Kyushu Synchrotron Light Research Center（ Form: No.5）

Experiment Report for Prefectural Beamline
Proposal no. : 1310118PT
Beamline no.: BL12
Report date: 12/10/2013

XPS analysis of Li-O 2 electrochemical reaction product for Li-O 2battery
Hye Ryung Byon, Nadège Bonnet Mercier
Byon Initiative Research Unit, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama
351-0198 Japan※１Long term users of Advanced Research User Proposals are requested to submit thisreport corresponding to each research term (I), (II), or (III), specifying each researchterm at the end of the Report Title.

※2Users of Non-proprietary proposals (Public User Proposal, Advanced Research User
Proposal with exception of both Trial Course and Industry-Academia Collaboration
Course, and Frontier Research User Proposal) are also requested to publish their resultsin the form of a refereed journal article or to submit their results in the form of aSAGA-LS Research Report refereed by SAGA-LS within two years from the end of thecurrent fiscal year．

１ ． Summary（ Note: Please include conclusions）
We report Li–O 2 electrochemical reaction products for reduction and oxidation with respect topotential, and for 5-times cycles on nanoporous gold electrode. Synchrotron X-ray
photoelectron spectroscopy (XPS) data reveals a main product of Li 2 O 2 during reduction,
which is completely decomposed after oxidation while new side products such as Li 2 CO 3 andlithium carboxylates are also produced. After 5 cycles, significant side-products are alsoobserved.
２．Purpose２．
of experiment and background
The Li-O2 battery is received great attention to apply for long-range-driving electric vehicles in the near futureowing to its high specific energy density (~3 kWh kg–1cell), which is more than 10 times higher than lithium ionbattery. The Li-O2 battery can be operated by O2 gas reduction and oxidation reactions, which produce
insoluble Li2O2 (2Li+ + O2(g) + 2e-  Li2O2(s)) and decomposition of Li2O2 (Li2O2(s)  2Li+ + O2(g) + 2e-),respectively. However, this Li-O2 electrochemical reaction is accompanied by parasitic reactions as aprotic
organic electrolyte and carbon electrode react with superoxide radical (O2-) as the reduced form of O2 gas,
which results in side products such as Li2CO3 and lithium carboxylates. Accordingly, one of the significant
challenges in the Li-O2 battery is to suppress the parasitic side reactions by using of supreoxide-tolerated
electrolyte and carbon-free electrode.
Here we investigate the reduction and oxidation products of Li-O2 reaction using nanoporous

gold (NPG) electrode and tetraethylene glycol dimethyl ether (tetraglyme) electrolyte usingsynchrotron XPS at BL12 of SAGA-LS. The combination of carbon-free electrode and electrolyte
is expected to produce less side products while acting as a platform to form and decomposethe predominant Li2O2 after reduction and oxidation. In particular, because the products formedon the NPG have very small quantity, light X-ray source of XPS can aid in identifying the chemical
information of thin product film.
３．Experimental（Note:
Description of sample，method of experiment and analysis, etc.）
３．
We prepared following 5 NPG samples via potential-controlled cyclic voltammetry (CV) with ametallic Li as the counter and reference electrodes (table). The clean NPG electrodes wereswept in 0.5 M LiTFSI (lithium bis(trifluoromethanesulfonyl)imide)) in O2-saturated tetraglyme
for oxygen reduction reaction (ORR) at 2.4 V and 2.0 V and for oxygen evolution reaction (OER)
at 3.8 V and 4.5 V, separately, at a sweeping rate of 1 mV s–1. The NPG electrodes were then washedout using dimethyl carbonate in an Ar-filled glove box and dried in vacuum chamber at 60 oC
for 6 h.
No.

Sample name

Description

1

NPG @OCV

Soaking

Preparation
into

Potential controlled electrochemicalelectrolyte

sweep, washing with anhydrous dimethyl2NPG@2.4V

Shallow reductioncarbonate (DMC) in an Ar-filled glove

3

NPG@2.0V

Deep reduction

box, followed by drying in vacuum chamber

4

NPG@3.8V

Shallow oxidation

at 60 oC for 6 h without air exposure.

