Lignin−Chi osan Gel Polyme Elec oly es o S able Zn
Elec odeposi ion
Na oa Almena a, Robin Gue e , Albe o J. Hue as-Alonso, Unnimaya Thalakkale Vee il,
Mika H. Sipponen,*and E lan z Lizundia*
Ci e This: ACS Sus ainable Chem. Eng. 2023, 11, 2283−2294
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ABSTRACT: Elec ochemical ene gy s o age echnologies o e
means o ansi ion owa d a deca bonized socie y and ca bon
neu ali y by 2050. Compa ed o con en ional li hium-ion
ba e ies, aqueous zinc-ion chemis ies do no equi e sca ce
ma e ials o oxic and lammable o ganic-based elec oly es o
unc ion, making hem a o able con ende s in he scena io o
in ensi ying clima e change and supply chain c isis. Howe e ,
en i onmen ally benign and bio-based ma e ials a e needed o
subs i u e ossil-based ba e y ma e ials. Acco dingly, his wo k
aps in o he possibili ies o lignin oge he wi h chi osan o o m
gel polyme elec oly es (GPEs) o zinc-ion chemis ies. A simple ab ica ion p ocess enabling ee-s anding sodium lignosul ona e−
chi osan and micella lignosul ona e−k a lignin−chi osan GPEs wi h diame e s exceeding 80 mm is de eloped. The GPEs combine
ensile s eng h wi h duc ili y, eaching Young’s moduli o 55 ±4 o 940 ±63 MPa and elonga ions a b eak o 14.1 ±0.2 o 43.9 ±
21.1%. Compe i i e ionic conduc i i ies anging om 3.8 o 18.6 mS cm−1and elec ochemical s abili y windows o up o +2.2 V s
Zn2+/Zn we e obse ed. Gi en he imp o ed in e acial adhesion o he GPEs wi h me allic Zn p omo ed by he anionic g oups o
he lignosul ona e, a s able cycling o he Zn anode is ob ained. As a esul , GPEs can ope a e a 5000 μA cm−2wi h no sho -ci cui
and Coulombic e iciencies abo e 99.7%, ou pe o ming con en ional sepa a o −liquid elec oly e con igu a ions such as he glass
mic o ibe sepa a o soaked in o 2 M ZnSO4aqueous elec oly e, which sho -ci cui s a e 100 μA cm−2. This wo k demons a es
he po en ial o unde u ilized bio e ine y side-s eams and ma ine was e as elec oly es in he ba e y ield, opening new al e na i es
in he sus ainable ene gy s o age landscape beyond LIBs.
KEYWORDS: lignin, chi osan, bioeconomy, ci cula economy, gel polyme elec oly e, zinc-ion ba e y (ZIB), zinc pla ing/s ipping
■INTRODUCTION
Ou socie y aces se ious global challenges associa ed wi h he
deple ion o ini e non- enewable esou ces, en i onmen al
pollu ion, and clima e c isis. The cu en linea economy,
ollowing a “ ake−make−dispose” app oach, equi es ex ensi e
amoun s o aw ma e ials ha a e hen p ocessed in o goods
and once used a e inally disca ded as a non-biodeg adable
was e. The ex ac ion and p ocessing o non- enewable
ma e ials such as me als o pe oleum-based polyme s is
associa ed wi h no able en i onmen al cos s, including la ge
ca bon dioxide oo p in s, ai and d inking wa e pollu ion,
biodi e si y loss, o eu ophica ion, among o he s.
1
The e o e,
ambi ious ini ia i es ha e been se o ansi ion owa d a
comple ely ci cula economy by 2050.
2
In a ci cula economy,
he ma e ials and p oduc s a e used o as long as possible,
which, coupled wi h lowe a es o ex ac ion and exploi a ion
o enewable na u al esou ces, educes esou ce deple ion and
a oids uncon olled was e accumula ion du ing he end-o -
li e.
3
The de elopmen o e icien enewable ene gy con e sion
and s o age echnologies is a p essing need o each sus ainable
p oduc ion and consump ion pa e ns ha egene a e na u al
sys ems.
4,5
The ene gy ansi ion is pi o al owa d clima e
change mi iga ion as i may shi om ossil-ene gy p oduc ion
o enewable-ene gy sou ces.
6
Elec ochemical ene gy s o age
sys ems a e pa icula ly ele an o enewable-ene gy ex-
ploi a ion gi en hei abili y o s o e and deli e on-demand
powe .
7
So a , echa geable li hium ion ba e ies (LIBs) ha e
been he p edominan solu ion gi en hei ela i ely high
ene gy-densi ies, low sel -discha ge a es, and long ope a ion
li espans.
8
Un o una ely, cu en LIBs ely on sca ce,
expensi e, and o en ha m ul ma e ials such as li hium, cobal ,
manganese, o nickel o unc ion. In addi ion, lammable
o ganic-based elec oly es a e o en equi ed, inc easing he
isk o undesi ed i es o explosions. As a esul , hei
Recei ed: Sep embe 29, 2022
Re ised: Janua y 16, 2023
Published: Janua y 30, 2023
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ab ica ion, use, and end-o -li e managemen poses a numbe o
challenges om an en i onmen al poin o iew, especially in
he long un.
9,10
Seeking o al e na i e ba e y chemis ies ha o e
accep able elec ochemical pe o mance, good sa e y, and use
o ea h-abundan and en i onmen ally iendly ma e ials is
becoming inc easingly u gen . Recha geable zinc-ion chem-
is ies ul ill hese s ingen equi emen s as hey ely on zinc
ions (Zn2+) as cha ge ca ie s.
11
Con en ional zinc-ion
ba e ies bene i om he la ge capaci y o Zn (820 mAh
g−1).
12
These ba e ies a e composed by a Zn me al nega i e
elec ode (anode), a posi i e elec ode (ca hode) ha enables
he e e sible Zn2+ (de)inse ion, and a glass mic o ibe
sepa a o soaked in o a liquid elec oly e.
