様式３S22027

Soft X-ray Absorption Near Edge Structure Studies onLithium Dodecyl Sulfate Effect for Distortions of LiCoO2in Aqueous Lithium-ion Batteries
Eunjin Choia and Hye Ryung Byona*aDepartment of Chemistry, Korea Advanced Institute of Science and Technology (KAIST),
Daejeon 34141, Republic of Korea
e-mail: qwerfcs@kaist.ac.kr, hrbyon@kaist.ac.kr*

LiCoO2 (LCO) is a representative positive electrode in lithium-ion batteries (LiBs). We utilized LCO toaqueous LiBs and found structure deformation by proton (H+) intercalation previously, causing capacity
fading. In this study, we added lithium dodecyl sulfate (LDS) to the electrolyte solution and investigated theLDS effect for LCO local structure change through slow and fast current density for cycling. The
short-range-order structural irreversibility was investigated by X-ray absorption near edge structure(XANES) analysis. The LDS suppressed surface degradation of LCO at 10 C compared to the absence ofLDS, revealing the protection of LCO.
Keywords: Aqueous lithium-ion batteries, LiCoO2, Lithium dodecyl sulfate, XANES, O K-edge, Co L-edge

Introduction
Aqueous LiBs have been studied to lower cost and the risk of fire. However, one of the challenges is waterand proton intercalation to lithium transition metal electrodes in salt-in-water, causing capacity fading andpoor cyclability. Our previous study revealed that 0.5 mol/kg Li2SO4 suppressed the proton intercalation toLCO compared to other electrolyte salts, such as 1 mol/kg LiTFSI, LiNO 3, and LiClO4. However, ratecapability was still poor with 0.5 mol/kg Li 2SO4[1].
In this experiment, the LDS additive was included in the aqueous electrolyte, and fast/moderatecharge-discharge processes were performed. Soft XANES verified the structural stability of LCO after
long-term cycles.
Experiments
LCO electrode was prepared by casting LCO slurry (LCO: Super P: poly(vinylidene fluoride) (PVdF)
binder = 8:1:1 wt% in 1-methyl 2-pyrrolidine (NMP)) on the etched Ti foil, followed by drying at 80 oC in athermal oven overnight. Ti foil was soaked in the etching solution (NH 4OH: H2O2 = 7:3 v/v%) for 10 min andwashed with acetone and distilled water before use. The loading mass of LCO active materials was 3~5 mg
cm-2.
We used three-electrode cells comprised of the LCO electrode, Pt coil, and Ag/AgCl (in 3 M NaCl) as theworking electrode, the counter electrode, and the reference electrode, respectively. All galvanostatic cycles
were performed under an Ar-filled glovebox to prevent contamination by oxygen.
After cycles with the below conditions ((1) ~ (4)), the LCO electrodes were washed three times by soaking
in DI water and dried under a vacuum at 80 ℃ overnight for XANES analysis.
(1) 50th cycled LCO at 0.5 C in aqueous electrolyte solution including 0.5 m Li2SO4 + 5 mM LDS(2) 100th cycled LCO at 10 C in aqueous electrolyte solution including 0.5 m Li2SO4 + 5 mM LDS(3) 50th cycled LCO at 0.5 C in aqueous electrolyte solution including 0.5 m Li2SO4(4) 100th cycled LCO at 10 C in aqueous electrolyte solution including 0.5 m Li2SO4(5) As-prepared LCO

Results and discussion
The O K-edge spectra of the cycled LCO electrodes ((1) ~ (4)) and the as-prepared LCO (5) were shown inFigure 1. As-prepared LCO showed sharp peaks at 529.6 eV, indicating the transition from O 1s to CoⅢ 3d

様式３and O 2p hybridization orbital, for both total electron yield (TEY) and partial fluorescence yield (PFY)
modes. In the TEY mode with ~10 nm surface depth, all cycled LCO electrodes showed a new signal at 528.5eV, indicated as O1, assigned to the transition from O 1s to CoⅣ 3d and O 2p hybridization orbital. It
indicated distortion of the CoO6 octahedron by the proton intercalation. The O1 peak of the LCO cycled at0.5 C for 50 cycles was negligible regardless of the presence and absence of LDS. For 10 C and 100 cycles,
however, the O1 intensity was different; The signal was attenuated with LDS, suggesting less structuraldegradation during the fast charging-discharging processes.
Figure 2 shows O K-edge spectra in the PFY mode. All electrodes exhibited the O1 signal with comparable
intensity, demonstrating distortions of LCO in the bulk structure regardless of the presence and absence ofLDS. Unlike 0.5 C cases, however, the capacity difference with (~100 mAh/g after 50 cycles) and without
LDS (< 30 mAh/g after 50 cycles) was significant at 10 C. Thus, this result suggested better stability of LCO
with LDS at high current rate.
Figures 3-4 showed the Co L-edge spectra. We found a slight blue shift of Co L2 and L3 edge for all cycled
electrodes in PFY modes, consistent with the bulk structural deformation in O K-edge modes with PFYmode.
as prepared LCO
0.5C
0.5C, +LDS
10C
10C,+LDSO K-edge
TEYO1528532536540

Energy (eV)

as prepared LCO
0.5C
0.5C, +LDS
10C10C,+LDS

O K-edge
PFY528532536540

Energy (eV)Figure 1. TEY mode of O K-edge XANES spectra.

Figure 2. PFY mode of O K-edge XANES spectra.

Figure- 3. TEY mode of Co L2&L3-edge spectra.

Figure 4. PFY mode of Co L2&L3-edge spectra.

Reference様式３[1] Oh, H.; Shin, S.; Choi, E.; Yamagishi, H.; Ohta, T.; Yabuuchi, N.; Jung, H.; Kim, H.; Byon, H. R.
Anion-induced interfacial liquid layers on LiCoO 2 in salt-in-water lithium-ion batteries. JACS Au. 2023,under revision.

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・This research result will be released at the 74th International Society of Electrochemistry.