様式３RS1503

Investigation of Li2O2 Film Growth on Carbon Nanotube Electrodes withCeO2 Nanoparticles
Chunzhen Yanga, Hye Ryung Byon,a Keisuke Yamanakab, Toshiaki Ohtab,
理化学研究所 Byon 国際主幹研究ユニット, b 立命館大学
Byon Initiative Research Unit, RIKEN, Wako, Saitama 351-0198 Japanb
Synchrotron Radiation (SR) Center, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japanaa

Unexpected Li2O2 film growth with thicknesses of ~60 nm is observed on a 3-D carbon nanotube (CNT)electrode incorporated with cerium dioxide (ceria) nanoparticles (CeO2 NPs) in Li-O2 battery. CeO2/CNT
electrodes are collected at different discharge/recharge states from Li-O2 batteries and investigated by C, Li,and O K-edge XANES. Bulk Li2O2 film is decomposed together with formation/decomposition of sideproducts.
Keywords: Li-O2 battery, XANES, lithium peroxide, Cerium oxide nanoparticles.
背景と研究目的 (Introduction)：
The rechargeable Li–O2 battery has been
intensively studied due to its promising potential
for providing superior energy density. 1 However,
the actual performance with respect to capacity
and round-trip efficiency has significantly
underperformed at the current stage, 1-3 which is
closely associated with the nucleation, growthand decomposition processes of Li 2O2.
It is generally believed that lithium peroxide(Li2O2) electrochemically forms thin films withthicknesses less than 10 nm resulting in capacity
restrictions due to limitations in charge transport.
However, we observed unexpected Li2O2 film
growth with thicknesses of up to ~60 nm on a
3-D carbon nanotube (CNT) electrode
incorporated with cerium dioxide (ceria)
nanoparticles (CeO2 NPs).4 The CeO2 NPs favor
Li2O2 surface nucleation owing to their strong
binding towards reactive oxygen species (e.g., O 2and LiO2). The subsequent film growth results inthicknesses of ~40 nm (a cut-off potential of 2.2
V vs Li/Li+), which further increases up to ~60
nm with the addition of trace amounts of H 2O
which enhances the solution free energy. To
understand the growth and decomposition
mechanisms of Li2O2, it is necessary to analyze
the surface and bulk properties of these film-like
Li2O2. Therefore, CeO2/CNT electrodes arecollected at different discharge/recharge statesfrom Li-O2 batteries and investigated by C, Li,and O K-edge XANES.

i) Synthesis and cell fabrication
CeO2 NPs were synthesized by thermal
decomposition of cerium nitrate precursor in thepresence of protecting agent of oleylamine
according to the literature.5 Around 0.6 mg CeO2
NPs and CNT (1.8 – 2.0 mg) were well mixed byultra-sonication in hexane, then made binder-free
positive electrode via vacuum filtration. The
electrode consisting of 3-D framework of CNTs
has many voids (more than 100 nm of size),
which is appropriate for O2 transport in Li-O2battery. The positive electrode was annealed at
350 oC in Ar gas and rigorously dried undervacuum, which was then assembled with metallic
lithium as the negative electrode, glass fiber
separator and tetraethylene glycol dimethyl ether(tetraglyme) with 0.5 M LiClO4 electrolyte in an
Ar-filled glovebox. The complete cell was filledwith O2 gas at atmosphere.
ii) Electrochemical evaluation
Galvanostatic discharge/charge was performed
using a battery cycler (WBCS3000, WonATech,Korea) at 25 oC. The applied current (mA/g) isbased on the CNT mass. All potentials werereferred to Li+/Li.
iii) Characterization
XANES experiments were performed at the SR
center in Ritsumeikan University. C, O and Li
K-edge XANES spectra were obtained at BL-02
and BL-11, where all electrode samples andreferences were loaded in a glove box anddirectly transferred into the measurement

実験 (Experimental)：17

様式３
chamber using a custom-made hermeticallysealed vessel.

4.0V). CeO2/CNT electrodes at 1DC (100%discharge, 2.0 mAh), 0.25RC (25% recharge)
0.5RC and 1RC are collected from Li-O2 cells
and characterized by O K-edge.

結果、および、考察 (Results and Discussion)：
In Li-O2 battery, the nucleation and growth of
Li2O2 occurs during discharge (DC) throughoxygen reduction reaction (ORR, O 2* + e– + Li+→ LiO2*, the asterisk mark (*) denotes
adsorption on the surface) and a second step via
either electrochemical reduction process (LiO 2*+ e– + Li+ → Li2O2(s)) or disproportionation(2LiO2(sol) → Li2O2(s) + O2).2, 3 The diffusion ofsuperoxide species (LiO 2*) on electrode surface
leads to the predominant surface nucleation andLi2O2 film growth (indicated as surface-mediatedgrowth). Morphology of the fully discharged
Li2O2 product formed on CeO 2/CNT electrode ischaracterized by SEM and TEM (Figure 1). A
thick film-like Li2O2 (thickness ~ 40 nm) is
observed on CeO2/CNT electrode when using thedry electrolyte (0.5 M LiClO4 in tetraglyme, with
H2O content < 20 ppm). With the presence of2000 ppm of H2O in the electrolyte, the film
thickness increased to ~60 nm. Results
demonstrate the contribution of increased
LiO2(sol).