13
Impo an ly, hey
do no equi e d y a mosphe ic assembly condi ions as zinc-ion
chemis ies could ope a e wi h aqueous-based elec oly es,
which u he enhances hei sa e y and en i onmen al
sus ainabili y o e LIBs and o he mul i alen sys ems (Mg,
Ca, and Al).
11
The eby, aqueous zinc-ion chemis ies p esen
signi ican ad an ages o s a iona y ene gy s o age.
14
The sepa a o −elec oly e pai is a ele an ba e y
componen because i de e mines o a la ge ex en i s
elec ochemical pe o mance (ene gy densi y o cycle s abili y)
and sa e y ( he mal s abili y o esis ance agains dend i e
punc u e).
15
This componen mus ensu e adequa e ion
ans e ence be ween elec odes a he same ime ha
elec onically and physically insula es he anode and he
ca hode so ha in e nal sho ci cui s and e en ual igni ion o
explosion isks a e a oided.
16
The subs i u ion o he po ous
non- enewable sepa a o soaked in o an aqueous elec oly e by
a gel polyme elec oly e (GPE) is now pu sued by bo h
academia and indus y o enhance ba e y sa e y by a oiding
elec oly e leakage.
17
GPEs ypically show an inc eased
compa ibili y wi h elec odes gi en hei mechanical lexibili y,
su passing liquid elec oly e designs in e ms o in e acial
compa ibili y and ope a ing li e ime.
18,19
In addi ion, GPEs
o e inc eased oom- empe a u e ionic conduc i i ies in
compa ison wi h solid-s a e Zn elec oly es, which usually
p esen conduc i i ies below 10−4mS cm−1due o he high
cha ge densi y o Zn2+.
20
Usually, GPEs a e ob ained om
pe ochemical sou ces, wi h polye hylene oxide, poly inylidene
luo ide and i s copolyme s, o polyac yloni ile being he mos
widely ound sys ems.
19
In con as , i he de elopmen o
GPEs is accompanied by he use o enewable sou ces,
addi ional en i onmen al bene i s may be ob ained. In e es -
ingly, ecen wo ks ha e p o en he sui abili y o biopolyme -
de i ed GPEs o achie e highly e e sible and s able me al
deposi ion in ba e ies.
18,21
Ano he ad an age o many na u al
polyme s is hei hyd ophilic na u e ha acili a es ion
exchange ac oss he GPE.
As he ba e ies a e becoming common in inc easing numbe
o e e yday applica ions, he e is a need o la ge scale biomass
esou ces o GPEs. Unde u ilized bio e ine y was es o e
plen y o ma e ials wi h abundan unc ional g oups and
ailo ed p ope ies o be exploi ed as ba e y elec oly es. So
a , mos o he e o s o de elop Zn2+ conduc ing ma e ials
ha e been di ec ed owa d he use o celluloses,
22
aga ose,
23
o
chi osan.
24
Among he bio-based ma e ials no ye ully
exploi ed, lignin is pa icula ly a ac i e gi en i s a oma ic
s uc u e and abundan p oduc ion as a by-p oduc om
indus ial biomass p ocessing.
25
Cu en ly, ∼98% o lignin
p oduc ion ( om he es ima ed 80 million on yea −1) is
combus ed o ene gy eco e y pu poses, which clea ly collides
wi h ci cula economy p inciples.
26
This esou ce was e
ep esen s a missed oppo uni y, pa icula ly when conside ing
ecen li e cycle assessmen (LCA) s udies highligh ing ha
when p ocessed in o ma e ials and (nano) ille s,
27
lignin can
lowe he en i onmen al impac s (including CO2 oo p in )
o e hei pe o-based coun e pa s. The e o e, he alo iza ion
o lignin in o high- alue-added p oduc s will posi i ely
con ibu e o economics and enhance ca bon e iciency o
bio e ine ies.
28
Among lignin-de i ed ma e ials, sodium
lignosul ona e (LS) and k a lignin (KL), p oduc s a ising
om he sul i e and sul a e pulping p ocess show in e es ing
p ope ies o GPEs. In addi ion o hei phenolic hyd oxyl and
sul onic acid g oups, he a oma ic g oups and e he bonds o
lignosul ona es can in e ac wi h di e en me al ions h ough
ca ion−πin e ac ions o enhance ion conduc ion,
29
yielding
GPEs wi h la ge ionic conduc i i ies. In addi ion, he adhesi e
p ope ies o lignin
30
can be exploi ed o ob ain ma e ials ha
adhe e on o me allic su aces, enhancing he s abili y o he
elec oly e−elec ode in e ace and enla ging he li espan o a
ba e y. Sul ona ion and subsequen chlo ina ion o pine acid
hyd olysis lignin and i s combina ion wi h poly( inyl alcohol)
has been epo ed o p oduce GPEs wi h an ionic conduc i i y
o 0.25 mS cm−1.
31
Howe e , he po en ial o sodium
lignosul ona e, which is he dominan echnical lignin based
on oday’s p oduc ion quan i ies, in combina ion o na u al
polysaccha ides has emained unexplo ed in GPEs.
Acco dingly, he e, we epo a simple ye e ec i e scalable
ab ica ion o ee-s anding GPEs consis ing o chi osan and
lignosul ona e o micella dispe sion o lignosul ona e wi h
so wood k a lignin, each o he o mula ions combining
enewabili y, low cos , he momechanical esis ance, ionic
conduc i i y, and elec ochemical s abili y. The gel cha ac e o
he biopolyme -elec oly e enhances ba e y sa e y and
en i onmen al sus ainabili y o e con en ional designs based
on mic opo ous ossil-based sepa a o s soaked in liquid
elec oly es. Impo an ly, he p esence o ee wa e inhe en
o glass mic o ibe s in he 2 M ZnSO4aqueous elec oly e
sys em is a oided, supp essing side eac ions on o Zn su aces
and hus ob aining longe ope a ion li espans in symme ic
Zn/Zn cells.
