues and he p inci-
ples o he ci cula economy, new solu ions need o be de el-
oped in he ield o ba e y elec oly es ha educe dependence
on he ex ac ion o ini e esou ces wi hou comp omising
elec ochemical pe o mance.
Renewable mac omolecules de i ed om na u al biomass,
also e e ed o as biopolyme s, show unc ional p ope -
ies equi alen o abo e hose o syn he ic polyme s, o e ing
addi ional a ac i e ea u es including enewabili y, biodeg-
adabili y, and lack o oxici y.[21,22] Among he wide a ie y
o biopolyme s anging om na u al polyme s (cellulose,
chi in, aga ose, e c.), p o eins (gela in, silk, e c.) o complex
3D b anched a chi ec u es (lignin), polysaccha ides hold a
p e alen ole. Thei unc ional g oups, o en including OH,
SO3H, CONH2, NH2, o CONH enable good a ini y
o liquids and ensu e p e e en ial in e ac ion wi h he sal
anion o imp o e sal solubili y and ca ion ans e ence in he
elec oly e.[12] Pa icula ly ele an a e biopolyme elec oly es
based on cellulose,[23,24] which o e en i onmen ally sus ain-
able and cos -e ec i e al e na i es o pe oleum-based con en-
ional choices. A s ep o wa d in he de elopmen o enewable
polyme s-based elec oly es may a ise om (nano) ib ous cellu-
losic s uc u es. Elec oly es based on nanocellulose can abso b
and con ine he liquid elec oly e. As an addi ional ad an age,
hey gi e access o 3D memb anes wi h con inuous in e con-
nec ed mesopo es ha ing good he mal,[25] mechanical,[26] and
elec ochemical s abili y,[27] oge he wi h la ge alues o elec o-
ly e up ake. This can p o ide e icien pa hways o ion ans e
ac oss he memb anes.[27,28] Con a ily o ino ganic solid elec-
oly es,[29,30] he mechanically so and duc ile cha ac e o cel-
lulosic GPEs ensu es a be e in e acial con ac wi h he elec-
odes and imp o ed dimensional s abili y upon he accommo-
da ion o any elec ode olume changes du ing Na+ inse ion/
ex ac ion. Addi ionally, biopolyme s can be easily unc ional-
ized o une hei unc ional p ope ies,[31] enla ging he ange
o oppo uni ies o de elop new ma e ials in he ene gy s o age
ield.
Thanks o hese ad an ages, di e en wo ks ha e ocused
hei a en ion on he de elopmen o ees anding po ous
ba e y sepa a o s comp ising cellulose nanoc ys als (CNCs),
highly c ys alline 100–1000 nm long needle-shaped nanopa -
icles (2–20 nm in diame e ) ex ac ed h ough a con olled
hyd olysis o cellulose.[25,27] These cellulosic nanopa icles ha e
he in iguing abili y o o m s uc u es wi h hie a chically
a anged mesopo es (po e diame e s om 2 o 50nm), ei he
in he o m o ees anding ilms,[27] hyd ogels, o c yogels.[32]
Impo an ly, mesopo ous mo phologies boos ion ans e ence
and ensu e uni o m ion- lux be ween he elec odes, p o iding
a s able and e e sible ion pla ing/s ipping wi hou dend i e
o ma ion.[33–36] Al hough s uc u es solely comp ising CNCs
could be used a p io i as GPEs o solid polyme elec oly es o
ba e ies, hei mechanical p ope ies a e no a pa . Imp o ing
he mechanical s abili y o CNC-based ma e ials upon blending
wi h o he cellulosic de i a i es may ensu e a good compa -
ibili y be ween elec oly e pa ne s h ough an de Waals in e -
ac ions and in e molecula hyd ogen bonding.[37,38] In an analo-
gous ashion o ha shown ecen ly o Li+ conduc o s,[25,27]
pa icula ly a ac i e a e he cellulosic de i a i es wi h Na+-
binding g oups, because hese coun e ions can ake pa in he
conduc ion mechanisms o he elec oly e. In his sense, cel-
lulose nano ibe s (CNFs) could be in e es ing o de elop Na+
conduc ing GPEs as he C6 p ima y hyd oxyl g oups exposed
on hei su aces a e selec i ely oxidized o sodium ca boxyla es
(COONa) h ough a TEMPO-media ed oxida ion p ocess.[39]
Following a ci cula “ euse, ecycle o biodeg adable” pe -
spec i e, he e we de elop o he i s ime a biopolyme -based
gel elec oly e o NIBs. The con inuous h ee-dimensional
s uc u e p o ided by CNCs hold he liquid elec oly e wi h
no leakage o s uc u e collapse. Pai ed wi h CNFs o boos
Na+ ans e ence, ionic conduc i i y alues up o 2.32 mS cm–1
wi h a ans e ence numbe o 0.637 a oom empe a u e
a e ob ained. Ob ained mesopo ous mo phology enables low
in e acial esis ance wi h me allic Na, which is ansla ed
in o a s able Na pla ing/s ipping a cu en densi ies up o
±500 µA cm–2. When assembled in o Na2Fe2(SO4)3/Na hal -
cells, he ba e y deli e s up o 80.6 mAh g–1 a e 25 cycles a
1C a e (g a ime ic ene gy densi y o 240 Wh kg–1). The highly
e e sible and s able Na me al deposi ion enabled by he nano-
cellulose gel elec oly e, combined wi h he supp ession o
dend i e g ow h and deli e y o high capaci y p omise u u e
ad ances in he de elopmen o long cycle li e beyond LIBs
based on enewable esou ces.