5

NPG@4.5V

Deep oxidation

All samples were transferred to XPS chamber using hermetic Ar-filled transfer vessels to avoid air exposure.
The synchrotron XPS measurements were performed using a photon energy of 650 eV and a basepressure of 10–8 Pa. Considering the escape depth of the photoelectron, the excitation energy
was selected to measure surface sensitive spectra, which allows us to obtain top thinner surface
information on the NPG electrodes. The XPS peaks were calibrated by Au 4f5/2 and Ag 3d3/2 and3d5/2 peaks from the as-prepared NPG. The peak intensity was normalized as referring to theinjection current (I0). The pass energy is 117.4 eV for survey and 23.5 eV for Li, O, and C
1s binding energy (BE) region. The scanning numbers are 100 times for Li 1s, 75 times for O
1s and 10 times for C 1s region with a time step for 50 ms.
４．Resultsand Discussions
４．
To investigate products during the Li–O2 electrochemical reaction, four NPG electrodes were prepared at
different reduction and oxidation stages: ORR at 2.4 (ORR–2.4V), 2.0 V (ORR–2.0V) and OER at 3.8 V(OER–3.8V), 4.5 V (OER–4.5V). Figure 1 shows synchrotron XPS spectra of survey, C 1s, O 1s, and Li 1s.
The survey spectra demonstrate the decreasing Au 4f7/2, 4f5/2 (84, 88 eV) and Ag 3d5/2, 3d3/2 peak intensities(368, 374 eV) upon the ORR, which totally disappear at 2.0 V due to deposition of Li–O2 reduction product onthe NPG. On the contrary, the Au and Ag peaks increase upon the OER as the oxidation (decomposition)
occurs. In the C 1s BE region, all NPG electrodes have identical carbon-related peaks (285.5 eV) deconvoluted
to sp3 hybridized carbon (285.2 eV) and C–O (286.1 eV). These peaks may arise from the tetraglyme residue
physically adsorbed. The O and Li 1s spectra reveal the predominant Li–O2 product of Li2O2. The O 1s and Li
1s peak at 532.4 and 56.3 eV, respectively, for the ORR–2.4V and ORR–2.0V correspond to those on the Li2O2

powder measured under the same condition (proposal no. 1310119R). Along with the Li2O2, the small tails
over 533.5 eV in asymmetric O 1s peaks on the ORR NPG electrodes demonstrate the presence of Li2CO3(proposal no. 1310119R) despite its insignificant quantity. The Li2O2 is complete decomposed after theoxidation as revealed by negligible Li 1s peak for the OER–4.5V. However, there is still significant carboxylate
and carbonate peaks (289.1 and 291.6 eV) in the C 1s BE region and a long tail of Li2CO3 (over 533.5 eV) inthe O 1s BE region. These Li2CO3 and lithium carboxylates presumably arise from the oxidized tetraglyme
over 4 V.

Figure 1. XPS spectra of NPG electrodes with different reduction (ORR) and oxidation (OER) stages insurvey, C 1s, O 1s, and Li 1s BE regions.

Over 5 time cycles, the side products, i.e., Li2CO3 and lithium carboxylates, are more significant. Figure 2
shows CV curves of 5 cycled NPG electrode and the corresponding XPS spectra. The ORR and OER peaks inthe CV curves decrease during 5 cycles due to the deposition of products, which cannot be decomposedcompletely. The XPS spectra of the 5-cycled ORR (ORR-5C) reveal largely increasing C, O, and Li 1s peaks incomparison with those of the 1st-cycled result (Figure 1). In particular, the additional peak at 289 eV in the C
1s BE region and the broad peak in the O 1s BE region demonstrate notable side products in addition to theLi2O2. After the OER at 4.5 V (OER-5C), all peak intensities decrease due to decomposition of products athigh oxidation potential while the small side products still remain.

Figure 2. CV (left panel) and XPS in survey, C 1s, O 1s, and Li 1s BE regions for 5 times-cycled NPG.

５．Future
issues５．
None

６．References
６．
(1) R. Wen, M. Hong and H. R. Byon, J. Am. Chem. Soc., 2013, 135, 10870–10876.
(2) D. M. Itkis, D. A. Semenenko, E. Y. Kataev, A. I. Belova, V. S. Neudachina, A. P. Sirotina, M. Hävecker, D.
Teschner, A. Knop-Gericke, P. Dudin, A. Barinov, E. A. Goodilin, Y. Shao-Horn and L. V. Yashina, Nano Lett.,
2013, 13, 4697–4701.
(3) S. Kundu, Y. Wang, W. Xia and M. Muhler, J. Phys. Chem. C, 2008, 112, 16869–16878
７．Publications,patents（Note: Typical deliverables related to this proposal．）
７．
Publication: Rui Wen and Hye Ryung Byon, “In situ monitoring of the Li-O2 electrochemical reaction on
nanoporous gold using electrochemical AFM”, Chemical Communications, 2014, 50, 2628-2631.
８．Keywords（Note:2-3 words about samples and experimental methods．）
８．
Li-O2 battery, nanoporous gold, XPS

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the publication of research results
９．
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