32
Ob ained esul s ep esen a s ep o wa d in he
de elopmen o ba e ies ha use upcycled bio e ine y was e
s eams.
■EXPERIMENTAL SECTION
Ma e ials. So wood k a lignin (KL, BioPi a 100 pine k a lignin
(UPM, Finland), lignosul ona e (LS, DS10, Domsjo, Sweden).
Chi osan (50,000−190,000 Da), glu a aldehyde (50 w % in H2O),
and zinc sul a e monohyd a e (ZnSO4·H2O) ha e been pu chased
om Sigma-Ald ich. The ca bon pape (TP-060-T5) was pu chased
om QuinTech. Elec oly es we e p epa ed using ul apu e wa e
(18.2 MΩcm). All chemicals we e used as ecei ed wi hou any
u he pu i ica ion. Fo elec ochemical s udies, glass mic o ibe
sepa a o s (GF/A, Wha man) we e employed as ecei ed. Zn oil
(0.25 mm hickness) was ob ained om The mo Fishe Scien i ic.
Fab ica ion o Gel Polyme Elec oly es. As summa ized in
Figu e 1, lignosul ona e−chi osan gel polyme elec oly es we e
syn hesized using LS o a micella dispe sion o lignosul ona e−
k a lignin (LSKL).
33
Fou samples o wo di e en composi ions o
LSKL−chi osan and LS−chi osan we e p epa ed, ob aining a lignin
con en o 10 and 30 w % in he inal memb ane. Chi osan was added
o a minimum o 70 w % o imp o e he ilm- o ming abili y while
ensu ing adequa e elec ochemical p ope ies (lowe chi osan
concen a ions ail o o m ee-s anding memb anes). Table S1 in
he Suppo ing In o ma iongi es addi ional de ails on each sample
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composi ion along wi h he nomencla u e. Glu a aldehyde was used
as a chemical c oss-linke o enhance he physical in eg i y o he gels.
P epa a ion o Lignosul ona e−K a Lignin−Chi osan Mem-
b anes. The chi osan solu ion (1 w % in 1 w % o ace ic acid) was
added o a ial con aining a p ede e mined amoun o KL-LS gel
dispe sion (43 w %). The mix u e was s i ed o 3 days un il
homogeniza ion was a ained. Then, o he c oss-linked samples, 50
w % glu a aldehyde was added o he homogenized solu ion and he
mix u e was s i ed a 500 pm o 30 min. A e ha , he samples
we e cas ed on polyp opylene Pe i dishes (⌀= 80 mm) and allowed
o c oss-link o 3 days a oom empe a u e in he ume hood. GPEs
o 250 ±70 μm hickness we e ob ained a e imme sing he ilms
in o a 2 M ZnSO4aqueous solu ion o e nigh .
P epa a ion o he Lignosul ona e−Chi osan Samples. Lignosul-
ona e solu ion (1 w %) was d opwise added o a chi osan 1 w %
solu ion in 1 w % o ace ic acid unde magne ic s i ing using a
sy inge pump a a low a e o 15 mL h−1. The mix u e was s i ed o
3 days un il homogeniza ion was a ained. Then, o he c oss-linked
samples, 50 w % glu a aldehyde was added o he homogenized
solu ion and he mix u e was s i ed a 500 pm o 30 min. A e ha ,
he samples we e cas ed on polyp opylene Pe i dishes (⌀= 80 mm)
and allowed o c oss-link o 3 days a oom empe a u e. GPEs o
250 ±70 μm hickness we e ob ained a e imme sing he ilms in o a
2 M ZnSO4aqueous solu ion o e nigh .
Memb ane Cha ac e iza ion. Field-emission scanning elec on
mic oscopy (FE-SEM) analyses we e ca ied ou using a Hi achi S-
4800 a an accele a ion ol age o 5 kV. Be o e he mo phological
obse a ions o he c yo- ac u ed su aces, he samples we e
spu e ed wi h a 10 nm- hin gold−palladium laye . Powde X- ay
di ac ion (XRD) pa e ns we e ob ained wi h a D8 Disco e
di ac ome e in e lec ion mode using Cu Kα adia ion (45 kV, 40
mA). A enua ed o al e lec ance Fou ie ans o m in a ed (ATR-
FTIR) spec oscopy esul s we e ob ained using a Va ian 610-IR FT-
IR spec ome e equipped wi h diamond ATR op ics.
The he mal deg ada ion beha io o gel elec oly es was s udied by
means o he mal g a ime ic analysis (The Disco e y TGA) in
pla inum oxide pans unde ai a mosphe e a a hea ing a e o 10 °C
min−1and 50 mL min−1 o each sample (2 o 7 mg). The mechanical
beha io o samples was s udied by uniaxial ensile es s using a
uni e sal es ing machine (T apezium Shimadzu AGS-X) equipped
wi h a 100 N load cell a a de o ma ion a e o 1 mmmin−1.
Specimens 20 mm long and 6 mm wide wi h hicknesses o 250 ±70
μm we e used. The mean a e age alue and s anda d de ia ion
Young’s modulus (E) ( om 0.5−1% s ain egion), s ess and s ain
a yield (σyand εy, espec i ely), and s ess and s ain a b eak (σband
εb, espec i ely) we e de e mined om ou measu emen s.
The elec oly e up ake (EU) o lignin gel elec oly es was measu ed
a e imme sion o he LSKL−chi osan and LS−chi osan memb anes
in 0.5, 1, and 2 M ZnSO4aqueous solu ion o 24 h as
m
m mEU 100 ( )
d y
we d y
= ×
(1)
whe e mwe and md y a e he weigh o he we and d y lignin gels,
espec i ely.