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2. Resul s and Discussion
2.1. Fab ica ion and Mo phology
The aim o his wo k is he ab ica ion o hie a chically s uc-
u ed GPEs based on nanocellulose ha acili a e a apid and
homogeneous Na+ ans e . In he las yea s, di e en biomi-
me ic app oaches ha e been explo ed o enginee he s uc u e
o ba e y sepa a o s wi h sui able nano- and mic opo ous mo -
phologies so ha e icien ion ans e wi h dend i e g ow h sup-
p ession and long- e m cycling pe o mance can be ob ained.
Fo example, Ma e al. used a op-down app oach o upcycle
eggshell memb anes in o a high-pe o mance sepa a o o
LIBs.[33] Howe e , he bo om-up app oach o e s an easie and
accessible p ocess wi h inc eased e sa ili y.[42] To bene i om
bo h app oaches, he e we ake inspi a ion om he hie a chical
s uc u e o he ees which con ain 1D building blocks wi h
plen y o oxygen-con aining g oups. Upon he combina ion o
op-down and bo om-up p ocesses, i is possible o achie e bio-
logical building blocks (nanocelluloses) ha a e sel -assembly
and chemical c osslinking can o m ionically conduc ing GPEs
sui able o sodium ion ba e ies. As summa ized in Scheme 1,
CNCs and CNFs a e i s ex ac ed om lignocellulosic biomass
h ough a op-down p ocess.[43] Using a chemically induced
hyd olysis (usually H2SO4-induced) o a mechanically induced
des uc u ing app oach assis ed by a TEMPO-media ed oxida-
ion, he mic o ib ils ha cons i u e he plan cell wall can be
selec i ely b oken down in o colloidal building blocks o ob ain
spindle-shaped CNCs wi h diame e s o 2–20nm and leng hs
below 1µm, o ilamen -like CNFs wi h diame e s o 5–30nm
and leng hs ex ending up o ew mic ome e s.[44] Du ing his
p ocess, he su ace o CNCs a e deco a ed wi h anionic sul a e
hal -es e g oups (−OSO3–),[27] while nega i ely cha ged ca boxyl
g oups ca ying Na+ as coun e ions (−COO–) a e p esen on
CNFs.[39] Gi en hei high s eng h and dimensional aniso opy,
bo h CNCs and CNFs a e highly a ac i e o he ab ica ion
o gels and hyd a ed physically and/o chemically c osslinked
3D ne wo ks ollowing a bo om-up app oach. Impo an ly,
cha ged su aces enable an e icien dispe sion, simul ane-
ously p o iding in e ac ing g oups ia elec os a ic o hyd ogen
bonding.[45] Once he gel is o med, he aqueous gel phase
can be easily eplaced by a gas phase h ough eeze-d ying,
esul ing in highly po ous ligh weigh ma e ials wi h a hie -
a chical po ous s uc u e and a e age po e size in he ens o
nanome e s. These c yogels can be applied as a ma ix o he
de elopmen o GPEs a e he in usion o ionically conduc ing
liquid elec oly es, which apidly pene a e in o he nanopo ous
ne wo k s uc u e ia capilla y e ec . The s ong a ini y o
pola liquid elec oly es o he nanocellulose s uc u e a ising
om he syne gis ic e ec s o CNCs and CNFs (OH, COO–,
OSO3– g oups, so / igid pa icle combina ion, hie a chical
s uc u e, e c.) makes hese ma e ials sui able o NIBs.[46]
Ideally, he nanocellulose concen a ion in he liquid sus-
pension should be kep as low as possible o maximize he
po osi y a e eeze-d ying. P elimina y expe imen s indica ed
ha he concen a ion should be a ound 4w % o CNCs and
2 w % o CNFs, because lowe concen a ions ail o o m
gels. To ob ain mechanically esilien GPEs a e he in usion
o he liquid elec oly e (and a oiding i s e en ual edispe sion),
glu a aldehyde is added as a chemical c oss-linke o o m
ace al linkages be ween he hyd oxyl g oups o he nanocellu-
loses.[47] Among he s udied concen a ions o glu a aldehyde
and CNC in wa e , ba e CNCs canno o m physically s able
s uc u es and collapse a e eeze-d ying (Figu e S1, Sup-
po ing In o ma ion). The inco po a ion o long and lexible
CNFs p o ides physically en angled ne wo ks ha acili a e
hyd ogel o ma ion,[45] which a e glu a aldehyde c osslinking
and eeze-d ying esul in mac oscopic po ous c yogels. These
c yogels a e he in usion o liquid elec oly e (1 m NaClO4 in
e hylene ca bona e/p opylene ca bona e (EC/PC) a 50/50 / )
yield physically s able GPEs (Figu e S2, Suppo ing In o ma-
ion). Figu e 1 shows he op- iew and c oss-sec ional scanning
elec on mic oscopy (SEM) images o CNC/CNF 80/20, 50/50,
and 20/80 c yogels. Highly po ous s uc u es wi h iso opically
dis ibu ed h ee-dimensionally connec ed po es a e achie ed
o all he composi ions, bo h a he su ace and ac oss he
en i e c yogel hickness. The e o e, he abundan OH su ace
g oups p esen on CNCs and CNFs will emain a ailable o
sol a e Na+ and o sepa a e i om ClO4–, acili a ing ion di u-
sion be ween elec odes.[23] In e es ingly, he 50/50 eeze-d ied
c yogel shows a ough mo phology, being composed o smalle
and mo e abundan po es, which is desi ed o GPEs o ob ain
high ionic conduc i i y.
