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Electrochimica Acta 391 (2021) 138877
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
In situ visualization of zinc plating in gel polymer electrolyte
Yuju Jeon
a , b , Yutong Wu
a , Yamin Zhang
a , Chihyun Hwang
a , Hyun-Wook Lee
b , ∗, Hyun-Kon Song
b , ∗, Nian Liu
a , ∗
a School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332 United States b School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, South Korea
a r t i c l e i n f o
Article history:
Received 25 January 2021
Revised 1 July 2021
Accepted 1 July 2021
Available online 9 July 2021
Keywords:
In situ optical microscopy ( in situ OM)
Gel polymer electrolyte
Zinc metal battery
Zinc metal plating
a b s t r a c t
Uniform zinc metal plating has been raised as a critical issue in zinc-based batteries. Randomly localized
ions lead to severe zinc dendrite formation in liquid electrolyte due to nonuniform ion flux caused by
electroconvective flow. One of the mitigating approaches is to use gel polymer electrolyte to regulate the
ion flux for suppressing zinc dendrites by imparting viscoelasticity to the electrolyte and improving the
ion transport along charged functional groups of polymer chains. However, to this date, the effectiveness
of gel polymer electrolyte has been visualized using ex situ methods (e.g., scanning electron microscopy)
that requires cell disassembly. And the underlying mechanism is poorly understood. Herein, we applied
in situ optical microscopy with dark-field illumination and a transparent glass slide cell to visualize zinc
metal plating in the gel polymer electrolyte. At a given current density, the morphological differences
of plated zinc metal between the liquid and gel polymer electrolytes were compared. Our in situ opti-
cal microscopy platform successfully showed that the gel polymer electrolyte supported by cross-linked
polyacrylic acid (PAA)/N,N’-methylenebisacrylamide (MBA) polymer framework significantly suppressed
the dendrite formation in contrast to the liquid electrolyte during plating. In addition, at various current
densities, the tendency of dendritic growth was observed and statistically compared in both electrolytes.
The findings will be useful for future design of rechargeable zinc-based batteries.
© 2021 Elsevier Ltd. All rights reserved.
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. Introduction
Zinc is a promising anode material for aqueous rechargeable
atteries due to its low reduction potential (–0.76 V vs. stan-
ard hydrogen electrode), high gravimetric and volumetric capac-
ty (820 mAh g −1 and 5855 mAh cm
−3 ), low cost (~$2 kg −1 ), in-
rinsic safety and low toxicity [ 1–5 ]. However, dendritic growth
f zinc during the charging process is a challenging problem in
queous zinc-based battery using liquid electrolyte ( Fig. 1 a) [ 6–
]. One of the speculations is that uneven electric field through
pace charge region localizes ions to a pristine electrode surface,
esulting in electroconvective fluid motion [ 9 , 10 ]. The electrocon-
ective flow causes nonuniform ion flux leading to the dendrite
ormation, which is aggravated by zinc oxide (ZnO) passivation and
ydrogen gas evolution [11] . ZnO by-products precipitated by zin-
ate ions ([Zn(OH) 4 ] 2 −) concentrated above the solubility limit pas-
ivate fresh zinc metal and then reduce the active surface area.
n addition, local high-energy-derived hydrogen gas evolution in-
∗ Corresponding authors.
E-mail addresses: [email protected] (H.-W. Lee), [email protected]
H.-K. Song), [email protected] (N. Liu).
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ttps://doi.org/10.1016/j.electacta.2021.138877
013-4686/© 2021 Elsevier Ltd. All rights reserved.
uces undesirable extra convective flow at the interface between
he electrolyte and electrode. Both detrimental side reactions dur-
ng zinc plating further increase the surface polarization and ac-
elerate the dendritic growth. The progressive growth of the zinc
endrites leads to separator penetration and eventually a short cir-
uit [12] .
