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RS1401

Elucidation of catalyst role in the lithium-oxygen battery
Hye Ryung Byona, Raymond A. Wonga,b, Chunzhen Yanga, Arghya Duttaa, Minho Oa, Misun Honga,c,
Keisuke Yamanakad, Toshiaki Ohtad,a

理化学研究所、Byon国際主幹研究ユニット,b東京工大, cPOSTECH, d 立命館大学SRセンター
ab

Byon Initiative Research Unit, RIKEN, Wako, Saitama 351-0198 Japan
Department of Energy Science, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku,
Yokohama 226-8502, Japanc
Department of Chemistry, POSTECH, 77 Cheongam-Ro, Namgu, Pohang 790-784, S. KoreabSynchrotron Radiation Center, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan

We report chemical identification of discharge and recharge products of lithium–oxygen (Li–O2) cellusing Au, Pt, Pd, Ru, Co3O4 and NiO nanoparticles on carbon nanotube (CNT) electrode. O and Li
K-edge XANES measurements reveal that lithium peroxide (Li2O2) is the main product during discharge,
which is decomposed during recharge. In addition, some side products such as lithium carboxylates andlithium carbonate (Li2CO3) are also emerged.
Keywords: Catalyst, Metal and metal oxide nanoparticles, Lithium-oxygen battery, Carbon nanotube,
XANES.
背景と研究目的(Introduction)：Despite hightheoretical capacity, a Li-O2 battery has sufferedfrom huge oxidation potential polarization on
carbonaceous positive electrode for recharge (RC,>4.2 V vs. Li/Li+), due to sluggish decompositionof non-conductive discharge (DC) product,
lithium peroxide (Li2O2 ↔ 2Li+ + O2 + 2e-)[1].
Such high potential triggers side reactions suchas degradation of electrolyte and carbonaceouselectrode, which results in poor cycle-ability[1].
To mitigate this problem during oxygen evolution
reaction (OER), solid-state metal or metal oxide
nanoparticles (NPs), which are expected to serve
as catalyst-like role, were introduced to theelectrode[2]. However, the specific role of theseNPs in the Li-O2 battery is little known due tocomplication from accompanying parasitic side
reactions[3]. In addition, reasonable comparisonof these NPs’ activities is not feasible under
different performance conditions when various
reports were referred.[2] Therefore, to gain areasonable assessment of their activities and anunderstanding of their role, it is necessary toexamine Li-O2 batteries with metal and metaloxide NPs under the same condition and analyze
their reaction processes in detail. Here we present
diagnosis of the true role of these NPs, such as
platinum (Pt), gold (Au), palladium (Pd),ruthenium (Ru), cobalt oxide (Co 3O4) and nickel
oxide (NiO), for OER in Li-O2 batteries undervery similar condition and analysis of DC andRC products using XANES spectroscopy.
実験(Experimental)：(1) Composite electrode synthesis: The Au, Pt, Pd,Ru, Co3O4 and NiO NPs were synthesized using
wet chemistry. Thereafter, stabilizing agents werethoroughly washed out. The metal and metaloxide NPs were then incorporated withmulti-walled carbon nanotubes (CNTs) following
by preparation of binder-free electrode using
vacuum-filtration technique. The CNTs serve as a3-D framework with abundant voids (more than100 nm), which are suitable to O2 gas transport inthe Li-O2 cell. We designed that all NPs accountfor ~40 wt% for the electrode weight (NP +CNT). The mass of CNT (~1.2 mg) iscomparable for all electrodes to reasonablycompare NPs’ role in the Li-O2 cells. The size ofsynthesized NP is as follows: Au (~9 nm), Pt(~3.5 nm), Pd (~3.5 nm), Ru (~1.5 nm), Co 3O4(~10 nm) and NiO (~2.5 nm).
(2) Galvanostatic tests: The Li-O2 cells wereassembled with metallic Li as the negativeelectrode, CNT with various metal/metal oxide
NPs as the positive electrode and 0.5M
LiTFSI/tetraglyme as the electrolyte solvent in an
Ar-filled glove box. Galvanostatic DC and RC
were performed after filling of O2 gas andmaintaining the open circuit potential for 3 h.
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The tests were examined at a current rate of 0.06
mA using a battery cycler (WonATechWBCS3000) in a constant-temperature chamber(25 oC, Yamada). A cut-off DC potential was 2.2
V and the RC was examined to the equivalentcapacity of the corresponding DC. All potentials
were referred to Li+/Li.
(3) Characterizations: After DC and RC, theLi-O2 cells were disassembled inside the Ar-filled
glovebox and the electrodes were directlytransferred to XANES chamber using a hermetic
vessel and without exposing to air. The XANES
spectral measurements were performed for O andLi K–edge at BL2 and BL11 at the SR center ofRitsumeikan University.