2.2. C yogel and GPE Physico-Elec ochemical P ope ies
A enua ed o al e lec ance-Fou ie ans o m in a ed (ATR-
FTIR) expe imen s we e pe o med on nanocellulose-based
GPEs o unde s and he speci ic in e ac ions be ween he nano-
celluloses and he sodium sal . ATR-FTIR spec a in Figu e 2a
show he cha ac e is ic abso p ion bands co esponding o cel-
lulose, wi h a b oad band a 3650–3200 cm–1 esul ing om
OH s e ching, na owe bands a 2902 cm–1 due o asym-
me ic and symme ic CH s e ching, a band a 1160 cm–1
(COC bending), a 1060 cm–1 ( ing ib a ion and COH
bending), and a 897 cm–1 (COC asymme ic s e ching).
The double appea ing in he ange 1840–1740 cm–1 o igina es
om he CO s e ching ib a ions o PC (a single peak a
1779 cm–1) and EC ( wo peaks a 1796 and 1768 cm–1).[48] Addi-
ionally, 50/50 and 20/80 GPEs show a small peak a ≈1605 cm–1
ypical o he dissocia ed ca boxyl g oups o CNFs,[49] while he
shoulde s a 1108 and 980 cm–1 (whose in ensi y dec eases wi h
CNF ac ion) a ise om he OSO3− g oups on CNCs. Finally,
a sha p mode associa ed o ClO4− appea s a 621 cm–1 o all
he c yogels soaked in liquid elec oly e (1 m NaClO4 in EC/
PC (50/50 / )). Al hough no shape di e ences a e obse ed
(Figu e S3, Suppo ing In o ma ion), he blueshi ing o he
band in compa ison o he 613 cm–1 o pu e NaClO4,[48] indi-
ca es an e icien sal dissocia ion in he p esence o nanocel-
lulose. Mo e p ecisely, he la ges blueshi s a e seen o 50/50
and 20/50 samples, sugges ing an imp o ed dissocia ion o
he sal in o Na+ and ClO4− ions.[50] Nanocellulose gel elec o-
ly es a e p edominan ly amo phous as con i med by he b oad
halo cen e ed a 2θ= 22° in he X- ay di ac ion pa e ns, co -
esponding o he (200) plane o cellulose I (Figu e 2b). This
amo phici y indica es he o ma ion o a polyme –sal complex,
whe e Na+ coo dina es wi h he oxygen-con aining unc ional
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g oups p esen on nanocelluloses.[48] The he mal s abili y o
nanocellulosic c yogels was s udied by he mog a ime ic anal-
ysis (TGA) as i is a p ime equisi e o imp o e ba e y sa e y.
TGA cu es in Figu e2c p esen a wo-s ep he modeg ada ion
p ocess cen e ed a ≈288 and ≈403 °C due o depolyme iza ion,
dehyd a ion and subsequen decomposi ion o glycosyl cellu-
losic uni s,[51] yielding a cha equi alen o 9w % o he ini ial
sample wi h CNFs. Nea CNC shows a educed he mal s abili y
due o he ca alyzing e ec o i s sul a e hal -es e g oups.[52]
Howe e , all syn hesized c yogels show accep able he mal
Small 2022, 18, 2107183
Figu e 1. Rep esen a i e SEM images showing he op- iew and c oss sec ion o CNC/CNF based c yogels a 80/20, 50/50, and 20/80 a ios.
Scheme 1. Schema ic ep esen a ion o he nanocellulose gel elec oly e ab ica ion: A op-down app oach is ollowed o ex ac biocolloidal CNCs and
CNFs om biomass, ollowed by bo om-up p epa a ion o he CNC/CNF c yogel using a combina ion o sel -assembly and sol-gel chemis y. GPEs
a e inally ob ained upon o ganic liquid in il a ion.
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s abili ies wi hin he expec ed ange o ba e y ope a ion as
p o ed by he onse o he mal deg ada ion ( empe a u e a
which he i s 10% weigh loss occu s) abo e 235 °C.
The po e mo phology o he c yogels was u he assessed
h ough N2 adso p ion–deso p ion expe imen s. Acco ding o
Figu e2d, eeze-d ied nanocellulose-based c yogels p esen a
B unaue –Emme –Telle su ace a ea (SBET) anging om 44.2
o 61.7 m2 g–1, whe e he c yogel comp ising he la ge ac-
ion o CNFs shows highe su ace a ea. These su ace a eas
ha emain well abo e he 10.8 m2 g–1 ob ained o he glass
mic o ibe sepa a o ,[53] may acili a e he in e ac ion wi h
liquid elec oly e o dissocia e Na+ and boos ionic conduc i i y.
