様式３
S18003

Surface protection of LiCoO2 positive electrodein aqueous lithium-ion batteries
Hyunjeong Oha, Hirona Yamagishib, Keisuke Yamanakab, Toshiaki Ohtab, Hye Ryung Byona*a Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro,bYuseong-gu, Daejeon 34141, Republic of Korea
Synchrotron Radiation (SR) Center, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan*e-mail: ohj2010@kaist.ac.kr, hrbyon@kaist.ac.kr

We have investigated cycling stability of lithium cobalt oxide (LiCoO2) as the positive electrode inaqueous electrolyte solution and tried to protect the LiCoO 2 surface from the adverse effect of water. First,
we attempted to form artificial coating layer of LITHion to minimize direct contact of water to LiCoO2
surface while allowing for Li+ ion transport. However the access of hydrated lithium ion through a coatinglayer eventually provided non-trivial quantity of water. Therefore LITHion coating gave a delay of capacity
fading while distortion of CoO6 layer of LiCoO2 arising from water became severe for cycling. Alternatively,
we tried to make a solution-phase protection layer using inner Helmholtz layer of SO42- in Li2SO4 electrolytesolution. In the absence of any artificial layer, the aqueous lithium-ion cells showed highly improved
capacity retention with 0.5 m Li2SO4(aq) (~100% even after 30 cycles) relative to that with 1 m LiTFSI(aq)(~30% capacity retention for the same cycling number). The O K-edge X-ray absorption near-edge structure(XANES) spectra revealed dramatic alleviation of CoO6 distortion and negligible side products with Li2SO4electrolyte solution.
Keywords: Aqueous lithium-ion battery, LiCoO2, XANES
背景と研究目的 (Introduction)：
Recently, aqueous electrolyte for rechargeable
lithium batteries (ARLB) have received a lot ofattention owing to the safety and cost issues 1.
Non-flammable and low cost of water could
solve the catching fire problems of battery
system. However, the positive electrode oflithium transition metal oxides has shown poor
cycling stability in the aqueous media than theones in the non-aqueous ones. For that reasons,especially in case of layered structured LiCoO 2(LCO) Xia group suggested preferential proton(H+) insertion into layered-structure of LCO atlow pH. The irreversible insertion of H + blocked
the pathway of Li+ movement while this issue
was relatively suppressed in alkaline solution2.
Yushin group demonstrated the adverse effect ofwater at the surface of LCO which resulted in alarger increase in ionic and electronicresistances3.
In this research, we tried to find methods toprotect the surface of LCO from undesired
reactions arising from water at neutral pH. First
we tried to design an artificial coating layer ofthe LCO surface using LITHion (Lithiatednafion). The structure of LITHion is shown in
Figure 1(a), when the layer deposited on thesurface. The large hydrophobic domain canminimize the water poisoning effect while theseparating narrow routes of SO 3- allow for Li+ion transport. In addition, we also investigatedanionic inner Helmholtz layer from SO42- byusing Li2SO4 solution, which remarkably could
protect the LCO surface even in the absence of
any artificial coating layer. Along withelectrochemical performance tests, we havestudied the surface and bulk structures of LCO
electrodes using X-ray absorption near-edge
structure (XANES) spectroscopy in PEY (partialelectron yield) and PFY (partial fluorescenceyield) to shed light on the effect of protectinglayers.
実験 (Experimental)：i) Electrode preparation(1) Commercial LiCoO2 (Sigma aldrich) powder
was mixed to Super P as the conducting agent
and polyvinylidene fluoride (PVDF, Kynar) as
the binder with 8:1:1 weight ratio. N-methyl
pyrrolidine (NMP) was used to make slurry,which was casted on the carbon paper (Toray,TGP-H-90) using doctor blade method in an
argon-filled glove box. The electrode dried at 60oC in vacuum overnight.
(2) Preparation of LITHion-coated LCO
10 wt% LITHion solution in isopropyl alcohol様式３(Ion power) was diluted 40 times by
N,N-dimethylforamide (DMF, Sigma aldrich).
The LCO electrodes were soaked in this solution
for 9 hours, followed by drying at 80 oC for 4 hand subsequent drying at 100 oC for 2 h in avacuum oven.
ii) Electrochemical test
Galvanostatic
chargeanddischarge
examinations were conducted using battery
testers (Biologic) and an Ar-filled glove box toavoid the adverse effect of O2 gas4. The LCO
electrode (area: 0.826 cm2) was used as theworking electrode (WE), and Pt coil was used as
the counter electrode (CE). Ag/AgCl (3 M NaCl)
was used as the reference electrode (RE) and thepotential range was set to –0.2 ~ + 0.8 V vs
Ag/AgCl. Two aqueous electrolyte solutions
including 1 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, Kishida) and 0.5 m
Lithium sulfate (Li2SO4, Alfa aesar) were used as
the aqueous electrolyte solutions.