To evenly distribute the ion flux and alleviate the problem of
inc dendrite growth, polymers have been coated on the surface
f zinc electrode [ 13 , 14 ] or used as electrolytes in zinc-based bat-
eries [ 15–20 ] ( Fig. 1 b). For example, functional groups in poly-
ers, such as amide group (-CONH 2 ) [13] , carboxyl group (-COOH)
21] or quaternary amine [ 22 , 23 ] have strong electrostatic inter-
ction with zinc or zincate ions, which improves the transport of
orking ions and thereby facilitates the uniform plating of zinc
24] . Furthermore, polyacrylic acid (PAA), polyacrylamide (PAM)
nd poly(vinyl alcohol) (PVA) have been gelated to serve as elec-
rolytes [25] . The entangled polymer chains impart viscoelasticity
o the electrolyte, which suppresses the electroconvection of fluid
nd regulates the ion flux [ 9 , 10 ]. Gel polymer electrolytes, due to
heir low tendency to mix, can form stable junction between elec-
rolytes, which enables the pairing of two different incompatible
lectrodes, one in each side of the junction, to achieve new possi-
ilities [20] .
Y. Jeon, Y. Wu, Y. Zhang et al. Electrochimica Acta 391 (2021) 138877
Fig. 1. Schematic illustration of zinc plating in (a) liquid electrolyte and (b) gel polymer electrolyte. Zinc forms large dendrites in liquid electrolyte, and flat film in the gel
electrolyte. (c) Chemical structure and formation mechanism of cross-linked PAA/MBA hydrogel. (d) Photo of PAA/MBA hydrogel at the bottom of a glass vial.
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To this date, zinc dendrite suppression by gel polymer elec-
rolyte has mainly been investigated using ex situ methods (e.g.,
canning electron microscopy) that analyze the sample after stop-
ing the reaction and disassembling the cell. It is desirable to have
eal-time, in situ tools to probe the dynamic information during
inc metal growth. In situ optical microscopy (OM) has recently
een considered a non-destructive and powerful analysis technique
hat displays real-time electrochemical operation in cells of differ-
nt chemistries and geometrical configurations [ 26–31 ]. In partic-
lar, in zinc-based batteries, only the effectiveness of adding ad-
itives to liquid electrolytes on dendrite suppression has been ob-
erved so far (Table S1) [ 15 , 32 , 33 ]. Herein, for the first time, we
tilized an optical microscope to visualize in situ the effect of gel
olymer electrolyte on zinc plating compared to the liquid elec-
rolyte [ 27 , 28 ]. In addition, the previous in situ OM works used
ire-type electrodes, which are not used in practical zinc-based
atteries. In our work, however, we used copper foil and zinc foil
s working and counter electrodes, respectively, which is a more
ommon practice in testing zinc-based batteries. Using this plat-
orm, zinc plating on copper foil was clearly monitored in situ from
he top view at various current conditions, visualizing morpho-
ogical differences of plated zinc metal and providing quantitative
nalysis of zinc dendrite formation in the liquid and gel polymer
lectrolytes.
The gel polymer electrolyte used in this study is from a pre-
ious report [20] . Its chemical structure and formation mecha-
ism are schematically illustrated in Fig. 1 c. To prepare the elec-
rolyte, acrylic acid (AA) and N,N’-methylenebisacrylamide (MBA)
ere dissolved in ZnO-saturated 45 wt% potassium hydroxide
KOH) solution. Then potassium persulfate (K 2 S 2 O 8 ) was added to
nitiate the radical polymerization, which was completed within
0 – 20 seconds at room temperature. The AA monomers were
hemically crosslinked with MBA crosslinkers, resulting in the
AA/MBA framework with amide (NH) and carboxyl (COOH) func-
ional groups. The PAA/MBA gel polymer electrolyte is transparent
nd viscous ( Fig. 1 d). 45 wt% KOH solution saturated with ZnO is
sed as a liquid electrolyte for comparison.