Elec ochemical s udies we e ca ied ou using a VMP3 Biologic
elec ochemical wo ks a ion. Gel elec oly es (diame e = 13 mm;
a ea = 1.327 cm2) we e assembled in o Swagelok- ype cells a oom
empe a u e. Fo ionic conduc i i y measu emen s, gel elec oly es
we e sandwiched be ween wo s ainless s eel ods. Elec oly e
esis ance was measu ed using a wo p obe AC impedance
spec oscopy analyze wi h a 5 mV ol age ampli ude in he equency
ange om 1 Hz o 5 MHz. The esis ance was measu ed om he
high- equency in e cep on he eal axis in he Nyquis plo s, and he
ionic conduc i i y (σi) was ob ained acco ding o
d
R A
i
b
=
×
(2)
whe e dis he gel hickness, Rb ep esen s he bulk esis ance
ex ac ed om he in e cep o he cu e wi h he eal impedance axis
in he Nyquis plo , and Aaccoun s o he con ac a ea o he gel
elec oly e and he s ainless s eel od.
In addi ion, he elec ochemical s abili y window was s udied by
ol amme ic measu emen s, whe e gel elec oly es we e sandwiched
be ween a ca bon pape as he wo king elec ode and a Zn me al disk
as he e e ence and coun e elec ode. The ol ammog ams we e
ob ained in he po en ial ange o −0.25 V o +2.4 V s Zn2+/Zn wi h
a scan a e o 1 mV s−1using a VMP3 Biologic ins umen .
Zn s ipping and pla ing pe o mance was s udied unde di e en
cu en densi ies om ±50 o ±500 μA·m−2. To ha end,
Figu e 1. Schema ic ep esen a ion o he ab ica ion p ocess o LS−chi osan and LSKL−chi osan gel polyme elec oly es.
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lignosul ona e−chi osan and LSKL−chi osan GPEs we e moun ed
be ween wo Zn-me al discs. Fo he sake o compa ison, Zn s ipping
and pla ing was also pe o med using a glass ibe sepa a o soaked in
a 2 M ZnSO4aqueous solu ion. Fo pos -mo em SEM s udies, he
cycled Zn su aces we e washed wi h wa e be o e d ying unde
acuum.
■RESULTS AND DISCUSSION
Mo phological and S uc u al Cha ac e iza ion. A
scalable ab ica ion p ocess o GPEs is de eloped using
unde u ilized bio e ine y was es as a sou ce ma e ial while
enabling a apid and s able Zn2+ anspo . Ou e o s ha e
been ocused on he use o lignosul ona e and chi osan as he
polyme ic cons i uen s o ee-s anding memb ane o ma ion,
which a e imme sion in 2 M ZnSO4H2O yield ionically
conduc ing GPEs. P elimina y expe imen s showed he
inabili y o achie e lignin-only GPEs due o hei poo ilm-
o ming p ope ies. In his sense, chi osan was inco po a ed a
70 and 90 w % as a model polysaccha ide o imp o e he ilm-
o ming abili y.
34
The mo phology o he esul ing GPEs a e
eeze-d ying was in es iga ed by scanning elec on mic oscopy
(SEM), and he c yo- ac u ed c oss sec ions a e shown in
Figu e 2a. The ob ained dense and smoo h s uc u e wi h no
oids o c ys alline agg ega es obse ed o all he composi-
ions indica es he uni o m dissocia ion o he zinc sal wi hin
he polyme ic gel. This ea u e is pa icula ly in e es ing as i is
conside ed a p ime equisi e o achie e a homogeneous ion
anspo ac oss he GPE and o e a s able ion elec o-
deposi ion on o Zn. An inc eased amoun o lignin yields
b ownish GPEs (Figu e S1). As shown in Figu es S2 and S3,
he de eloped app oach is simple and scalable enough o esul
in homogeneous, ee-s anding, and mechanically lexible GPEs
wi h diame e s abo e 80 mm.
Amo phous elec oly es a e p e e ed as he c ys alline
egions gene ally p esen an inc eased esis ance o he
anspo o ions.
35
Lignins a e known o be amo phous, bu
chi in, he p ecu so o chi osan, can o m c ys alline
domains.
36
Acco dingly, he occu ence o c ys alline phases
has been assessed by X- ay di ac ion expe imen s. As shown
in Figu e 2b, all he GPEs p esen an amo phous halo wi h a
b oad peak cen e ed a 2θ= 20.5°o igina ing om he o e lap
o he (110) c ys al plane o chi osan (2θ=∼19.2°),
37
oge he wi h he b oad band a 2θ= 22°o s acked lignin
a oma ic laye s in o he (002) plane (see he di ac ion
pa e ns o he biopolyme GPE componen s in Figu e S4).
38
As opposed o ba e ZnSO4sal , which o ms a c ys alline
s uc u e wi h well-di e en ia ed and na ow di ac ion peaks
(Figu e S4),
39
he GPEs he e syn hesized p esen low in ensi y
sha p peaks, indica ing he coexis ence o la ge amo phous
egions wi h ew c ys alline phases. O e all, he p edominan
amo phous halo o all he GPEs sugges s he abili y o lignin−
chi osan blends o complexa ion and dissolu ion o ZnSO4,
enabling a low ba ie o ion di usion o he mig a ion o
Zn2+ h ough hese amo phous phases (no e, on he con a y,
he highly c ys alline cha ac e o he ZnSO4sal in Figu e
S4).
40
A enua ed o al e lec ance−Fou ie ans o m in a ed
(ATR-FTIR) spec a in Figu e 2c p esen he cha ac e is ic
abso p ion bands o lignin, wi h a band a 1000 cm−1assigned
o C−O de o ma ion and a oma ic C−H in plane de o ma ion,
1550 and 1650 cm−1(a oma ic skele al ib a ions and C�O
s e ch), 2800−3000 cm−1C−H s e ch in me hyl g oups in
LS and me hylene g oups o LS and chi osan, and a b oad
band a 3100−3500 cm−1assigned o hyd oxyl g oups in
chi osan and lignin.