Figure 2. Galvanostatic discharge (DC)/recharge(RC) curves at a current density of 0.1 mA with alimited DC capacity of 2.0 mAh using CeO 2/CNT
electrode as cathode in Li-O2 battery. The watercontent in the electrolyte is < 20 ppm.

Figure 3. O K-edge XANES spectra of soaked(as prepared and soaked in the tetraglyme
electrolyte), 1DC (Q = 2.0 mAh), 0.25RC, 0.5RC
and 1RC CeO2/CNT electrodes with (a)surface-sensitive total electron yield (TEY) modeand (b) bulk-sensitive partial fluorescence yield(PFY) mode. The reference spectra are Li 2O2,HCO2Li, CH3CO2Li
andLi2CO3. The
galvanostatic conditions are shown in Figure 2.

Figure 1. SEM and TEM (inset) images ofCeO2/CNT electrode surfaces after full discharge(DC) with (a) dry tetraglyme and (b) 2000 ppm
of H2O. The white dashed line in TEM image is a
guide to indicate the CNT surface.

O K-edge XANES spectra of as-prepared
CeO2/CNT electrode exhibits three main peaks
related to O2p-Ce4f (529.5 eV), O2p-Ce5d-eg(532.2 eV) and O2p-Ce5d-t2g (536.6 eV)
hybridization in CeO 2 NPs (Figure 3).6 After
1DC, σ* (O–O) absorption peak of Li2O2 is
predominant at ~530 eV, 7,8 which is partiallydiminished at 0.25RC and 0.5RC suggesting
Li2O2 decomposition, as demonstrated byappearance of CeO2-related peaks such as theO2p-Ce5d-eg and O2p-Ce4f, and completely

Galvonostatic discharge/charge profiles with alimited discharge capacity of 2 mAh is shown inFigure 2. The discharge of CeO2/CNT electrodein Li–O2 cell shows a stable potential plateaus at~2.63 V (vs. Li/Li+), whereas recharge of Li2O2
thick films required a high overpotential (over18様式３
disappear after 1RC. Along with the primary
product of Li2O2, side products are also observed.
The 1DC spectrum with the TEY mode reveals ashoulder corresponding to lithium carboxylates(HCO2Li and CH3COOLi) at 531.7-532 eV. After
0.25RC, an absorption peak at ~532 eV issignificant, which is attributed to both lithium
carboxylates and CeO2 (O2p-Ce5d-eg). After0.5RC,lithium carboxylates aremore
pronounced than the Li2O2 being decomposed.
Besides, there is an additional small shoulder
related to π* (C=O) absorption of lithium
carbonate (Li2CO3) appears at 532.8 eV 8 with theTEY mode. These side products result fromdegradation of electrolyte solution and CNT.9,10
After 1RC, the side product peaks disappear
almost completely and the three main peaks fromCeO2 are visible.

K.; Ohta, T.; Byon, H. R. Nano Lett 2016, 16,2969.
Meetings[1] Chunzhen Yang, and Hye Ryung Byon,
Structural effect of nano ceria incorporated tocarbon nanotube for the formation ofnon-crystalline Li2O2 in Li-O2 batteries, 55thBattery Symposium Japan 2014, Electrochemical
Society of Japan, Kyoto, Japan[2] Chunzhen Yang, and Hye Ryung Byon, CeO2
Nanoparticle-Guided
Formation
andDecomposition of Amorphous Li 2O2 on Carbon
Nanotube Cathode in a Li-O 2 Cell, 227th ECS
Meeting 2015, Electrochemical Society, Chicago,
USAIn summary, we investigated the electrochemical
performance of Li-O2 cells with CeO2 NPs
decorated on the CNT. An unexpected film
product is observed on the discharged electrode,with thickness of ~ 40 nm, and increased to ~ 60
nm when enhancing the solvation of LiO 2 byaddition of H2O in the electrolyte. Through theanalysis of O K-edge XANES spectra, thedischarge product is mainly Li2O2, with trace
amount of carbonaceous side product on thesurface. The recharge process is accompaniedwith further formation and decomposition of sideproducts, which are mainly lithium carbonate andlithium carboxylates.
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論 文 ・ 学 会 等 発 表 （ 予 定 ） (Articles andmeetings)
Articles[1] Yang, C.; Wong, R. A.; Hong, M.; Yamanaka,19