様式３R1414 and R1417

Investigation of discharging products (Li 2O2) stemming from side reaction inLi-O2 battery using various nanocatalysts
CeO2 Nanoparticle-Guided Formation and Decomposition of Amorphous Li 2O2 on Carbon
Nanotube Cathode in a Li-O2 Cell
Chunzhen Yanga, Keisuke Yamanakab, Toshiaki Ohtab, Hye Ryung Byonaa 理化学研究所 Byon 国際主幹研究ユニット, b 立命館大学 SR センター
a Byon Initiative Research Unit, RIKEN, Wako, Saitama 351-0198 Japanb The SR Center, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan

Nanosized cerium oxide (CeO 2) particles incorporated carbon nanotube (CNT) is employed as a binder-freecathode for Li-O2 batteries. Unique film-like morphology of Li2O2 is formed on the discharged electrodewith poorer crystallinity and crystal size comparing to typical toroidal-shaped Li2O2. O and Li K-edge
XANES spectra are conducted to identify the chemical species present on the discharged and recharged
electrodes.
Keywords: Li-O2 battery, XANES, Lithium peroxide, Cerium oxide (ceria) nanoparticles.
背景と研究目的 (Introduction)：
Rechargeable
non-aqueous
lithium-oxygen(Li-O2) batteries have been the focus of recent
research due to their high theoretical energy
density which is about one order of magnitude
higher than those commercial Li-ion batteries.1,2
However, the Li-O2 battery is still under its
infancy stage, as various fundamental issues need
to be addressed before their practical application,
including poor cycle life, low round trip
efficiency, and poor rate capability.
The structure and morphology of Li2O2 in a
Li-O2 cell have been widely explored by theircorrelation to recharge potential. The bulk sizeand poor conductivity of Li2O2 toroids incursluggishdecomposition
duringrecharge,resulting in a rapid potential rise to more than 4.2
V vs. Li/Li+ concomitant with severe sidereactions. In this study, we introduce nanoceria(CeO2) as structural promoter onto carbon
nanotube cathode (i.e. positive electrode) toalternate the structure, morphology andcrystalline size of discharge product of Li 2O2. It
is known that nanosized CeO2 nanoparticle (NP,~5 nm) has strong affinity towards the reactive
oxygen species (ROS, such as O 2, O2- and O22-),
which can affect the nucleation behaviors of
Li2O2, thus the morphology. As a result, unique
Li2O2 film is observed on discharged cathode,which has much poorer crystallinity and smaller
size than typical toroidal-shaped Li2O2. To

identify the chemical species formed during
discharge and recharge in Li-O2 cells, O and Li
K-edge X-ray absorption near-edge structure(XANES) spectrum are conducted.
実験 (Experimental)：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.3 Subsequently, ~0.6
mg CeO2 NPs (with an average particle size of~5.3 nm) and CNT (1.8 – 2.0 mg) were well
mixed by ultrasonication in hexane, thenassembled as a binder-free porous carbon cathodeby vacuum filtration. The cathode was rigorously
dried under vacuum, and assembled with ametallic lithium anode, glass fiber separator andtetraethylene glycol dimethyl ether (tetraglyme)
with 0.5 M LiClO4 electrolyte in an Ar-filled
glovebox to fabricate Li-O2 cell.
ii) Electrochemical evaluation
Galvanostatic discharge/charge was performed
using a battery cycler (WBCS3000, WonATech,Korea). The applied current (mA.g -1) andcalculated capacity (mAh.g -1) were calculated
based on the total cathode mass (including
covalently bonded perfluorinated chains).
iii) Characterisation
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様式３O and Li K-edge XANES data were collected at
BL2 and BL11 lines of The SR Center,
Ritsumeikan University.
結果、および、考察 (Results and Discussion)：
From the SEM image in Figure 1a, it is shownthat nanosized CeO2 particles are homogeneously
distributed on CNT network (CeO2/CNT). Afterthe 1st discharge (1DC), unique film-like DC
product is observed on the electrode more thantypical toroidal-shaped Li2O2 (Figure 1b). This
product disappears in following recharge (RC).

