Photon Factory Activity Report 2014 #32 (2015) B

BL-9C/2014G055

A Study on Reaction Mechanisms by X-ray Absorption Spectroscopy in NaxNi1/3Mn2/3O2
as Positive Electrode Materials for Sodium-ion Batteries
Kei Kubota, Issei Ikeuchi, Koichi Hashimoto and Shinichi Komaba*
Department of Applied Chemistry, Tokyo University of Science,
1-3 Kagurazaka, Shinjuku, Tokyo 162-8601 Japan

3 Results and Discussion
From X-ray absorption near edge structure(XANES) spectroscopy of P3- and P2-type Na2/3x[Ni1/3Mn2/3]O2 at Ni K-edge, energy shift toward higher
energy was observed after charge to 4.5 V and reversibly
shifted back by discharge to 2.0 V, which indicates that
Ni was oxidized and reduced during charge and discharge
processes for both materials. Next, XANES spectra at
Mn K-edge during charge to 4.5 V for P3- and P2-phases
are shown in Fig. 1(a) and (b), respectively. No energy
shift was observed before and after the charge to 4.5 V in
P3-type phase, suggesting that Mn does not participate in
the redox reaction during charge and discharge processes

Normalaized Absorption / a.u.

2 Experiment
Composite positive electrodes consisted of 80 wt%
active materials, 10 wt% acetylene black, and 10 wt%
polyvinylidene fluoride (PVdF), coated on aluminum foil
as a current collector. The electrolyte solution used was 1
mol dm-1 NaPF6 dissolved propylene carbonate (Kishida
Chemical Co.Ltd., Japan). Electrochemical testing wasconducted using R2032-type coin cells with Na metal as anegative electrode. The coin cells were assembled in an
Ar-filled glovebox and cycled at a rate of C/20. After theelectrochemical tests, composite electrodes were takenout from the cells, and the electrodes were rinsed with
diethyl carbonate (DEC) and sealed in a water-resistantpolymer film in the Ar-filled glovebox.

for P3-type phase. On the other hand, the spectrum ofP2-type phase shifted toward higher energy after charge
to 4.0 V. However, no shift was observed after furthercharge to 4.5 V. These results indicate existence of Mn3+
in initial composition of P2-type phase and the reversible
redox reaction during charge and discharge even though
Mn was expected to exist as tetravalent ion in initial
composition of P2-type Na2/3[Ni1/3Mn2/3]O2. In spite of
the same composition between P3 and P2 phases, valenceof manganese ion before electrochemical Na extraction isdifferent, which would influence on the electrochemical
properties as positive electrode materials for Na-ionbatteries. Consequently, the difference in electronic stateof manganese ions in between P2- and P3-type phases isfound by XAS study. To elucidate the charge/discharge
mechanism between P3- and P2-type Na2/3[Ni1/3Mn2/3]O2,
further detailed analysis on XAS, especially EXAFS
combined with ex-situ XRD, is required.

:pristine:4.0 V ch:4.5 V ch:after 1 cycle
6550

6560

6570

Energy / eV

Normalaized Absorption / a.u.

1 Introduction
Na-ion batteries are promising candidates as a next
generation energy storage device because of the materials
abundance and high energy performance. However,averaged operating potential of NaxMeO2 is usually lower
than that of LixMeO2, resulting in lower energy density
compared with that of Li-ion system. Among the layeredoxide cathode materials, P2-type Na2/3[Ni1/3Mn2/3]O2 is apromising candidate as a high potential positive electrodematerial because of the operating potential of 3.6 V vs.
Na/Na+ based on the Ni2+/4+ redox couple.[1] This
material has another crystal polymorph of P3-type layeredoxide reported as a low temperature phase.[2] In thesematerials, valence of Ni and Mn is expected to divalentand tetravalent, respectively.
P3-type material is,therefore, also expected to exhibit high operating
potential. In this study, we examined the electronicstructure of transition metals in P3- and P2-type Na2/3during charge/discharge by X-rayx[Ni1/3Mn2/3]O2
absorption spectroscopy (XAS) and revealed relationshipbetween crystal structure and electronic structure.

pristine
4.0 V ch
4.5 V chafter 1 cycle
6550

6560
Energy / eV

6570

Fig. 1 XANES spectra of (a) P3- and (b) P2-type Na2/3x[Ni1/3Mn2/3]O2 at Mn K-edge during 1st cycle.

Photon Factory Activity Report 2014 #32 (2015) B

References[1] Z. Lu and J. R. Dahn, J. Electrochem. Soc., 148(11),A1225 (2001).
[2] Z. Lu, J. R. Dahn et al., Chem. Mater., 12, 3583
(2000).
* komaba@rs.kagu.tus.ac.jp