w2
. Experimental
.1. Materials
Zinc oxide (ZnO), potassium hydroxide (KOH), acrylic acid
AA), N,N’-methylenebisacrylamide (MBA) and potassium persul-
ate (K 2 S 2 O 8 ) were purchased from Sigma-Aldrich. Copper foil
Shenzhen Jingliang Copper Industry Co., LTD), zinc foil (Alfa Ae-
ar), slide glass (Corning, 75 × 25 mm), cover glass (Ted Pella,
nc., 24 × 12 mm), vacuum grease (Dow Corning) and silver paint
Ted Pella, Inc.) were prepared for glass cell assembly. Transparent
ouch (Ampac, SealPAK), glass fiber (GE Healthcare, Whatman
TM
0370 0 03) and titanium wire (Goodfellow) were purchased for
ouch cell assembly. Carbon on nickel mesh (MEET) and hy-
rophilic polypropylene membrane (Celgard, 3501) were prepared
or zinc-air cell assembly.
.2. Electrolyte preparation
ZnO-saturated 45 wt% KOH was used as the liquid electrolyte
ith pH of 15.77. To prepare PAA/MBA gel polymer electrolyte,
mg of MBA was dissolved into 0.6 ml of AA and stirred for
0 minutes. Then, the mixture was added to 3 ml of ZnO-saturated
5 wt% KOH and stirred for 15 minutes. Some ZnO will precipi-
ate due to the slight change in pH to 15.74. The solution was cen-
rifuged and syringe-filtered to completely remove the precipitated
nO. Another 4 wt% of K 2 S 2 O 8 solution was prepared to initiate
elation. The resultant liquid and PAA/MBA gel electrolytes con-
ained 55 wt% and 47 wt% of water, respectively.
.3. In situ optical microscopy (OM) measurement
The structure of the homemade OM cell is schematically shown
n Fig. 2 . Glass slides and cover glasses were first washed with soap
o remove surface contaminants. Copper foil and zinc foil with a
ize of 3 cm × 0.3 cm were placed on each side of the slide glass
ith 0.5 mm gap as working and counter electrodes, respectively.
Y. Jeon, Y. Wu, Y. Zhang et al. Electrochimica Acta 391 (2021) 138877
Fig. 2. (a) Schematic illustration of in situ dark-field optical microscopy setup. (b)
Photo of a glass slide cell used in this study.
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Fig. 3. Dark-field optical microscopic snapshots of zinc plating on copper foil in (a–
c) liquid and (d–f) PAA/MBA gel electrolytes. Zinc metal was plated at 1.7 mA cm
−2 .
Snapshots were obtained after plating (a, d) 0 mAh cm
−2 , (b, e) 0.17 mAh cm
−2 and
(c, f) 0.34 mAh cm
−2 . In the liquid electrolyte, black mossy dendrites were gener-
ated and grown isotropically during plating. However, dendrites were significantly
suppressed in the PAA/MBA gel electrolyte during the entire plating process.
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over glass was fixed in the middle of the anode and cathode by
sing vacuum grease, leaving small openings for electrolyte injec-
ion. For OM cells with liquid electrolyte, 40 μl of ZnO-saturated
5 wt% KOH was injected to fill the gap between glass slide and
over glass. For OM cells with gel polymer electrolyte, 40 μl of
A/MBA-dissolved ZnO-saturated 45 wt% KOH was firstly injected
nd 2 μl of 4 wt% K 2 S 2 O 8 was subsequently added for gelation,
ollowed by immediate polymerization of AA and MBA completed
ithin 10 – 20 seconds. Then, the cover glass was completely
ealed with the vacuum grease, and copper tape was attached to
nds of the anode and cathode to tightly fix both electrodes and
revent damage by test clips. Finally, a droplet of fast-drying sil-
er paint was coated on the copper tape to reduce the contact re-
istance between the copper tape and the test clip. The area of
opper electrode in contact with electrolyte was about 0.29 cm
2 .
he copper foil side was observed by an optical microscope (Leica
MC2700) with an objective lens (Leica N PLAN L 50X 0.50 BD)
n dark-field or bright-field mode. Zinc plating was conducted gal-
anostatically for 0.34 mAh cm
−2 at current densities of 0.34, 1.7,
.4 and 10 mA cm
−2 , using a potentiostat (Bio-Logic SP-200).
