X-Ray Absorption Spectroscopic Analysis of Cobalt-Doped Lithium Oxide Cathode Material for a Lithium-Peroxide Battery during Charge and Discharge
Authors: Ogasawara Yoshiyuki , Kobayashi Hiroaki , Hibino Mitsuhiro , Yamaguchi Kazuya , Kudo Tetsuichi , Asakura Daisuke , Okuoka Shinichi , Ono Hironobu , Yonehara Koji , Sumida Yasutaka , Mizuno Noritaka 
Published: February 2020

1. Introduction
Recently, cathode materials using redox of oxide anion draw increasing attention for development of rechargeable batteries with high energy density. We have proposed a new rechargeable battery system using a redox between oxide and peroxide ions in a cathode material.1 The theoretical energy density is 2570 Wh/kg (voltage: 2.87 V, specific capacity: 897 mAh/g), assuming that the redox between Li2O and Li2O2. We have demonstrated that the battery system with Co-doped Li2O (CDL) as a cathode material exhibits a reversible capacity of 270 mAh/g operating on the redox of oxide and peroxide ions.1,2 But this obtained capacity is far less than the theoretical one. Therefore, there must be room for enhancing the specific capacity. For further improving the battery system, deeper knowledge and better understanding of charge/discharge mechanism of the CDL are required.
X-ray absorption spectroscopy (XAS) is an effective method to investigate electronic states and coordination structures of elements of interest in a sample.1,3 In this study, we investigated the states of cobalt and oxygen in the CDL during charge and discharge by using both hard and soft XAS.
2. Experimental
The CDL (Co/(Li+Co) = 0.09) was prepared from Li2O and LiCoO2 by using a planetary ball-mill.2 The CDL was mixed with acetylene black and polytetrafluoroethylene at a weight ratio of 75:20:5 or 85:10:5 to serve as cathodes. The cathode material pressed onto aluminum mesh, lithium metal as an anode, an electrolyte solution of 1 M LiBF4 dissolved in a mixture (1:1 by volume) of ethylene carbonate (EC) and diethyl carbonate (DEC), and a glass-fiber filter as a separator were assembled in a 2032 coin-type cell. All these procedures were operated in an argon-filled glove box.
The cell was charged to 270 mAh/g and then discharged to 270 mAh/g at a current density of 4.5 mA/g at 298 K. The cathodes charged or discharged to various depths were removed from the cell, washed with EC-DEC once and with DEC twice, and dried under vacuum for XAS measurements.
The hard XAS measurements at Co K-edge were carried out at the beam line BL9A of the Photon Factory in High Energy Accelerator Research Organization in a transmittance mode. The samples were packed in laminated aluminum foil bags. The soft XAS measurements at O K-edge were carried out at the beam line BL2 of Ritsumeikan University SR Center in both total electron yield (TEY) and partial fluorescence yield (PFY) modes. The samples were transferred to high-vacuum chamber of BL2 by using a transfer vessel filled with argon gas.
3. Results and discussion
XAS spectra at Co K-edge were measured for the CDL charged and discharged in increments of 45 mAh/g. In the charge process, the absorption edge shifts to higher energy by 1.0 eV from 0 to 135 mAh/g and remains in almost the same position from 135 to 270 mAh/g. In the discharge process after charging to 270 mAh/g, the absorption edge gradually shifts to lower energy by 1.0 eV from 0 to 225 mAh/g and sharply shifts to lower energy by 1.3 eV from 225 to 270 mAh/g.
XAS spectra at O K-edge were measured for the CDL charged and discharged in increments of 90 mAh/g. In the charge process from 0 to 90 mAh/g, a broad peak at 528 eV, which is attributed to electron transition from O 1s to Co 3d-O 2p hybridized states, increases in its intensity. In the charge process from 90 to 270 mAh/g, a new peak appears at 530 eV. This peak is attributed to peroxide in CDL since Li2O2 shows a peak attributed to electron transition from O 1s to σ*(O-O) at the same energy. In the discharge process, the peak at 530 eV weakens and disappears from 0 to 180 mAh/g and the broad peak at 528 eV decreases in its intensity from 180 to 270 mAh/g.
These results suggest that the Co 3d-O 2p hybridized states contribute to charge transfer in the earlier charge period and the later discharge period. The formation and decomposition of peroxide species in the later charge period and the earlier discharge period, respectively, are spectroscopically traced, which are consistent with our previous results of chemical peroxide analysis.1

Acknowledgements
This work was supported by JSPS Grant-in-Aid for Young Scientist (B) Grant Number 15K18326 and JSPS Grant-in-Aid for Scientific Researches (B) Grant Number 26289371. The XAS measurements were carried out under the approvals of the Photon Factory Program Advisory Committee (Proposal No. 2015G698).
References
1. S. Okuoka et al., Sci. Rep., 4, 5684 (2014).
2. H. Kobayashi et al., J. Power Sources, 306, 567 (2016).
3. Y. Ogasawara et al., J. Power Sources, 287, 220 (2015).

Figure 1