XANES spectroscopy with Li and O K-edge
were performed. If these NPs can trigger moreunintended reactions instead of catalytic role,
which can drop the RC potential, the sideproducts are predominant compared to Li2O2.
Figs. 2–4 displays O and Li K-edges XANES
spectra. Fig. 2 shows O K-edge of DC electrodes(afterfulldischargedelectrode)withbulk-sensitive PFY and surface-sensitive PEY
modes. It is very clear that all electrodes havemain σ*(O-O) peak of Li2O2 at ~530 eV in PFY
mode,
demonstrating
predominant
Li-O2
electrochemical reaction. Interestingly, the NiO
electrode has an apparent shoulder at ~531.5 eV,
which corresponds to the peaks from lithium
carboxylates (LiCO2H and LiCO2CH3). This canbe further corroborated by the result from PEY
mode. The PEY spectra reveal formation oflithium carboxylates-like side products at surface
for almost all electrodes. In particular, NiO, Ruand Au electrodes have greater side products on
the surface compared to Li2O2. This ispresumably due to decomposition of electrolyte
at the Li2O2 surface.

結果、および、考察(Results and discussion)：Fig. 1 shows the 1st cycled galvanostatic curves
with CNT (red), Au/CNT (blue), Pt/CNT(orange), Pd/CNT (green), Ru/CNT (grey),
Co3O4 (violet) and NiO/CNT (dark yellow) in 0.5
M LiTFSI/tetraglyme electrolyte. All electrodes
have similar discharge (DC) potential to ~2.7 V,
which suggests negligible effect of metal/metaloxide NPs for DC. The DC capacities aredifferent from electrodes: The Pt, Ru and NiO on
the CNT electrodes have lower capacity thanmetal/metal oxide-free CNT (~1900 mAh/g-CNT)
whereas the Li-O2 cells with Co3O4, Pd and Au
deliver greater capacity (over 2000 mAh/g -CNT).
The reason for the different capacity with respect
to the metal and metal oxide NPs is not very clear.
In contrast to the comparable DC potential,different RC potentials appear. Referring to thepotential of CNT (~4.4 V) at the end of RC, Ru(3.95 V), Co3O4 (4.07 V), Pd (4.14 V), Pd (4.15V), NiO (4.15 V) and Au (4.37 V) have lower
potentials. It seemingly shows that themetal/metal oxide lowers the RC potential, i.e.,enhanced OER from the DC-RC profiles.

Fig. 2. O K-edge XANES spectra of DC electrodes
with PFY and PEY mode. The bottom spectra arereference powders of Li2O2, LiOH, LiCO2H,LiCO2CH3, Li2CO3, and LiTFSI.

Fig. 1. Electrochemical performance of various
electrodes in 0.5 M LiTFSI/tetraglyme at a current
rate of 0.06 mA and cut-off potential of 2.2 V for
DC in O2 atmosphere. The electrodes are CNT (red),
Au/CNT (blue), Pt/CNT (orange), Pd/CNT (green),
Ru/CNT (grey), Co 3O4 (violet) and NiO/CNT (darkyellow).

Fig. 3 displays O K-edge spectra after RC (fullyrecharge). It is obvious that the Li2O2-related
peak completely disappear as it was decomposed.
The black graph and dashed area indicate thestandard reference of ‘as-prepared’ CNTelectrode. It is ideal that all electrodes return toTo investigate true role of metal and metal oxide,2様式３its original form (like as-prepared CNT) without
any degradation. However, we could find thetrace of side products. There is a pronounced
shoulder at ~532 eV for CNT, Ru, Co 3O4 andNiO, which arises from lithium carboxylates. Forthe Co3O4, the Li2CO3-associated peak at ~533
eV is also apparent. The Pt and Pd have relatively
low side product-related peak. In addition, theAu/CNT has even lower peak at ~531.5 eV than
the as-prepared CNT, of which specific reason is
not clearly revealed at the current stage. It isnoted that the Co3O4 and Ru show pre-edge
absorption peak at ~527.5 eV, which result from
Co3O4 itself and RuO2 (outer layer of Ru),respectively. All other electrodes have similar
pre-edge to the as-prepared CNT.

