R1349

XANES investigation of side products in Li-O2 battery
using solid polymer electrolyte
Nadege Bonnet-Merciera, Chihiro Yogib, 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
ab

Byon Initiative Research Unit, RIKEN, Wako, Saitama 351-0198 Japan
Synchrotron Radiation Center, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan

We report chemical identification of discharge and recharge products of lithium–oxygen (Li–O2) cellusing a solid polymer electrolyte combined with a multi-walled carbon nanotubes electrode. O, C and Li
K-edge XANES measurements reveal that products are mixtures of lithium hydroxide (LiOH), lithium
carboxylates, lithium peroxide (Li2O2) and lithium carbonate (Li2CO3).
Keywords: Lithium-oxygen battery, Solid polymer electrolyte, Carbon nanotube, XANES.
背景と研究目的(Introduction)： Non-aqueous
Li-O2 batteries have received considerable
attention because of their remarkably higher
theoretical specific energy density than otherLi-ion batteries.[1] However, one of the greatchallenges for practical applications is low
stability of liquid-type non-aqueous electrolyteby superoxide radical attack. To improvechemical and physical stability of electrolyte,
solid-state Li ion conductive materials using
polymer or ceramic have been extensively
studied.[2] In particular, solid polymer electrolyte(SPE)[3] has been the promising medium for Li
batteries, for example poly(ethylene oxide)(PEO) with Li salt complex (PEO-based SPE)
which has exhibited wide electrochemical
window, appreciable stability and reasonable
solvation ability for alkali metal salts. In thiswork, we performed chemical identification ofproducts using a PEO-based SPE incorporatedwith carbon nanotubes (CNT) electrode after
discharge and recharge by XANES. The spectra
show that a main discharge product is not theexpected Li2O2 but a mixture of Li based
compounds stemming from side reaction due toSPE decomposition.
実験(Experimental)：(1) Composite electrode synthesis: The
homogeneous, flexible and semi-transparent
PEO-based SPE was prepared by a solvent-freeprocedure using ball-milling and hot-press
techniques. The SPE was then merged with thefree-standing CNT film by using the same
hot-press technique, which resulted in a
two-phase membrane having the SPE and theCNT/SPE. In the CNT/SPE phase, the conductive
network of CNT film was coated by the thin
PEO-based SPE, which maintained the CNT film
pores required for the smooth O2 gas transportand accommodation of discharge products.
(2) Galvanostatic tests: The Li-O2 cells wereassembled with metallic Li disk (diameter ≈ 12mm, Honjo) as the anode, CNT/SPE as thecathode/electrolyte. The O2 gas was then filledwith the Li-O2 cells at atmosphere. Galvanostaticdischarge and recharge examinations wereconducted in a potential range of 2.0–4.5 V at acurrent rate of 0.05 mA.cm–2 using a battery
cycler (WonATech WBCS3000) after maintaining
the open circuit potential for 3 h. Thetemperature was kept at 55°C during themeasurements using a constant temperaturechamber (Isuzu seisakusho). All potentials werereferred to Li+/Li.
(2) Characterizations: After discharge andrecharge, the cells were disassembled inside
glovebox and the CNT/SPE electrodes wereconveyed using a custom-made hermetic vessel
for XANES spectral measurements for O, C andLi K–edge, performed at BL2 in the SR center ofRitsumeikan University. All samples andreferences of Li2O2 (99% purity), LiOH (98%purity), Li2CO3 (99% purity) CH3CO2Li (98%purity) and HCO2Li H2O (98% purity) and PEO
were loaded directly transferred into the chamber
for XANES measurements without air exposure.
結果、および、考察(Results and discussion)：Fig. 1 shows galvanostatic the 1st cycleddischarge-recharge profile at 55oC. The Li-O2
cell delivers a capacity of ~300 mAh.g–1 at anaverage discharge potential of 2.65 V. The
recharge potential reaches to ~4.5 V at the end ofrecharge. The capacity of the SPE-employed
Li-O2 cell is lower than the cell with typicalliquid-type electrolyte. This is probably due tolow conductivity of SPE.

attributed to LiOH at 532 eV is shown. After the
1st recharge, the Li2CO3-related peak at 533 eV
newly appears in addition to the LiOH peak.

Fig. 1.
Electrochemical performance using
CNT/SPE composite electrode at 0.05 mA.cm–2 and55oC in O2 atmosphere

Figs. 2–4 display O, Li and C K-edges XANES
spectra of as-prepared (black), 1st discharged(1DC, pink) and 1st recharged (1RC, green)
CNT/SPE with the TEY mode. The bottom
spectra indicate reference powders of LiOH,
Li2O2, Li2CO3, HCO2Li, CH3CO2Li and PEO.

Fig. 3.
Li K-edge XANES spectra using TEY
mode of as-prepared, 1DC, 1RC CNT/SPE andreferences of Li2O2, Li2CO3 and LiOH

The Li K-edge spectra in Fig. 3 corroborate the O
K-edge results. The edge of 1DC CNT/SPE at~57.5 eV arises from the LiOH and that of 1RC
at ~58 eV is associated to the Li2CO3.
Interestingly, a sharp peak and broad shoulder at60–62 eV for the 1DC CNT/SPE are observed,
which is not be shown from the LiOH spectrum.
It is attributed to the Li2O2, as the ideal Li–O2
reaction product. Consequently, the Li K-edge
spectra determine the discharge products to LiOH
and Li2O2 that probably distributed on thesurface.

Fig. 2.
O K-edge XANES spectra using TEY
mode of as-prepared, 1DC, 1RC CNT/SPE andreferences of PEO, Li2O2, Li2CO3 and LiOH

The O K-edge spectrum of as-prepared CNT/SPEin Fig. 2 exhibits ether-correlated peak at 531.5
eV arising from the PEO. At the end of 1DC,such a PEO peak is silence probably due todeposition of discharge product but the new peak

The C K-edge spectra in Fig.4 demonstrate
additional formation of lithium carboxylates suchas formate (HCO2Li) and acetate (CH3CO2Li) for
the 1DC and 1RC CNT/SPE as evidenced byappearance of a sharp peak at 288.5 eV. A
predominant peak at 285.5 eV for the as-prepared
CNT/SPE decreases after 1DC and 1RC caused
by deposition of Li compounds.

Fig. 3. C K-edge XANES spectra using TEYmode of as-prepared, 1DC, 1RC CNT/SPE andreferences of Li2CO3, HCO2Li and CH3CO2Li.

In summary, XANES spectra reveal that the maindischarge product in Li–O2 cells using SPE is a
mixture of Li compounds such as LiOH, HCO2Li
and CH3CO2Li, which are accumulated inaddition to the Li2CO3 formed during recharge.
This result implies acute parasitic side reaction
for the SPE-employed Li–O2 cell, and also
instability of ether-based electrolyte even forliquid type.
文 献(references)[1] P.G. Bruce, S.A. Freunberger, L.J. Hardwick,J.M Tarascon, Nat. Mater., 2012, 11, 19-29[2] F. Li, H. Kitaura, H. Zhou, Energy Environ.
Sci., 2013, 6, 2302-2311[3] F. Gray, M. Armand, Handbook of battery
materials, 2011, 2, 627-656
論 文 ・ 学 会 等 発 表 （ 予 定 ） (Articles andmeetings)[1] N. Bonnet-Mercier and H. R. Byon, The 54th
Battery Symposium in Japan. (Oral Presentation)[2] N. Bonnet-Mercier and H. R. Byon, 2014
MRS Spring Meeting (Poster Presentation)