41
The mal and Mechanical P ope ies. The de elopmen
o GPEs wi h accep able he mal s abili ies is an essen ial
p e equisi e owa d ba e y sa e y so ha he likelihood o
he mal unaway is educed. The mog a ime ic analysis
(TGA) is commonly used o assess he he mal s abili y o
ba e y sepa a o s. The TGA cu es unde ai a mosphe e
Figu e 2. (a) Rep esen a i e SEM mic og aphs showing he c oss sec ions o he GPEs wi h a ied composi ions. (b) XRD pa e ns and (c) ATR-
FTIR spec a o syn hesized GPEs. The scale ba size o SEM images a ies due o he sligh di e ences in ilm hickness.
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shown in Figu e 3a o LSKL−chi osan and LS−chi osan
memb anes (be o e ZnSO4soaking and no Zn sal s) show an
ini ial weigh loss o 10−15 w % a 75 °C, esul ing om he
adso bed mois u e e apo a ion. A ma ked he modeg ada ion
e en occu s in he empe a u e ange o 245−400 °C. This
weigh loss a ises om py olysis o he biopolyme in ol ing
agmen a ion o he in e -uni linkages be ween phenolic and
ca bonyl g oups and he elease o monome ic phenols in
lignin
30
as well as he deace yla ion and clea age o glycosidic
linkages in chi osan.
42
Finally, a decomposi ion o he esidual
ca bon occu s. In addi ion, Figu e 3b shows he he mode-
g ada ion aces o he GPEs, which a e ob ained a e soaking
he memb anes in o 2 M ZnSO4H2O. The mass loss e en a
nea ly 200 °C a ises om he wa e loss esul ing om ZnSO4
sal c ys alliza ion.
43
In e es ingly, ZnSO4exe s a e a ding
e ec on mass loss o all he s udied o mula ions, pa icula ly
du ing he second and hi d deg ada ion s ages a empe a u es
abo e ∼300 °C. In pa icula , he GPEs each hei 50% weigh
loss a 390−438 °C (depending on he composi ion) in
compa ison wi h he 311−408 °C obse ed o he memb anes
wi hou he elec oly e. Fo he sake o compa ison, as
highligh ed by he dashed line, he mic opo ous polyole in
Celga d 2325 sepa a o (composed by pe o-based poly-
p opylene and polye hylene) eaches he 50% weigh loss a
284 °C.
44
This lame e a dancy e ec o lignin is consis en
wi h p e ious s udies, which asc ibed such beha io due o he
o ma ion o in umescen cha laye s e icien ly limi ing he
hea and lammable ola ile ans e .
43,45
In addi ion, he
ma ked inc ease in he esidual mass a 700 °C om 2.5 w %
o KL-LS/Chi 10/90 o 36 w % o i s GPE coun e pa is
explained by he e ec i e lame e a dan ole o zinc sul a e by
p omo ing he o ma ion o cha ing laye s. A simila e ec was
also ound in polyp opylene-based composi es.
45
The amoun
o cha esidue sligh ly inc eases wi h lignin amoun (ei he KL
o LS) gi en he endency o a oma ic ings o yield
ca bonaceous s uc u es. The imp o ed he mal s abili y a e
imp egna ion wi h ZnSO4 oge he wi h he la ge cha esidue
a high empe a u es indica es ha LSKL−chi osan and LS−
chi osan GPEs a e appealing o physically isola e ba e y
elec odes (anode and ca hode) a high empe a u es, a oiding
he isk o sho -ci cui upon he mal unwaway.
46
Mechanical p ope ies o GPEs should be also conside ed
when designing sa e ba e ies wi h long-ope a ion li e spans. In
pa icula , mechanically adap able and duc ile elec oly es
ypically show an enhanced in e acial con ac wi h me allic
elec odes, while s i GPEs o e a physical ba ie agains
undesi ed dend i e g ow h.
47
Uniaxial ensile s ess−s ain
expe imen s we e conduc ed o e alua e he Young’s modulus
(E), ensile s ess a yield and a b eak (σyand σb), and
elonga ion a yield and a b eak (εyand εb). Rep esen a i e
s ess−s ain cu es can be seen in Figu e 4a, while Table S2
summa izes he main cha ac e is ic pa ame e s. The glass
mic o ibe sepa a o p esen s a s i and b i le cha ac e , wi h a
modulus o 1470 ±120 MPa and εbo ∼5.8 ±0.1%.
48
On he
con a y, as schema ized in Figu e 4b, LSKL−chi osan and
LS−chi osan GPEs show a semi-duc ile beha io cha ac e ized
by E alues anging om 55 ±4 o 940 ±63 MPa and εb
alues o 14.1 ±0.2 o 43.9 ±21.1% (Figu e 4c,d). Fo he
sake o compa ison, he mos widely exploi ed GPE, he
polye hylene oxide/LiTFSI blend, displays a Young’s modulus
o ∼100 MPa.
15
Independen ly o he composi ion, we ound
ha glu a aldehyde c oss-linking inc eases s i ness and lowe s
duc ili y due o he o ma ion o ex ended imine linkages in he
elec oly e. An inc eased ac ion o lignin lowe s bo h Eand εb
alues as epo ed o chi osan-lignin ilms,
49,50
while he
micella LSKL dispe sion esul s in s i e bu mo e b i le
GPEs in compa ison wi h LS−chi osan GEPs.
51
O e all, he
GPEs he e de eloped show an enla ged duc ili y while keeping
accep able ensile modulus alues, o e ing an ad an ageous
esis ance agains dend i e pene a ion.