Impo an ly, all he c yogels show a ype IV iso he m wi h
H2 hys e esis, whe e mos o he po es ha e a diame e om
4 o 10 nm (see he DFT po e size dis ibu ion in Figu e S4,
Suppo ing In o ma ion). This mesopo ous mo phology can
ensu e a homogeneous Na+ lux be ween elec odes, which is
ansla ed in o dend i e- ee s able ion pla ing and s ipping.[54]
The abili y o he de eloped c yogels o adso b liquid elec o-
ly e was e alua ed by elec oly e up ake measu emen s based on
Equa ion (2). As shown in Table S2 (Suppo ing In o ma ion),
all he samples p esen liquid up ake alues exceeding 2000%
(wi h a maximum o 4441% o he 20/80 sample), well-abo e
he low alue o 125% shown by he Celga d sepa a o . These
ema kably la ge alues can be explained by a combina ion o
high elec oly e a ini y o nanocellulose, low su ace ension
and capilla y ac ion p o ided by he mesopo ous hie a chical
s uc u e.[34] These cha ac e is ics a e ideal o boos he ionic
conduc i i y abo e 10−3 S cm−1, which is equi ed o p ac ical
applica ions and a e usually ha d o ealize wi h con en ional
GPEs.[55] To con i m he po en ial o nanocellulose gel elec-
oly es as e icien medium o Na+ anspo , we e alua ed
he ionic conduc i i y acco ding o Equa ion (3) based on he
Nyquis impedance plo s shown in Figu e 3a (see Table S2, Sup-
po ing In o ma ion, o u he de ails). All he GPEs p esen
an ionically conduc ing na u e as sugges ed by he s aigh lines
wi h no semici cles, which is asc ibed o coo dina ion si es o
he mobile Na+ p o ided by he OH g oups on he CNC and
CNF su aces.[12] The ionic conduc i i y inc eases om 1.467 o
2.323 mS cm–1 wi h inc easing CNF ac ion om 20 o 50w %,
sugges ing ha he sodium ca boxyla e (-COONa) g oups on he
CNFs enhance he coun e ion mobili y.[56–58] Fo compa ison,
1 m NaClO4 in EC/PC has an ionic conduc i i y o 8 mS cm–1,[5]
con i ming ha nanocellulose acili a es Na+ di usion in he
sys em. This la ge ionic conduc i i y alue o he 50/50 GPE co -
ela es well wi h he enhanced ee ion concen a ion shown in
FTIR esul s. Howe e , u he inc ease in CNF con en esul s
in d op o he ionic conduc i i y o 0.672 mS cm–1, as bo h he
su ace unc ional g oups and su ace a ea a e he dominan
d i e s o achie e a la ge ionic anspo . The achie ed ionic
conduc i i ies a e well-abo e he esul s epo ed o o he
sepa a o –liquid elec oly e sys ems (1.35 mS cm–1 o aga ose/
PVA,[34] 0.12 mS cm–1 o Celga d,[34] 0.4 mS cm–1 o cellulose
ace a e,[59] o 0.07 mS cm–1 o chi in nano ibe memb anes),[60]
o mos o he GPEs o NIBs de eloped so a (see Table S3,
Suppo ing In o ma ion, o u he de ails).
Small 2022, 18, 2107183
Figu e 2. a) ATR-FTIR spec a and b) XRD pa e ns o CNC/CNF GPEs composed o CNC/CNF c yogels soaked in o ganic elec oly e (1 m NaClO4 in
EC/PC a 50/50 / ). c) The mog a ime ic aces and d) N2 adso p ion–deso p ion iso he ms o eeze-d ied CNC/CNF c yogels. The inse shows he
calcula ed B unaue –Emme –Telle speci ic su ace a ea.
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Single-ion conduc ing elec oly es (also known as ionome s)
o e he addi ional ad an age o ensu ing s able alkali me al
deposi ion.[61] We hus quan i ied he Na+ ans e ence numbe
( Na+) o ou GPEs acco ding o he B uce–Vincen me hod[62]
IVIR
IVIR
Na
ss 00
0s
sss
()
()
=∆−
∆−
+ (1)
whe e I0 and Iss a e he cu en s in he unpola ized and pola -
ized s a es, espec i ely; R0 and Rss a e he esis ances in he
unpola ized and pola ized s a es, espec i ely; and ΔV ep e-
sen s he applied po en ial s ep (see Figu e S5 and Table S4,
Suppo ing In o ma ion, o he da a). A maximum Na+ o
0.860 is achie ed o he 20/80 GPE. In compa ison, 80/20 and
50/50 GPEs p esen lowe Na+ ans e ence numbe alues o
0.324 and 0.637, espec i ely. In any case, Na+ emains well-
abo e he 0.066 obse ed o he comme cial polyole in sepa-
a o soaked in liquid elec oly e. The ans e ence numbe
inc eases wi h CNF concen a ion, and i ep esen s one o he
la ges epo ed so a o GPEs applied in sodium ion ba e ies
(0.3 o poly( inylidene di luo ide-co-hexa luo op opylene),[63]
0.48 o poly( inylidene luo ide-co-hexa luo op opylene)/
poly(me hyl me hac yla e)/β-alumina,[64] 0.24 o poly(e hylene
glycol) me hyl e he me hac yla e/glass ibe ,[65] o 0.39 o
poly(e hylene oxide),[66] see Table S3, Suppo ing In o ma ion),
indica ing ha Na+ mo ion h ough he CNC/CNF gel elec o-
ly e is acili a ed by he Na+ dissocia ion p o ided by TEMPO-
modi ied CNF su aces.