Fig. 2. The normalized potential profiles of Li-O2
cell employing CeO2/CNT and CNT at a current
density of 50 mA g-1-CNT in 0.5 M
LiClO4/tetraglyme electrolyte. The cut-off
discharge voltage is 2.2 V.
To investigate chemical identification of DC andRC products on CeO2/CNT electrode, O K-edge
and Li K-edge XANES spectra are conducted.
Figure 3 and 4 display the normalized O and Li
K-edge XANES spectra of CeO2/CNT in bulk
sensitive partial or total fluorescence yield (PFYor TFY mode) and surface-sensitive total electron
yield (TEY mode).
In Figure 3, the O K-edge spectra of as-prepared
CeO2/CNT shows three main peaks at 529.5,
532.2 and 536.6 eV, which are related toO2p-Ce4f, O2p-Ce5d-eg and O2p-Ce5d-t2g
hybridization in CeO 2, respectively. Anabsorption peak at ~530 eV appeared after 1DC(i.e., full discharge), suggesting that the Li2O2 isthe predominant DC product. The Li2O2 partiallydisappears at 0.5RC (~50% RC) and completely
eliminated at the end of 1RC (full recharge). A
small shoulder at 531.7 eV is also noticed on theTEY spectra of 1DC electrode, whichcorresponds to lithium carboxylates. After 0.5RC,
lithium carbonate-associated absorption peak
appears at 532.8 eV. These side products areapparent in the TEY mode, indicating that thedegradation of electrolyte and CNT may occur on
the topmost surface of electrode. After 1RC, alllithium carboxylates and lithium carbonate peaks
cannotbeobserved,indicating
fulldecomposition of side products. Accordingly, theCeO2 is visible whereas the absorption peaks of
O2p-Ce4f and O2p-Ce5d-eg are largely decreased
relative to that of O2p-Ce5d-t2g when comparing
to as-prepared electrode.

Fig. 1. SEM images of 25% CeO 2/CNT
electrodes: (a) as-prepared and (b) after 1stdischarge (1DC).
The 1st DC and RC potential profiles ofCeO2/CNT electrode is compared to (CeO2-free)
CNT in Figure 2, at a current density of 50 mAg -1
in 0.5 M LiClO4/tetraglyme. Both electrodes
demonstrate comparable discharge voltages at
2.63 V, whereas difference in RC potentialappears. The RC potential at ~25% RC process is
3.59 V for CeO2/CNT and 3.85 V for CNT, which
demonstrates higher oxygen evolution reaction
efficiency in the presence of CeO2 NP.

130

様式３
Commercial Li2O2, CH3COOLi, and HCO2Lipowders are employed as standard references.
Black and dark green lines are PFY mode, grayand light green lines are TEY mode.
In summary, CeO2/CNT composite electrode isused as the cathode in a Li-O2 battery. The
galvanostatic potential profiles show plateauswith considerably lower initial RC potential than
CNT electrode. O and Li K-edge XANES wereemployed as the analytical tool to identify thechemical species on the DC and RC electrodes. It
is found that the major DC product with film-likemorphology is Li2O2, whereas minor sideproducts of lithium carboxylates and carbonateare also formed. These products are decomposed
after RC.
Fig. 3. The O K-edge XANES spectra ofCeO2/CNT electrodes at different DC and RC.
1DC, 0.5 RC and 1RC is the 1st full discharge,
50% recharge and full recharge, respectively.
Commercial Li2O2, CH3COOLi, Li2CO3 andHCO2Li are employed as standard references.
Black and dark red lines are PFY mode, gray andlight red lines are TEY mode.

文 献 (References)[1] J. Christensen et al., J. Electrochem. Soc.,
2012, 159, R1-R30.
[2] R. Black et al., Adv. Energy Mater., 2012, 2,801-815.
[2] S. Lee et al., ACS Nano, 2013, 7, 9693-9703.
論 文 ・ 学 会 等 発 表 （ 予 定 ） (Articles andmeetings)
Articles[1] C. Yang, R. A. Wong, K. Yamanaka, T. Ohta
and H.R Byon, Understanding the Role of
Structural Promoter in Lithium-Oxygen Battery
using CeO2 nanoparticle on CNT. (Inpreparation).
Meetings[1] Chunzhen Yang, and Hye Ryung Byon,
Structural effect of nano ceria incorporated tocarbon nanotube for the formation of
non-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 Li2O2 on Carbon
Nanotube Cathode in a Li-O 2 Cell, 227th ECS
Meeting 2015, Electrochemical Society, Chicago,
USA

In Figure 4, the Li K-edge spectra of 1DC
electrode also show peaks corresponding to Li2O2
at 60.7 and 62.1 eV. However, unlike the standard
Li2O2 powder, the TEY mode of 1DC spectra hasmuch stronger absorption at 62.1 eV, which ispossibly arising from the existence of lithiumcarboxylates. These peaks vanish at 0.5RC,implying decomposition at local area around Li
after the initial RC.

Fig. 4. The Li K-edge XANES spectra ofCeO2/CNT electrodes at different DC and RC.
1DC, 0.5 RC and 1RC is the 1st full discharge,
50% recharge and full recharge, respectively.
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