Fig. 4. Li K-edge XANES spectra of DC and RC
electrodes with TFY mode. The bottom spectra arereference powders of Li2O2, LiCO2H, LiCO2CH3,
Li2CO3, LiOH, LiF and LiTFSI.
In summary, we study DC and RC products on
positive electrodes with various metal and metaloxide NPs in Li-O2 cells after the first cycle. The
XANES spectra demonstrate that the main DC
product is Li2O2 for all electrodes, which is also
completely decomposed after 1RC. All electrodes
cover with thin side products such as HCO2Li
and CH3CO2Li after 1DC, which is likely to formchemical reaction at the interface of
Li2O2/electrolyte. Therefore, the formation ofthese side products cannot be taken into accountof metal/metal oxide NPs’ effect. After 1RC, suchlithium carboxylate-related products are observed
for all electrodes, which are particularly intense
for CNT, Ru and Co3O4. The Li2CO3 is also
found from the Co3O4 electrode. This suggeststhat (1) the Ru and Co 3O4 electrodes remainmuch side products due to low RC potential,
which is not as much as high to decompose
insulating side products or (2) they can trigger
further unintended reactions compared to othermetal/metal oxide NPs. The pronounced sideproducts on the metal/metal oxide-free CNT canbe understood from the highest RC potential,
which enhances degradation of electrolyte andCNT. The study of XANES spectra correlatedwith DC-RC galvanostatic profiles reveal theeffect of metal/metal oxide NPs for RC.

Fig. 3. O K-edge XANES spectra of 1RC electrodes
with PFY and PEY mode. The black graph anddashed-line area indicate the reference of
as-prepared CNT.

The Li K-edge spectra for DC and RC with TFYmode in Fig. 4 are in good agreement with the O
K-edge results. After DC, all electrodes have twomain peaks of 60.5 and 62 eV. For the Pd, Ru andNiO on the CNT, the second peak of ~62 eV isintense, which is likely due to additional sideproducts of lithium carboxylates and LiF
obtaining by decomposition of LiTFSI salt. After
RC, all Li2O2 peaks disappear. In addition, wecannot find any side-product peak possibly due tolower sensitivity of Li K-edge relative to O
K-edge XANES.

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文 献(references)[1] (a) G. Girishkumar, B. McCloskey, A. C.
Luntz, S. Swanson and W. Wilcke, J. Phys.
Chem. Lett., 1, 2193 (2010); (b) P. G. Bruce, S.
A. Freunberger, L. J. Hardwick and J.-M.
Tarascon, Nat. Mater., 11, 19 (2012).
[2] (a) Y. –C. Lu, H. A. Gasteiger and Y.
Shao-Horn, J. Am. Chem. Soc., 133, 19048(2011); (b) Z. Peng, S. A. Freunberger, Y.
Chen and P. G. Bruce, Science, 337, 563(2012); (c) F. Li, D. –M. Tang, Y. Chen, D.
Golberg, H. Kitaura, T. Zhang, A. Yamada and
H. Zhou, Nano Lett., 13, 4702 (2013); (d) R.
Black, J.-H. Lee, B. Adams, C. A. Mims and
L. F. Nazar, Angew. Chem. Int. Ed., 52, 392(2013).
[3] B. D. McCloskey, R. Scheffler, A. Speidel, D.
S. Bethune, R. M. Shelby and A. C. Luntz, J.
Am. Chem. Soc., 133, 18038 (2011).
論 文 ・ 学 会 等 発 表 （ 予 定 ） (Articles andmeetings)[1] H. R. Byon, R. A. Wong, C. Yang, A. Dutta,M. O, M. Hong, M. L. Thomas, K. Yamanaka
and T. Ohta, 電気化学会第８２回大会 inYokohama National University, Japan.
(March/17/2015. Oral Presentation)[2] H. R. Byon, C. Yang, R. A. Wong, A. Dutta,
M. O, M. Hong, M. L. Thomas, K. Yamanaka
and T. Ohta, 201% Electrochemical Society
Spring Meeting, Chicago, USA (May/27/2015,Oral Presentation)4