The elec ochemical s abili y o he elec oly e is a key
pa ame e o he inal con igu a ion o he ba e y as he
Small 2022, 18, 2107183
Figu e 3. a) Nyquis impedance plo o he GPEs. Elec ochemical s abili y window o GPEs ob ained by linea sweep ol amme y om 0.3 o 5.0V wi h
a s ainless-s eel wo king elec ode and me allic Na as coun e elec ode a a scan a e o 1 mV s−1 wi h b) anodic scan and c) ca hodic scan. d) Schema ic
illus a ion o he p oposed Na+ conduc ion mechanism ac oss he nanocellulose GPE. e) Compa ison o he 50/50 CNC/CNF composi ion wi h he
s a e-o - he-a GPEs applied in NIBs. Fu he de ails including he co esponding e e ences a e gi en in Table S3 (Suppo ing In o ma ion).
16136829, 2022, 43, Downloaded om h ps://onlinelib a y.wiley.com/doi/10.1002/smll.202107183 by Uni e sidad del Pais Vasco, Wiley Online Lib a y on [10/05/2023]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License
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elec ochemical s abili y window (maximum ope a ing ange)
o he elec oly e de e mines he choice o he ca hode. Acco d-
ingly, ol amme ic measu emen s in he po en ial ange o
0.3–5V e sus Na/Na+ we e pe o med. As shown in Figu e3b,
du ing he anodic scan all GPEs exhibi a ise in he oxida i e
cu en a a ound 4.7V e sus Na/Na+ ha co esponds o he
oxida i e decomposi ion o he elec oly es (simila o he s a-
bili y o he ba e o ganic liquid elec oly e).[5] Such high anodic
s abili y can be asc ibed o he good apping abili y o he
liquid elec oly e by he hie a chical nanocellulosic s uc u e.
In he ca hodic scan shown in Figu e 3c, he GPEs display a
educ i e decomposi ion cu en a a ound 0.3 V e sus Na/
Na+. The 50/50 GPE shows he bes ca hodic s abili y, ou pe -
o ming o he GPEs based on he moplas ics.[67] Such la ge
elec ochemical s abili y enables he implemen a ion o ca h-
odes wi h highe ol ages han he well-known Na3V2(PO4)3
wi h a wo king ol age window o 2.8–3.8 V e sus Na/Na+.
When combined wi h Na2Fe2(SO4)3, an alluaudi e- ype ca hode
ha has an excep ionally high ol age pla o m o 3.8V hanks
o he highly elec onega i e [SO4]2– as well as he unique
Fe2O10 dime geome y,[68] ene gy densi ies o o e 400 Wh kg−1
can be eached.[41] As schema ically shown in Figu e 3d, he
inc eased Na+ conduc i i y and Na+ alues o he nanocellulose
GPE a e achie ed hanks o he syne gis ic e ec s o CNCs and
CNFs, which combine OH, OSO3–, and COO– g oups. As
a esul , an imp o ed dissocia ion o he sal in o he mobile
Na+ is achie ed, which can apidly mo e h ough he GPE in
he p esence o he o ganic liquid elec oly e. As summa ized
in Figu e3e, he 50/50 nanocellulose gel elec oly e de eloped
he e o e s a good balance be ween ionic conduc i i y, elec o-
chemical s abili y and ans e ence numbe , making his ma e-
ial sui able o p ac ical implemen a ion.
Solid polyme elec oly es based on cellulose wi h a Li+ con-
duc i i y o 1.5 mS cm–1, a ans e ence numbe o 0.78 and
elec ochemical s abili y window up o 4.5V e sus Li/Li+ ha e
been ecen ly epo ed.[69] Cu2+ was coo dina ed wi h CNFs o
open he molecula channels and enla ge he spacing be ween
he polyme chains o enable Li+ inse ion and apid anspo .
Al hough solid-s a e elec oly es a oid undesi ed liquid elec o-
ly e leakage issues, he epo ed uni e sal app oach (i could be
ex ended o o he polyme s and ca ions) elies on he modi ica-
ion o he c ys alline s uc u e o he ma e ial, which can com-
p omise he mechanical p ope ies o he sepa a o -elec oly e
pai . In compa ison, ou app oach chemically c oss-links CNCs
and CNFs in o highly po ous s uc u es o p o ide e icien Na+
conduc ing pa hways.