.4. Pouch cell test
Round disks of copper foil and zinc foil with diameter of 10 mm
ere used as the anode and cathode, respectively. A 350 μm-
hick glass fiber sheet was used as a separator between the an-
de and cathode. 100 μl of ZnO-saturated 45 wt% KOH solution
as injected on the separator for liquid electrolyte test. In the case
f cell using gel polymer electrolyte, 100 μl of AA/MBA-dissolved
nO-saturated 45 wt% KOH was firstly injected and 5 μl of 4 wt%
2 S 2 O 8 was subsequently added for gelation. Zinc plating was con-
ucted galvanostatically for 0.34 mAh cm
−2 at current densities of
.34, 1.7 and 3.4 mA cm
−2 , using LANHE battery tester.
.5. Zinc-air cell test
Zinc foil was used as the anode with electrolyte-contact area
f 1.13 cm
2 (diameter = 12 mm). Pt/C:Ir/C (1:1 = w:w) loaded at
arbon on nickel mesh was used as the cathode with electrolyte-
ontact area of 0.95 cm
2 (diameter = 11 mm). A 25 μm-thick
olypropylene membrane was used as a separator between the an-
de and cathode. 1 ml of ZnO-saturated 45 wt% KOH solution was
njected into the cell. In the case of cell using gel polymer elec-
rolyte, 952.4 μl of AA/MBA-dissolved ZnO-saturated 45 wt% KOH
as firstly injected and 47.6 μl of 4 wt% K 2 S 2 O 8 were subsequently
dded for gelation. Discharge and charge was conducted galvanos-
atically for 2 mAh cm
−2 at current density of 2 mA cm
−2 , using
io-Logic BCS-805.
3
.6. Morphology and material characterization
Surface and cross section of the plated copper electrodes were
bserved by SEM (Hitachi SU 8230). Depth profiling analysis on
econdary ions with negative polarity was conducted by time-of-
ight secondary ion mass spectroscopy (TOF-SIMS; TOF-SIMS 5,
ON TOF). An ion beam of 25 keV Bi 1 + in high current mode
as applied for the depth profiling. The measured area for the
epth profiling and analysis was 100 × 100 μm. The target area of
0 0 × 30 0 μm was sputtered by a 2 keV Cs + beam for 800 sec-
nds. After sputtering, the mapping images for the surface area of
00 × 100 μm were obtained using an ion beam of 50 keV Bi 3 2 +
n burst alignment mode. Fourier transform-infrared spectra (FT-IR;
lpha, Bruker) was obtained at 400 – 4000 cm
−1 in transmittance
ode.
. Results and discussion
.1. In situ OM and SEM visualization at 1.7 mA cm
−2
Zinc metal plating on the copper foil was visualized in situ at
.7 mA cm
−2 for 0.34 mAh cm
−2 and recorded at 30 frames per
econd simultaneously. As shown in Fig. 3 , Fig. S1 and Movie S1,
he growth behaviour of zinc metal in the liquid electrolyte was
ifferent from that in the PAA/MBA gel electrolyte. In the liquid
lectrolyte, black mossy dendrites were generated during plating
inc metal up to 0.17 mAh cm
−2 ( Fig. 3 b). Upon further plating to
.34 mAh cm
−2 , the dendrites grew isotropically and merged with
eighboring dendrites ( Fig. 3 c). This is because the tip of the den-
rite served as a lightning rod where charges were concentrated.
he size of dendrites after 0.34 mAh cm
−2 plating was 13.7 μm ( ±.7 μm). In contrast, dendrites were significantly suppressed in the
AA/MBA gel electrolyte during the entire plating process ( Fig. 3 e
nd f). The zinc metal layer uniformly grew without dendrites. In
he cross-section view of zinc metal layer plated at the front edge
f copper electrode, the morphological difference between the liq-
id and PAA/MBA gel electrolytes was clearly shown (Fig. S2). In
he case of the liquid electrolyte, coral-like zinc dendrites with
thickness of 187.4 μm were grown. However, the PAA/MBA gel
lectrolyte showed densely plated 92.3 μm-thick zinc metal layer.
