Photon Factory Activity Report 2017 #35 (2018)

BL-9C/2016G679

XAS study of intercalation mechanism of electrode materials
with the hollandite-type structure
Norihito KIJIMA1,* Kunimitsu KATAOKA1, and Mikito MAMIYA1
Advaced Industrial Science and Technology (AIST), Tsukuba, 305-8565, Japan

1

1 Introduction
Hollandite-type oxides have a tunnel structure
comprising double chains of MO6 (M=metal) octahedra
forming (2×2) tunnels, as depicted in Fig. 1. These large+(2×2) tunnels are privileged pathways for Li iondiffusion. Therefore, several oxides with hollandite-typestructure work as electrode materials for lithium ion
batteries [1,2]. We have recently succeeded in the+synthesis of the K -extracted hollandite-type KxTi0.9Nb0.1O2
specimens for the first time. In this work, we examined
the K extraction and Li intercalation mechanisms by Xray absorption spectroscopy (XAS).
2 Experiment
The hollandite-type specimens were prepared by thesolid state reaction. Mixtures of K2CO3, TiO2, and Nb2O5
powders were calcined at 1200 °C for 1 h under H2+atmosphere. To extract K ions form the tunnel structure,
the resultant specimens were then treated by piranhasolution at 80 °C for 4 h.
Composite working electrodes consisted of 80 wt %hollandite-type specimen, 15 wt % acetylene black, and 5wt % polyvinylidene fluoride, pasted on Al foil as acurrent collector. The electrolyte solution used was 1
3mol/dm LiPF6 dissolved in blended ethylene carbonate(EC) and diethyl carbonate (DEC) at a volumetric ratio of
1:1. Electrochemical testing was conducted using coincells with Li foils as counter electrodes, which wereassembled in an Ar-filled glovebox and cycled at a rate of
10 mA/g. After the electrochemical tests, composite
electrodes were taken out from the cell, and were thenrinsed with DEC and sealed in a gas barrier film in theAr-filled glovebox.
The XAS spectra of powder specimens andhermetically-sealed electrodes were measured bytransmission mode using synchrotron radiation at
beamline BL-9C of Photon Factory in KEK. The
computer program, ATHENA, [3] was used for the
analysis of the XAS data.
3 Results and Discussion
Figure 2 shows Ti and Nb K-edge X-ray absorption
near-edge structure (XANES) spectra of the KxTi0.9Nb0.1O2+
powders before and after K extraction. As Fig. 2 presents,+
XANES spectra were slightly shifted after K extraction.
+This finding shows that K ions in the tunnel space wereextracted by oxidation of both Ti and Nb rather than ionexchange mechanism.
Figure 3 presents Ti and Nb K-edge XANES spectra of+
the K0.03Ti0.9Nb0.1O2 electrode under initial Li insertiondeinsertion. As Fig. 3 shows, evidence of reduction to+lower valent Ti and Nb ions after Li insertion wasdetected by the XANES spectra. The mean valence stateof Ti at 1.0 V is between 3+ and 4+. On the other hand,the valence state of Nb at 1.0 V is about 4+ because thechemical shift of the spectrum was close to NbO2. After+Li deinsertion up to 3.0 V, the XANES spectra agreed
fairly well with initial spectra at OCV, which wereconsistent with the excellent charge-discharge property of
the hollandite-type TiO2 electrode.

Fig. 1: Crystal structure of the hollandite-type KxTiO2.

Fig. 2: (a) Ti K-edge and (b) Nb K-edge XANES spectraof the KxTi0.9Nb0.1O2 (x=0.2 and 0.03) powders before and+after K extraction.

Fig. 3: (a) Ti K-edge and (b) Nb K-edge XANES spectra+of the K0.03Ti0.9Nb0.1O2 electrode during the initial Li
insertion-deinsertion.
References[1] M. Sakao et al., Solid State Ionics 243 (2013) 22.
[2] N. Kijima et al., Solid State Ionics 262 (2014) 14.
[3] B. Ravel and M. Newville, J. Synchrotron Radiat. 12(2005) 537.
* n-kijima@aist.go.jp