2.3. Elec ochemical Pe o mance o GPEs
Thanks o he hie a chical s uc u e, la ge ionic conduc i i y,
Na+ ans e ence numbe and b oad elec ochemical s abili y
window, nanocellulose gel elec oly es ul ill all he physico-
elec ochemical equi emen s o unc ion as e icien NIB elec-
oly e. The nex s ep owa d high-pe o ming and sa e NIBs
is o ensu e an e icien long- e m Na+ deposi ion. The e o e,
sodium elec odeposi ion was assessed in symme ic Na/Na
cells by pe o ming successi e pla ing and s ipping cycles a
cu en densi ies a ying om ±50 o 500 µA cm–2. Each Na
pla ing o s ipping p ocess was main ained o 1 h. As shown
in Figu e 4a, when cycled a 50 µA cm−2 ( he posi i e and nega-
i e po en ials co espond o Na pla ing and s ipping, espec-
i ely), he 50/50 GPE shows an inc easing o e po en ial which
s abilizes a nea ly 47mV. The CNC/CNF 20/80 and 80/20 com-
posi ions show la ge o e po en ials o 190 and 75mV, espec-
i ely (a 50 µA cm−2, Figu e 4b,c), indica ing ha he 50/50
o mula ion ensu es an imp o ed Na ion ans e ence e e s-
ibili y and homogeneous Na elec odeposi ion be ween elec-
odes. Mo eo e , he cu es in Figu e4d show o e po en ials
o ≈316mV o he Celga d memb ane soaked in 1 m NaClO4 in
EC/PC (50/50 / ) liquid elec oly e. As compa ed in Figu e4e,
he nanocellulose gel elec oly e enables a i e old o e po en ial
dec ease wi h he squa e-wa e shape o he pola iza ion cu e,
indica ing a smoo h Na deposi ion wi h low o uosi y.[53]
The sudden ol age d op obse ed a e 40, 100, and 242 h
(highligh ed by a yellow ligh ning symbol) o 20/80, 80/20,
and Celga d (cu en densi ies o 50, 200, and 200 µA cm−2,
espec i ely) is ela ed o he p ema u e cell ailu e by a den-
d i e-induced sho -ci cui . Con e sely, no ol age d op is seen
o 50/50 un il he cu en densi y inc eases up o 500 µA cm−2.
Sho -ci cui occu ed only a e 329 h o Na pla ing/s ipping,
demons a ing ha he 50/50 GPE can signi ican ly smoo hen
and homogenize Na+ elec odeposi ion while supp essing den-
d i e o ma ion. Pos -mo em SEM images o he 80/20 and
20/80 GPEs ob ained a e symme ic cycling in Na/Na cells
in Figu e S6 (Suppo ing In o ma ion) con i m he p esence o
mic oscale high-su ace a ea, non-uni o m Na deposi s indica-
i e o inhomogeneous sodium elec odeposi ion. In compa -
ison, he SEM images o he cycled 50/50 GPE display highly
uni o m Na deposi s, which a ise as a esul o he homoge-
nous cu en dis ibu ion ac oss he elec ode/elec oly e in e -
ace (mani es ed as a s able pola iza ion ol age in Figu e4a).
Ene gy-dispe si e X- ay spec oscopy analysis u he con i ms
he p esence o a hick laye composed o Na, Cl and O on he
20/80 and 80/20 GPEs. As summa ized in Figu e4 , his pe -
o mance ep esen s a signi ican imp o emen o e he Na
pla ing and s ipping esul s epo ed so a o symme ic Na/
Na cells wi h sepa a o s soaked in a liquid elec oly e ( u he
de ails in Table S5, Suppo ing In o ma ion). The 50/50 GPE
de eloped he e shows p omising esul s, because low o e po-
en ials (gua an eeing an e icien u iliza ion o he ac i e ma e-
ials) wi h long ope a ion li e spans a e desi ed. We asc ibe
hese esul s o:
• he combina ion o mechanical duc ili y p o ided by CNFs,
which esul s in a s able in e ace wi hou delamina ion due
o he hyd oxyl g oups in ol ed,[70] and inhe en mechanical
s i ness o CNCs, which o e s esis ance o dend i e g ow h;
• he mesopo ous s uc u e boos s a s able and uni o m ion
ans e ence ac oss he elec oly e (supp essing dend i e
g ow h),[33] while he unc ional COO g oups on he su -
ace o CNFs s ongly in e ac wi h he ions in he elec oly e,
acili a ing ion pai dissocia ion, hus imp o ing he Na ion
anspo ( Na+ up o 0.86); and
• he adhesi e cha ac e o nanocellulosic ma e ials as ecen ly
p o ed by Ta dy e al.,[71] who showed he po en ial o CNCs
o o m supe s uc u es ha sel -assemble ac oss mul i-
ple leng h scales o p o ide adhesion h ough non-co alen
Small 2022, 18, 2107183
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in e ac ions. This abili y, also ound in o he na u al biopoly-
me s such as chi in nanoc ys als o amyloids,[72] enables ioni-
cally conduc ing gels o success ully adhe e on o me allic Na
su aces ia sup amolecula in e ac ions.