To reveal the detailed morphology of plated zinc, ex situ scan-
ing electron microscopy (SEM) was conducted ( Fig. 4 ). 10 to 20
icrometer-sized mossy dendrites consisting of the needle-like
inc whiskers, and the porous electrode surface were observed in
he liquid electrolyte case ( Fig. 4 a and b). Under the PAA/MBA gel
lectrolyte, on the other hand, the zinc deposit layer was uniform
ith thickness of 2.2 μm that is 8 times denser than the plated
Y. Jeon, Y. Wu, Y. Zhang et al. Electrochimica Acta 391 (2021) 138877
Fig. 4. Ex situ SEM images of the top view and cross section of plated zinc metal
layer in (a, b) liquid electrolyte and (c, d) PAA/MBA gel electrolyte. Zinc metal was
plated at 1.7 mA cm
−2 for 0.34 mAh cm
−2 in both cases.
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Fig. 5. Dark-field optical microscopic snapshots of zinc plating on copper foil in
liquid electrolyte at 0.34 mA cm
−2 and 3.4 mA cm
−2 plating current densities. (a–c)
0.34 mA cm
−2 and (d–f) 3.4 mA cm
−2 . The total zinc plating capacity was controlled
to be 0.34 mAh cm
−2 .
Fig. 6. Dark-field optical microscopic snapshots of zinc plating on copper foil in
PAA/MBA gel electrolyte at 0.34 mA cm
−2 and 3.4 mA cm
−2 plating current den-
sities. (a–c) 0.34 mA cm
−2 and (d–f) 3.4 mA cm
−2 . The total zinc plating capacity
was controlled to be 0.34 mAh cm
−2 .
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inc layer in the liquid electrolyte ( Fig. 4 c and d). These results
ere highly consistent with those from in situ OM ( Fig. 3 ).
The different behaviour of zinc plating in the liquid and gel
olymer electrolytes stemmed from the different transfer process
f zincate ions. During plating in the liquid electrolyte, the ion lo-
alization at the electrode surface generated by the uneven electric
eld led to the growth of mossy dendrites (Fig. S3a). From a mi-
roscopic view, the mossy dendrites were composed of lots of indi-
idual zinc whiskers that were hundreds of nanometers thick (Fig.
3b). Each zinc whisker was intimately covered with insulating
assivation layer [34] . According to the time-of-flight secondary
on mass spectroscopy (TOF-SIMS) depth profiling, the increasing
n
− versus the decreasing ZnO
− and ZnOH
− with sputtering time
ndicated that the plated layer was composed of zinc metal pas-
ivated by the ZnO and zinc hydroxide (Zn x OH y ) by-products (Fig.
4a). In particular, the surface mapping images after the sufficient
puttering for 800 seconds showed concentrated chemical signals
f ZnO
− and ZnOH
− at the dendrite spots in comparison with the
idely distributed Zn
− signals, supporting the by-product passiva-
ion along the zinc dendritic structure with a large surface area
Fig. S4b). Therefore, the mossy structure could not be changed to
uniform metal layer structure, meanwhile it was thickened in the
urther plating process. In the presence of the PAA/MBA polymer
rameworks providing the viscoelasticity in the electrolyte, zincate-
on flux was evenly distributed so that the number of nucleation
ites increased. Thus, zinc granules plated in the PAA/MBA gel elec-
rolyte were smaller than those in the liquid electrolyte (Fig. S5)
13] . Consequently, the uniform and dense zinc metal layer was re-
lized.