52
The mechanical
lexibili y o he GPEs also acili a es ba e y cell assembly
while ensu ing a good in e acial con ac wi h me allic Zn due
o he abili y o accommoda e elec ode olume changes upon
Zn2+ inse ion/ex ac ion.
53
Elec ochemical Cha ac e iza ion. The p edominan ly
amo phous cha ac e o he GPEs oge he wi h he abundan
hyd oxyl unc ional g oups (4.12 mmol g−1)
33
a ailable o
in e ac wi h Zn2+ o he GPEs encou age hei use as ionic
conduc o s o ba e ies. The elec oly e up ake was quan i ied
based on eq 1 a e soaking he memb anes in o 0.5, 1, and 2
M ZnSO4H2O o 24 h. EU alues anging om 78 o 319 w
% we e ob ained o a ious memb ane composi ions (Figu e
5a). No ma ked changes depending on KL o LS a e achie ed.
Glu a aldehyde c oss-linking lowe s elec oly e up ake alues
as he amoun o g oups o in e ac wi h wa e a e educed.
54
An inc ease in he lignin con en (a expenses o a dec ease o
Figu e 3. TGA aces unde ai a mosphe e o (a) LS−chi osan and LSKL−chi osan memb anes and (b) co esponding GPEs. Dashed line:
Celga d 2325 memb ane. The inse s in (a) show he LS10Chi90 sample be o e and a e TGA measu emen .
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chi osan) also esul s in a lowe ed elec oly e up ake despi e
he phenolic hyd oxyl g oups and ca bonyl g oups being ac i e
wa e so p ion si es in lignin.
55
Though KL and LS a e no
hyd ophobic ma e ials as such, he p esence o hyd ophobic
ings as opposed o he abundan pola g oups o chi osan
lends explana ion o he lowe elec oly e up ake in he
p esence o lignin.
56
I is also obse ed ha la ge elec oly e
up ake alues a e achie ed when inc easing ZnSO4concen-
a ion. I is impo an o no e ha measu ed wa e up ake
alues a e la ge han he ∼14−16 w % shown o o ganosol /
Na ion/polye hylene oxide o o ganosol /Na ion mem-
b anes
57
o he 28.5−34.5 w % ob ained o sul ona ed
poly(e he e he ke one)/lignin memb anes
58
and compa able
o he 230 w % shown by lignin memb anes imp egna ed in
solu ions o o ganic ca bona es.
59
These esul s indica e he
sui abili y o K a lignin−chi osan and lignosul ona e−
chi osan o p o ide e icien medium o Zn2+ anspo .
Zn2+ conduc i i y alues ha e been ob ained om he
Nyquis impedance plo s in Figu e S5 using he eq 2 ( u he
de ails a e p o ided in Tables S3−S5). All he GPEs display
s aigh lines wi h no semici cles as a esul o hei ionically
conduc ing cha ac e . As summa ized in Figu e 5b, ionic
conduc i i ies om 3.8 o 18.6 mS cm−1a e achie ed o he
GPEs. La ge alues o 15.0 and 59.3 mS cm−1a e obse ed o
he glass mic o ibe sepa a o soaked in 0.5 and 2 M ZnSO4
H2O, espec i ely. Howe e , lignin−GPE conduc i i ies a e
supe io o he 0.12 mS cm−1 epo ed o a ca boxyme hyl
cellulose/ZnSO4GPE,
60
* he 14.6 mS cm−1shown o
xan han gum/ZnSO4/MnSO4GPE,
61
o he 8.9 mS cm−1o
chi osan/choline ni a e GPEs.
62
We also achie ed la ge
alues compa ed o he epo ed lignin-de i ed GPEs in ended
o LIBs, including 3.73 mS cm−1showed by a GPE ob ained
upon soaking lignin ibe s in o 1 M LiPF6e hylene ca bona e/
dime hyl ca bona e/e hyl me hyl ca bona e
59
o 2.52 mS cm−1
ob ained o a poly inylpy olidone/lignin soaked in o he
same sys em as abo e.
63
Simila ly, ob ained conduc i i ies a e
abo e he numbe s epo ed o po ous memb anes soaked in
zinc sal aqueous solu ion, such as he 9.1 mS cm−1ob ained
o a Na ion/lignin memb ane soaked in o 2 M ZnSO4H2O.
64
A plausible explana ion may be he coo dina ion o Zn2+
mobile cha ge species wi h he hyd oxyl g oups in chi osan
and lignin as well as ca boxylic and sul ona e g oups o lignin,
oge he wi h he lone pai on he N a om and O a om in
chi osan coo dina ing wi h Zn ions and he phenol hyd oxyl
g oups o lignin ha dissocia e he anion om he sal ,
59
acili a ing Zn2+ mo emen . In addi ion, he 3D s uc u e o
LS, ha ing as sul ona e, pola e he , and hyd oxyl g oups
o ms ionic domains wi h highly mobile wa e molecules,
54
u he boos ing Zn2+ conduc i i y. These cha ac e is ics
enable ob aining GPEs wi h a good comp omise be ween
mechanical p ope ies (s i bu duc ile) and ionic conduc i -
Figu e 4. Mechanical cha ac e iza ion wi h (a) ep esen a i e s ess−s ain cu es oge he wi h (b) a schema ic illus a ion showing he
mechanical cha ac e is ics o glass mic o ibe sepa a o and lignin GPEs. Mean alues o (c) Young’s modulus and (d) elonga ion a b eak o
LS−chi osan and LSKL−chi osan GPEs. Da a co esponding o glass mic o ibe is shown o compa ison.
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i ies in he mS cm−1o de , o en con lic ing equi emen s in
he polyme elec oly e ield.
65,66
Aqueous Zn cell ailu e is usually igge ed by hyd ogen
e olu ion eac ions and co osion eac ions o he me allic Zn
anode, limi ing he p ac ical abili y o zinc-ion ba e ies.