Ano he ele an ea u e o he designed nanocellulose
gel elec oly e is ha i does no con ain luo oe hylene ca -
bona e, an elec oly e addi i e ha is known o o m a obus
and s able solid–elec oly e in e phase by p e en ing elec o-
ly e b eakdown and consump ion (which in u n is ansla ed
in o mo e s able me al elec odeposi ion).[73,74] As luo oe h-
ylene ca bona e p esen s ad e se en i onmen al and human
heal h e ec s, limi ing i s use and hus educing po en ial
human and en i onmen al exposu e o haza dous chemi-
cals should be a p io i y in es ablishing a mo e sus ainable
ba e y indus y.[75,76] Al hough ou elec oly e s ill con ains
o ganic sol en s wi h some oxici y and he e is a s ong
need o eplace all elec oly e chemicals wi h en i onmen ally
iendly al e na i es in he longe e m, his wo k ep esen s
an impo an s ep owa ds g eene elec oly es wi h compe i-
i e elec ochemical pe o mance, which could be po en ially
applied o ansien o deg adable ba e ies.[77]
2.4. Ba e y Pe o mance
As he CNC/CNF 50/50 GPE o e s he mos a ac i e balance
be ween mechanical, s uc u al and elec ochemical p ope ies,
we e alua ed i s po en ial as a NIB elec oly e. Figu e 5a shows
he gal anos a ic cha ge–discha ge cu es (1.9–4.3V s Na/Na+)
a 1C a e (120 mA g–1) o his GPE in a Na2Fe2(SO4)3/Na hal -
cell. The alluaudi e- ype Na2Fe2(SO4)3 ca hode, i s ly epo ed
in 2014,[78] was selec ed because i p o ides an en i onmen ally
sus ainable al e na i e o he commonly s udied Na3V2(PO4)3
ca hode, which equi es anadium, a geno oxic ma e ial ha
can cause se ious oxici y issues o human and animal li e.[79]
In addi ion, all he elemen s (Na, Fe, and S) a e ea h-abundan
(making i economically a ac i e), and i s excep ionally high-
wo king ol age can deli e no ably high ene gy densi ies.[41] An
ini ial discha ge capaci y o 91.4 mAh g–1 is achie ed, and he ini-
ial Coulombic e iciency o 97.2% inc eased o 99.1% a e i e
cycles. A e 50 cycles, he capaci y dec eased o 69.7 mAh g–1,
co esponding o 76.3% capaci y e en ion. In compa ison, he
Celga d sepa a o (2325, i-laye polyp opylene-polye hylene-
polyp opylene) p esen s a poo e elec oly e a ini y and a low
po osi y o 39%. These ea u es a e ansla ed in o long pa hways
Small 2022, 18, 2107183
Figu e 4. Room empe a u e ol age e sus ime cu es o a symme ic Na/Na cell o Na pla ing/s ipping a cu en densi ies anging om ±50 o
500 µA cm−2 and wi h a discha ge/cha ge ime o 1 h o : a) 50/50, b) 20/80, c) 80/20 GPEs, and d) Celga d memb ane soaked in a 1 m NaClO4 in EC/
PC (50/50 / ) liquid elec oly e. The yellow ligh ning signals he occu ence o sho -ci cui . e) Magni ied iew o he ol age e sus ime cu es in
he 190–200 h ange o CNC/CNF 50/50 GPE (blue line) and Celga d (black line). ) Compa ison wi h he s a e-o - he-a sepa a o –liquid elec oly e
pai s. Fu he de ails including he co esponding e e ences a e gi en in Table S5 (Suppo ing In o ma ion).
16136829, 2022, 43, Downloaded om h ps://onlinelib a y.wiley.com/doi/10.1002/smll.202107183 by Uni e sidad del Pais Vasco, Wiley Online Lib a y on [10/05/2023]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License
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o ions o c oss he sepa a o .[80] Acco dingly, Na2Fe2(SO4)3/Na
hal -cells wi h Celga d exhibi la ge pola iza ion e ec s be ween
cha ge and discha ge cycles, which a ec s he elec ochemical
pe o mance a ela i ely as cha ge–discha ge a es. As a
esul , Celga d soaked in 1 m NaClO4 in EC/PC (50/50 / ) can
only deli e 0.6 mAh g–1 a C/4.
The e e sible capaci y p o ided by he nanocellulose gel
elec oly e in combina ion wi h he high wo king ol age o
he ba e y esul in a g a ime ic ene gy densi y (based on he
ac i e mass o he ca hode) o 240 Wh kg–1 a a a e o 1C (cal-
cula ed by in eg a ing he second cycle in he capaci y- ol age
cu e), making his sys em compe i i e agains cu en LIBs.