Moreover, it was suggested that the transport of zincate ions to-
ards the electrode surface was improved by the electrostatic in-
eraction of zincate ions with the amide (NH) and carboxyl (COOH)
unctional groups of PAA/MBA polymer chains. During plating in
he liquid electrolyte, the zincate ion complexes ([Zn(OH) 4 ] 2 −) with
egative charge are difficult to approach to the negatively charged
lectrode surface due to electrostatic repulsion [22] . Thus, the zin-
ate ion pathways cannot be controlled throughout the electric
ouble layer at the electrode surface. In the case of the PAA/MBA
el electrolyte, the elemental mapping for nitrogen and carbon in-
icated that the polymer frameworks covered the plated zinc gran-
les well even after washing the electrode (Fig. S6). It was esti-
ated that the adsorbed polymer chains provided zincate-ion con-
uctive pathways to electrode surface along the partially positively
harged sites of nitrogen in the amide group and oxygen in the hy-
roxyl group [ 22 , 23 ]. Therefore, the coordination site of zincate ion
4
as investigated by Fourier-transform infrared spectroscopy (FT-
R) measurement on the PAA/MBA-based gel electrolyte with and
ithout ZnO salt (Fig. S7). The identified FT-IR peaks represented
he existence of functional groups in the PAA/MBA polymer frame-
orks: OH and NH at 2800 to 3700 cm
−1 ; amide at 1639, 1554
nd 1320 cm
−1 ; COO
− at 1404 cm
−1 ; C-OH at 1054 cm
−1 [ 35–38 ].
n the presence of zincate ions, the peak shift to lower wavenum-
er was observed at amide Ⅲ (1320 to 1317 cm
−1 ) and C-OH (1054
o 1052 cm
−1 ) compared to the absence of zincate ions, indicating
hat the zincate ions were coordinated to the positively charged
itrogen in the amide group and oxygen in the hydroxyl group.
.2. In situ OM and SEM visualization at 0.34 and 3.4 mA cm
−2
To study the relationship between the current density and zinc
orphology during plating, additional zinc plating in the liquid
nd PAA/MBA gel electrolytes was monitored in situ at smaller
0.34 mA cm
−2 ) and bigger (3.4 mA cm
−2 ) current densities as
hown in Figs. 5 , 6 and S8. With the liquid electrolyte, the den-
ritic growth of zinc metal was still observed at both current
onditions ( Figs. 5 and S9). Compared to the dendrite size of
3.7 ± 4.7 μm after plating at 1.7 mA cm
−2 for 0.34 mAh cm
−2
Fig. 3 c), the dendrites were larger (28.5 ± 18.5 μm) at
.34 mA cm
−2 current density, and smaller (10.8 ± 4.1 μm) at
.4 mA cm
−2 current density (Movie S2). As a higher current
as applied with larger overpotential, smaller dendrites were pro-
uced. This phenomenon follows the metal electro-deposition the-
ry that there is an inverse relationship between dendritic nuclei
metal nuclei) size and plating overpotential; r ∝ η−1 ( r : nuclei
ize, η: plating overpotential) [ 39 , 40 ]. Meanwhile, the PAA/MBA gel
lectrolyte effectively suppressed the dendrite formation and re-
lized densely plated zinc metal layer ( Figs. 6 and S10). Even at
he lowest current density (0.34 mA cm
−2 ), only a small dendrite
as slowly generated from the middle of plating (Movie S3). It
Y. Jeon, Y. Wu, Y. Zhang et al. Electrochimica Acta 391 (2021) 138877
Fig. 7. Dendrite size distribution versus applied current densities after plating for
0.34 mAh cm
−2 in liquid and PAA/MBA gel electrolytes. The liquid electrolyte
showed zinc dendrites at all current densities. Smaller current density results in
larger dendrites and wider size distribution. In the PAA/MBA gel electrolyte, den-
drites only grew at very small current density (0.34 mA cm
−2 ), but not at other
higher current densities.
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as been shown before that dendrites are more likely to grow at
ow current due to lower nucleation overpotential and less nucle-
tion sites [41] . Additionally, at extremely high current density of
0 mA cm
−2 , the benefit of PAA/MBA gel electrolyte on the plated
orphology of zinc was also confirmed (Fig. S11). It seemed that
he zinc metal was evenly plated in both electrolytes (Fig. S11a–f).
owever, the dendrites with size of 2.5 ± 1.4 μm were identified
n the liquid electrolyte by ex situ SEM measurement while there
as no dendrite in the PAA/MBA gel electrolyte (Fig. S11h and i).