67
As
he ex en o he ini ial o e po en ial ob ained o Zn pla ing
on a smoo h Zn oil is ela ed o he hyd ogen p oduced, he
elec ochemical s abili y window o GPEs in he po en ial
ange o −0.25 o +2.4 V s Zn2+/Zn has been assessed. As
indica ed by he low cu en peaks in Figu e 5c, he o e all
s abili y window expands up o +2.2 V s Zn2+/Zn wi h
hyd ogen and oxygen e olu ion eac ion po en ials enla ged in
compa ison o he glass mic o ibe sepa a o soaked in he
liquid elec oly e. Pa icula ly, he onse o e po en ial o
hyd ogen e olu ion eac ion is no ably shi ed ca hodically by
20−50 mV o all GPEs. As a esul , he sys em he e explo ed
can be use ul wi h high- ol age ca hodes such as cobal
hexacyano e a e.
11
This obse a ion ma ching p e ious
epo s ha highligh he wide elec ochemical s abili y o
lignin in LIBs
59,68
is asc ibed o he adhesion o he GPE o he
elec ode su ace o p o ide a passi a ion laye .
11
Impo an ly,
lignosul ona e−K a lignin−chi osan and lignosul ona e−
chi osan GPEs p esen a 4.5- o 18- old educ ion on he
anodic and ca hodic in ensi y peaks a ∼0 V in compa ison
wi h he glass mic o ibe sepa a o soaked in o 2 M ZnSO4
H2O as a esul o he hyd ogen e olu ion eac ion deple ion
by lignin GPEs.
67
The educed esis ance o Zn anodic
dissolu ion and Zn ca hodic deposi ion p ocess p o ided by
he biopolyme GPEs is u he con i med by Zn/Zn
symme ical cell esul s.
The elec ochemical pe o mance o he GPEs has been
u he in es iga ed using a symme ic Zn/Zn cell con ig-
u a ion a oom empe a u e. Impo an ly, his echnique
p o ides in o ma ion ega ding he e e sibili y o he Zn2+
anspo h ough he GPEs.
69
Ob ained esul s a e depic ed in
Figu e 6, whe e nega i e and posi i e po en ials ep esen Zn-
me al s ipping and pla ing, espec i ely. A a low cu en
densi y o 50 μA cm−2, all he samples display a e e sible Zn-
me al pla ing and s ipping beha io , including he glass
mic o ibe sepa a o soaked in 2 M ZnSO4H2O. Vol age
luc ua ions a e obse ed as cu en densi y inc eases,
sugges ing he g ow h o a esis i e solid elec oly e in e phase
(SEI) on o he su ace o he Zn me al.
70
The glass mic o ibe
sepa a o shows a sudden ol age d op iden i ied as an in e nal
sho -ci cui wi h dend i e pene a ion jus a e se ing he
cu en a 200 μA cm−2(42 h es ). This p ema u e sho -
ci cui o igina es om he inhomogeneous ion anspo
be ween Zn su aces acili a ed by he mic ome e -sized
po es o he sepa a o .
69
On he con a y (magni ied iew in
Figu e S6a), he non-c oss-linked LS-Chi 30−70 sample
p o ides a nea ly squa e wa e shape e en a a high cu en
densi ies o 500 μA cm−2(>100 h es ), indica ing he
occu ence o homogeneous me al ion elec odeposi ion wi h
e e sible Zn/Zn2+ edox eac ions also a ele a ed cu en
densi ies.
20,69
We conduc ed a symme ic cycling es unde mo e se e e
condi ions o he LS30Chi70 GPE and obse ed no sho -
ci cui o a eal cu en densi ies as high as 5000 μA cm−2
(Figu e 7a), sol ing he anode e e sibili y and s abili y issues
obse ed in con en ional ZIBs. This long- e m s abili y and
p ocess e e sibili y is also con i med by he high Coulombic
e iciency alues o abo e 99.8% o he majo i y o he
composi ions s udied (Figu e S7). Mo e p ecisely, Coulombic
e iciencies app oaching 99.5% a e obse ed when he a eal
cu en densi y is below 300 μA cm−2, which is abo e he
alues o 99.6% ob ained by cyclohexanedodecol-modi ied
ZnSO4elec oly es
71
o he 99.3% showed by he Zn(ClO4)2
elec oly e.
72
We asc ibe his pe o mance o he e e sible
Zn2+ inse ion/ex ac ion o igina ing om he good elec oly e
up ake and he ionic conduc i i y o lignin-GPEs, oge he wi h
hei mechanically adap a i e pe o mance (duc ili y), adhesi e
p ope ies gi en by he phenolic hyd oxyls,
30
and esis ance
agains dend i e g ow h ( ela i ely high Young’s modulus
alues). In addi ion, we also obse ed ha he GPEs ha ing
lowe lignin ac ions p esen a egion o la ge pola iza ion
du ing he ini ial h ee o six s ipping/pla ing cycles o each
Figu e 5. (a) Elec oly e up ake; (b) ionic conduc i i y and (c) elec ochemical s abili y window LS−chi osan and LSKL−chi osan GPEs. The
inse shows an expanded iew o he oxygen e olu ion eac ion egion.
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cu en densi y be o e eaching a s eady-s a e pola iza ion
shape, sugges ing a poo e ini ial in e acial con ac (Figu e
S8).
73
The ex en o o e po en ial is also a ele an pa ame e
because i indica es he cha ge anspo cha ac e is ic, an
essen ial equisi e o a oid dend i e o ma ion.
74
Despi e he
lowe ionic conduc i i y o lignin GPEs o e he glass
mic o ibe sepa a o soaked in 2 M ZnSO4H2O, compa able
o e po en ial alues o ∼160 mV a e obse ed a o 50 μA
cm−2 o he majo i y o he samples. I may be concluded hus
ha he as e Zn2+ anspo in he glass sepa a o is o se by a
smoo he and mo e homogeneous Zn pla ing/s ipping
p o ided by he GPEs, which enables a s able SEI laye .