LITHion was coated on LCO with several
nanometer of thickness. The first galvanostaticcharge and discharge curves of LITHion coatedLCO (LITHion@LCO) was compared with bare
LCO in Figure 1(b). Electrochemical reaction ofLITHion@LCO are identical to that of pristine
LCO, showing similar lithiation/delithiation
potential plateaus at 0.67 V vs Ag/AgCl and no
additional plateaus. However, for 30 cycles, thecapacity retentions from two electrodes arenotably different. Figure 1(c) showing
cyclability of LCO in 1 m LiTFSI aqueoussolution reveals ~85% capacity retention fromLITHion coating and ~20% from bare LCO.
Higher columbic efficiency for LITHion@LCO
also supported reversible Li+ ion (de)lithiation.
However, Figure 1(d) demonstrates poor ratecapability in high current density of 10 C evenwith LITHion coating and unstable capacity
retention once the current density increased.

iii) Characterizations
The cycled LCO electrodes in LiTFSI solution
were washed with 1,2-dimethoxyane (DME)
while H2O was used as the washing solution
when LCO cycled in Li2SO4 solution. All
samples dried at 60 oC in vacuum overnight andwere directly transferred into the measurement
chamber using a custom-made hermeticallysealed vessel. The measurements of O K-edge
and Co L-edge spectra using XANES
spectroscopy were performed at BL-11 of the SR
center in Ritsumeikan University.
結果、および、考察 (Results and Discussion)：

Figure 1. Galvanostatic examinations of bare
LCO (gray) and LITHion@LCO (green) in 1 m
LiTFSI solution (a) Structure of LITHion. (b) 1st
galvanostatic charge and discharge curves (b)Comparison of cyclability and columbic
efficiency during 30 cycles with 0.5 C current
density. (d) Rate capability result.

Figure 2. XANES spectra of as-prepared LCO(black), 30th cycled LITHion coated LCO (green),and 30th cycled bare LCO (gray) with 1 m
LiTFSI aqueous solution (pH ~7). PFY mode(top) and PEY (bottom) of (a) cobalt L3-edge and(b) oxygen K-edge region
To investigate the structural degradation of LCO
from aqueous electrolyte, we analyzed the local
structure change of LCO using XANES
spectroscopy. Figure 2 shows Co L3-edge and O
K-edge regions of LCO electrodes in PEY andPFY modes. The Co L3-edge spectrum for theas-prepared pristine LCO shows the Co 2p to 3dtransition peak indicated as indicated by A1 at
780.3 eV, which slightly shifts positively in both
PEY and PFY mode after 30th cycle (Figure 2(a)).
It indicates the delihitated Li+ ions are not totallylithiated again for cycling, which is consistent
with the result of remaining Co4+ in LCO5,6.
Concomitantly, the new peak of A2 that is related
to Co2+ appears at the surface of PEY mode. The

様式３
reason for the formation of Co 2+ is understood forthe reduction of Co3+ at the space charge regionof LCO surface 3, 7.
For the O K-edge region in Figure 2(b), the Co
3d-O 2p hybridization peak at ~530 eV is
assigned to B1. In addition, the O 1s transition tothe hybridized states of O 2p and Co 4sp appears
to the broad peaks over 535 eV, which areindicated as C and D. After 30th cycles, theXANES spectra are totally different from PEY
and PFY modes. At the surface from the PEY
mode, the B2 peaks of 521.8 eV associated withthe partial oxidation of oxygen (O –2+δ) areenhanced for both LiTHion coated and bare LCO
electrode surface. This peak results from theinterfacial reaction between LCO and electrolyte
solution. On the other hand, at the bulk region
from the PFY mode, the pre-edge B3 peak of529.9 eV significantly emerges for bothelectrodes. It indicates the distortion of CoO 6
octahedral structure8, 9 as the delithiation occurs.
Namely, the distortion of CoO 6 octahedral
structure irreversibly takes place due to lesslithiation process for cycling.
Unfortunately, although LITHion coating on
LCO shows enhanced cyclability, distortion ofCoO6 local structure and surface degradation areanalogous to those of bare LCO electrode in 1 m
LiTFSI aqueous solution. Therefore we conclude
that LITHion is not capable of preventing thewater poisoning efficiently although it can delay
it in some extent. Presumably, the hydrated Li+ion (Li(H2O)4+) itself can approach LCO
electrode through the SO3- channel in theLITHion layer, and the remaining water after
dehydration of Li+ for lithiation process destructs
LCO structure.
Alternatively, we designed the new concept ofsolution-phase protection layer using surface
adsorption-favorable anions. It was reported thathigh concentration of anion (TFSI-) inwater-in-salt electrolyte made a dense inner
Helmholtz layer at the surface of positivelycharged electrode and prevented the access ofwater.10,11 The molecular dynamic (MD)simulation revealed that the oxygen atoms ofTFSI- were attractive to the positively charged
surface12. However when the anion concentration
is lower and competes with much higher
concentration of water in a typical electrolyte
solution, the oxygen atoms linked with a bulky
CF3 group in TFSI- may not be easily exposed
toward electrode surface. Therefore, the water
has a high chance to accommodate the electrodesurface and induces the water-poisoning resultfor LCO. Therefore, we tried to find the newanion which has high affinity to adsorb on thepositive electrode surface compared to water. The
tetrahedral structure of SO42- anion contains fouroxygen atoms at the vertexes, which can directly
approach to electrode surface without any
hinderance. Its smaller size and resonancestructures are also attractive to form stable andsolution-phase protection layer.