Addi ionally, he cha ge/discha ge cu es o he ba e y wi h he
50/50 GPE p esen s a lowe elec ochemical pola iza ion ( he
di e ence be ween cha ge and discha ge pla eaus) as compa ed
o ha obse ed o Celga d, sugges ing ha he nanocellulose
gel elec oly e p omo es Na+ di usion be ween he Na and
Na2Fe2(SO4)3. The shape o he ol age p o ile ba ely changes,
indica ing ha he c ys al s uc u e o he ca hode ma e ial
emains unchanged. I is impo an o no e ha he nanocellu-
lose gel elec oly e he e de eloped su passes he speci ic capaci-
ies p o ided by o he sepa a o -liquid elec oly e sys ems wi h
Na3V2(PO4)3 as ca hode, such as he 91 mAh g–1 a 24 mA g–1
o polyp opylene,[81] he 101 mAh g–1 a 24 mA g–1 p o ided by
polysul onamide,[81] he 70 mAh g–1 a 12 mA g–1 p o ided by a
chi in nano ibe memb ane,[60] o he 116 mAh g–1 a 12 mA g–1
gi en by an aga ose-based sepa a o .[34] Ye , he de eloped nano-
cellulose gel elec oly es s ill ha e oom o imp o e he elec o-
chemical pe o mance by educing hei hickness o 25–30µm.
This would educe he o e all esis ance o he cell,[82] al hough
he limi ed esis ance o hin GPEs agains dend i e punc u e
may p esen s issues ega ding ba e y du abili y and sa e y.
The in e connec ed po ous nanocellulose s uc u es, he
c yogels as well as he gel elec oly es a e capable o holding
la ge amoun s o liquid. This ea u e, oge he wi h he non-
oxic and biodeg adable cha ac e o nanocellulose, enables he
de elopmen o ionically conduc ing gels ha p o ide speci ic
mechanical and chemical condi ions o mimic he ex acel-
lula ma ix.[83,84] As a esul , hey can be applied as 3D mic o-
en i onmen s o cell cul u e,[85] as biocompa ible inks o 3D
biop in ing,[86] wound d essing,[87] o o biosensing pu poses
in diagnos ics and bioimaging.[88] Mo eo e , he ease o nano-
cellulose unc ionaliza ion can be exploi ed o con ol biological
in e ac ions, enabling hei implemen a ion as on-demand
d ug deli e y sys ems ha a e able o elease loaded d ugs
unde ex e nal s imuli such as pH changes o nea -in a ed
exposu e.[89]
3. Conclusions
The de elopmen o sa e, g een, and elec ochemically e icien
ba e ies is an u gen ask owa d elec i ica ion and imple-
men a ion o he decoupling and deca boniza ion agenda. In
his con ex , he e we show a s a egy o de elop en i onmen-
ally sus ainable, economically a ac i e, and elec ochemically
ele an sodium ion ba e ies. Biomass esou ces a e used o
he i s ime o syn hesize a biopolyme -based gel elec oly e
o e ing a good balance be ween he o en mu ually exclusi e
enewabili y and elec ochemical pe o mance. A gel elec o-
ly e wi h a mesopo ous s uc u e is achie ed a e c osslinking
mechanically s i cellulose nanoc ys als and lexible cellulose
nano ibe s wi h glu a aldehyde. The COONa su ace g oups
p esen on cellulose nano ibe s inc ease he amoun o mobile
Na+, boos ing coun e ion mobili y and o e ing ionic conduc-
i i ies up o 2.32 mS cm–1 and a Na+ ans e ence numbe o
0.637. The hie a chical po osi y and e icien Na+ ans e ence
enable a homogeneous and s able Na pla ing/s ipping wi h
educed o e po en ial. These cha ac e is ics a e ansla ed in o
a dend i e- ee Na elec odeposi ion capable o ope a ing a cu -
en densi ies up o 500 µA cm−2. The anodic s abili y o 4.7V
e sus Na/Na+ makes nanocellulose gel elec oly es adequa e
o high-ene gy ca hode con igu a ions, including beyond
li hium ion chemis ies. When assembled in Na2Fe2(SO4)3/Na
hal -cells, nanocellulose gel elec oly es p esen a good capaci y
e en ion wi h a g a ime ic ene gy densi y (based on ca hode
mass) o 240 Wh kg–1 a a a e o 1C, su passing he elec o-
chemical pe o mance o he con en ional sepa a o -elec oly e
pai s elying on ossil esou ces.
4. Expe imen al Sec ion
Ma e ials: Mic oc ys alline cellulose wi h a pa icle size o 20 µm
(310697-500G), sul u ic acid (H2SO4, 95%–97%), glu a aldehyde, sodium
pe chlo a e (NaClO4), EC, and PC we e ob ained om Sigma-Ald ich.
I on (II) sul a e hep ahyd a e (FeSO4·7H2O) and sodium sul a e
Small 2022, 18, 2107183
Figu e 5. a) Gal anos a ic cha ge–discha ge p o iles a 1C (1C = 120 mA g−1) in Na2Fe2(SO4)3/Na hal -cell con igu a ion o he CNC/CNF 50/50 GPE
oge he wi h b) i s co esponding e olu ion o he discha ge capaci y and Coulombic e iciency. Gal anos a ic cha ge–discha ge p o iles a C/4 o
Celga d sepa a o soaked in 1 m NaClO4 in EC/PC (50/50 / ) (in magen a) a e p o ided o compa ison. The inse in (a) magni ies he speci ic capaci y
egion o Celga d (up o 2 mAh g−1).
16136829, 2022, 43, Downloaded om h ps://onlinelib a y.wiley.com/doi/10.1002/smll.202107183 by Uni e sidad del Pais Vasco, Wiley Online Lib a y on [10/05/2023]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License