75
Mo eo e , no able o e po en ial di e ences a e obse ed a e
100 h (500 μA cm−2; see Figu e S6b), wi h alues anging om
130 o 318 mV. O e all, glu a aldehyde c oss-linking and he
subs i u ion o LS by KL seem o inc ease he o e po en ial.
We asc ibe his beha io o he o ma ion o a he e ogeneous
solid elec oly e in e phase on he Zn anode su ace.
74
In any
case, i is impo an o no e ha he low o e po en ial ob ained
by he LS,Chi 30/70 GPE enables a high ol age e iciency o
he ba e y, delaying undesi ed decomposi ion eac ions ha
may yield o a p ema u e cell ailu e o ine icien cha ge/
discha ge.
74
I is wo hy o no e ha he obse ed o e -
po en ial alue o LS30Chi70 GPE is simila o ha shown by
a cellulose−aga ose GPE ecen ly de eloped by ou g oup,
which o e s a li espan ex ending o e 8500 h (one yea
cycling) in a symme ic Zn|Zn con igu a ion.
23
In ac , we
pos ula e ha de eloped lignin-based GPEs supp ess he
p esence o ee wa e as opposed o con en ional memb ane/
aqueous elec oly e sys ems, shielding Zn su aces om he
ee wa e -induced Zn co osion and hus enabling long- e m
cycling.
32
No ing his composi ion p esen s he lowe Young’s
modulus (55 ±4 MPa), i can be concluded ha achie ing a
good in e acial compa ibili y and in ima e con ac o he GPEs
wi h he me allic Zn (also p omo ed by he hyd oxyl and e he
Figu e 6. Room- empe a u e ol age ( e ical axis) s ime (ho izon al axis) cu es o symme ic Zn/Zn cells o Zn pla ing/s ipping a di e en
cu en densi ies o lignosul ona e and lignosul ona e/k a lignin GPEs. De ails on he composi ion o each GPE oge he wi h he sample code
a e p o ided o each composi ion. Colo s om blue o g een ep esen cu en densi ies om ±50 o 500 μA cm−2, while he ligh ning indica es
he occu ence o sho -ci cui . The cu es co esponding o he glass mic o ibe soaked in o 2 M ZnSO4H2O a e shown o compa ison.
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g oups in lignin) de ini i ely con ibu e o educe he
o e po en ial. In ac , pos -mo em SEM and XRD analyses
o cycled Zn su aces in Figu e 7b and Figu e S9, espec i ely,
p o e ha he smoo h and homogeneous elec odeposi ion
ea u e o he LS30Chi70 and LS30Chi70GA GPEs is
ansla ed in o a la and dend i e- ee zinc s uc u e
(JCPDS: 36-1451 o me allic zinc). This mo phology con as s
wi h he mic oscale high-su ace a ea needle-like Zn deposi s
obse ed when he symme ic cells sepa a ed by a glass
mic o ibe soaked in o 2 M ZnSO4H2O (sho -ci cui a 100
μA cm−2). The mo e abundan deposi s obse ed o he
LS30Chi70 GPE o e he LS30Chi70GA one may o igina e
om he no ably la ge cu en densi ies, igge ing sho -
ci cui (5000 s 200 μA cm−2).
Conside ing he unc ional p ope ies and hei enewable/
biodeg adable na u e, we en isage he ollowing po en ial
applica ions o ab ica ed lignin-con aining GPEs. Fi s , he
lexible cha ac e o de eloped GPEs and, in pa icula , o he
samples con aining lignosul ona e, is a ac i e o implemen
solid-s a e ZIBs o lexible and wea able elec onic de ices,
whe e ba e ies could ope a e unde epea ed s e ching and
bending condi ions.
76
Gi en he e sa ili y o he gela ion
(physical o chemical), he la ge ionic conduc i i ies, and
mechanical esis ance achie ed, we o esee a b igh u u e o
biopolyme -elec oly es in o highe -ene gy densi y zinc-ion
chemis y ba e ies. Fo example, u he e o s a e needed o
inco po a e lignin GPEs in seconda y zinc-ai ba e ies
77
o in
ZIBs comp ising me al−o ganic amewo k (MOF) ca ho-
des.
78
When lignin−chi osan GPEs a e pai ed wi h o ganic
ca hode ma e ials, ully enewable ZIBs could be ob ained,
a oiding he use o sca ce and oxic c i ical aw ma e ials ha
a e causing se ious esou ce deple ion and supply chain
bo leneck issues.
79
Addi ionally, he biodeg adable cha ac e
o elec oly e cons i uen s makes hese GPEs suscep ible o
deg ade unde compos ing condi ions, opening new oppo -
uni ies o ab ica e ansien echa geable ba e ies ha a e
apidly deg aded in o ha mless by-p oduc s (once he adequa e
igge is ac i a ed) a e a pe iod o s able ope a ion.
23
The eby, lignin−chi osan GPEs ha e he po en ial o lessen
he inhe en en i onmen al impac o con en ional LIBs
equi ing non-biodeg adable and ha m ul ma e ials.
■CONCLUSIONS
The p oduc ion o ba e y elec oly es adhe ing o ci cula
economy p inciples equi es a cau ious balance be ween he
Figu e 7. (a) Room- empe a u e ol age ( e ical axis) s ime (ho izon al axis) cu es o symme ic Zn/Zn cells o he LS30Chi70 GPE a high
a eal cu en densi ies. (b) Pos -mo em SEM images o he Zn su aces a e cycling in symme ic Zn/Zn cells using a glass mic o ibe sepa a o
soaked in o 2 M ZnSO4H2O oge he wi h hose co esponding o he LS30Chi70 and LS30Chi70 GPEs. The SEM image co esponding o he
p is ine Zn oil is shown o compa ison.
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