To evaluate the relevance of the conclusions in this study, the
oltage profiles of the OM cells were compared to those of con-
entional pouch cells, as shown in Fig. S12. Despite the difference
n cell configurations (two electrodes head-to-head versus facing-
ach-other), the voltage profiles of the OM cells at given current
ensities (0.34, 1.7 and 3.4 mA cm
−2 ) were consistent with those
n pouch cells. This result was expected because electron and mass
ransport in the bulk electrolytes are faster than the reaction at
he electrode surface, at the current densities used in this study.
e also tested zinc-air cells configured of Pt/C:Ir/C (1:1 = w:w)
oaded at carbon on nickel mesh as a cathode and zinc foil as an
node to investigate the benefit of using PAA/MBA gel electrolyte
Fig. S13). The capacity of 2 mAh cm
−2 was discharged and charged
t 2 mA cm
−2 . The zinc-air cell with the PAA/MBA gel electrolyte
xhibited improved cyclic performance compared to the cell with
he liquid electrolyte, which was estimated to be attributed to the
table plating and stripping of zinc metal in the presence of the
AA/MBA gel electrolyte.
. Conclusion
In summary, in situ optical microscopy was successfully used to
isualize the effect of gel polymer electrolyte on the suppression
f dendrite formation. In contrast to the ex situ analysis such as
EM, the in situ observation provided real-time information of zinc
lating in a non-destructive way. The tendency of dendrite growth
or all current conditions in both electrolytes was summarized in
ig. 7 . In the liquid electrolyte, dendrites were observed at all cur-
ent densities. Larger current led to smaller dendrites with inverse
roportionality. At a given current, the dendrites had wide size dis-
ribution especially at the small current (0.34 mA cm
−2 ). In the gel
lectrolyte with the PAA/MBA polymer chains, on the other hand,
he dendrite growth was effectively suppressed, with the exception
f the small current (0.34 mA cm
−2 ).
Recently, various kinds of gel polymer electrolytes have been
pplied to zinc-based batteries in order to alleviate the problem
f zinc dendrite formation on the anode. Up to now, however, our
tudy was the first to visualize the effect of the gel polymer elec-
5
rolyte on the dendrite suppression with the optical microscope
nd compare the results with the liquid electrolyte. The fundamen-
al findings in our study are expected to provide guidance on the
esign of high-performance rechargeable zinc anode for high en-
rgy aqueous batteries. For example, for an aqueous zinc battery
sing acid-alkaline dual electrolyte, applying polymer frameworks
o alkaline electrolyte is predicted to lead to uniform zinc plating
s well as widening the electrochemical stability window.
eclaration of Competing Interest
The authors declare that they have no known competing finan-
ial interests or personal relationships that could have appeared to
nfluence the work reported in this paper.
redit authorship contribution statement
Yuju Jeon: Conceptualization, Data curation, Investigation,
roject administration, Visualization, Writing – original draft, Writ-
ng – review & editing. Yutong Wu: Writing – review & editing.
amin Zhang: Writing – review & editing. Chihyun Hwang: Data
uration, Investigation. Hyun-Wook Lee: Supervision. Hyun-Kon
ong: Supervision. Nian Liu: Conceptualization, Supervision, Writ-
ng – review & editing.
cknowledgments
Yuju Jeon and Hyun-Wook Lee acknowledges support from
he Ministry of Trade, Industry & Energy/Korea Institute of
nergy Technology Evaluation and Planning (MOTIE/KETEP)
20194010 0 0 010 0). Nian Liu acknowledges support from faculty
tartup funds from the Georgia Institute of Technology. The
uthors thank Sanjoy Banerjee and Gautam Yadav for useful
iscussions. Material characterization and device fabrication were
erformed in part at the Georgia Tech Institute for Electronics
nd Nanotechnology, a member of the National Nanotechnology
oordinated Infrastructure, which is supported by the National
cience Foundation (Grant ECCS-1542174). Yuju Jeon and Nian Liu
hank Sara E Bowles for proofreading the manuscript.
upplementary materials
Supplementary material associated with this article can be
ound, in the online version, at doi:10.1016/j.electacta.2021.138877 .
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