Figure 3. Galvanostatic electrochemical test ofpristine LCO in 1 m LiTFSI solution (gray) and
0.5 m Li2SO4 solution (light green) (a) 1 stgalvanostatic charge and discharge curves (b)Comparison of cyclability and Columbic
efficiency during 30 cycles with 0.5 C current
density. (c) Rate capability test
Figure 3 shows galvanostatic charge anddischarge curves with 0.5 m Li2SO4 solution,
which are similar to the one with 1 m LiTFSI
solution. In addition, the cell with 0.5 m Li2SO4
solution shows superior cyclability and ratecapability. The capacity retention maintained
100% even after 30th cycles (Figure 3(b)). For
rate capability test, although capacity drop isincreased with increasing current density, thecapacity retention is much improved without any
destruction even after fast cycling (Figure 3(c)).
The structure change of LCO was also
investigated from XANES. With 5 m Li 2SO4
solution, the shift of A1 peak is suppressed asshown in Co L3-edge region (Figure 4(a)). In
addition, emergence of B2 and B3 peaks in O
K-edge spectra is significantly alleviated,indicating less CoO6 distortion caused by theprotection of LCO surface from SO 42- ions.
Therefore, the SO42- anion (0.5 m) could makeless damage of LCO structure and lower side
reaction compared to TFSI- anion (1 m). This
stable LCO structure is consistent with reversible
lithiation/delithiation process for cycling.

様式３
2017, 8(18), 4362.
論 文 ・ 学 会 等 発 表 （ 予 定 ） (Articles andmeetings)

Figure 4. XANES spectra of pristine LCO(black), 30 cycled LCO with 1 m LiTFSI (gray)and 30 cycled LCO with 0.5 m Li2SO4 (blue). (a)PEY (bottom) and PFY mode (top) of cobalt
L3-edge spectra (b) oxygen K-edge spectra

In summary, we studied surface protection
methods for LiCoO2 positive electrode, (1)LITHion coating and (2) in-situ formation ofsolution-phase SO42- anion layer. The LITHion
could not efficiently prevent water poisoning andit resulted in severe structural distortion in bothbulk and surface region. The alternative method
using SO42- anion in the electrolyte solution
could much more suppress CoO6 distortion andenhance reversible lithiation/delithiation process.
It may be attributed to high adsorption affinity ofSO42- anion to LiCoO2 electrode and hindering
the access of water.
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[5]. T. Mizokawa, et al., Phys. Rev. Lett., 2013,111, 056404.
[6]. L. Dahéron, et al., Chem, Mater. 2008, 20,583.
[7]. K. Yamanoto, et al., J. Phys. Chem. C, 2014,
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Meetings[1] Hyunjeong Oh, Hirona Yamagishi, Keisuke
Yamanaka, Toshiaki Ohta and Hye Ryung Byon,
Degradation mechanism of LiCoO 2 in aqueous
lithium-ion batteries, Korean society of industrial
and engineering chemistry 2018 Spring, Daegu,
Republic of Korea[2] Hyunjeong Oh, Hirona Yamagishi, Keisuke
Yamanaka, Toshiaki Ohta and Hye Ryung Byon,
Study of the degradation mechanism of LiCoO2
layered-structure positive electrode in aqueous
lithium-ion batteries, The 19th international
meeting on lithium batteries, Tokyo, Japan[3] Hyunjeong Oh, Hirona Yamagishi, Keisuke
Yamanaka, Toshiaki Ohta Hirona Yamagishi,
Keisuke Yamanaka, Toshiaki Ohta and Hye
Ryung Byon, Understanding interfacial reactionof layered LiCoO2 positive electrode in aqueous
lithium-ion batteries, International BatteryAssociate, San Diego, USA.