The Charge/Discharge Properties of Vinylene Carbonate-Treated Cobalt-Doped Lithium Oxide Cathode for Lithium Peroxide Batteries
Authors: Kobayashi Hiroaki , Hibino Mitsuhiro , Ogasawara Yoshiyuki , Yamaguchi Kazuya , Kudo Tetsuichi , Okuoka Shinichi , Yonehara Koji , Ono Hironobu , Sumida Yasutaka , Mizuno Noritaka 
Published: February 2020

Introduction
We recently proposed a novel lithium-ion battery system based on a redox reaction between oxide ions and peroxide ions at a cathode, and demonstrated the system with a Co-doped Li2O cathode, in which Co ions were located at the same sites as Li+ ions.1 In these earlier studies, the Co-doped Li2O exhibited a specific capacity of 270 mAh g−1 with 50-cycle durability.2 However, the specific capacity of 270 mAh g−1 was much smaller than the theoretical specific capacity of 897 mAh g–1 on the basis of Li2O2 + 2Li = 2Li2O. In the present study, we aimed at improving the cell performance with the Co-doped Li2O cathode and clarifying the mechanism of reactions during charging/discharging.
Experimental
The sample was prepared by pulverization of mixture of Li2O and LiCoO2 with a planetary ball mill at 600 rpm for 10 h. The starting materials were set so that the atomic ratio (Co/(Co+Li)) in the mixture was 0.09, which had been optimized in our previous study.2 XRD analysis revealed that the prepared sample contained (Li0.82Co0.058□0.12)2O (Co-doped Li2O) and LiCoO2 at the molar ratio of 95:5, where □ represents a Li+-site vacancy. The sample was mixed with ketjen black and polytetrafluoroethylene at the weight ratio of 75:20:5 and typically 10 mg of the mixtures were pressed onto an Al mesh current collector. Thus fabricated electrode was immersed in vinylene carbonate (VC) for 1 h and dried in vacuum. The VC-treated cathode, Li metal as an anode, an electrolyte, and a glass-fiber filter as a separator were assembled in a 2032 coin-type cell. The electrolyte solution was 1 M LiPF6 dissolved in a mixture (1:1 by volume) of ethylene carbonate (EC) and dimethyl carbonate (DMC), with a 5vol% VC additive. The aforementioned operations were conducted in an Ar-filled glove box. Charge/discharge tests between 1.7 and 3.5 V were carried out at 25 °C in a constant-current mode at the current density of 50 mA g–1. Pressure change in a cell was monitored using a pressure gauge connected to the cell. The lower detection limit of the pressure was 5×102 Pa, which corresponded to 6 mmol per 1 mol of the empirical formula of Co-doped Li2O.
Results and discussion
Figure 1(a) shows voltage curves of the cathode during charge/discharge and pressure in the cell during charge. The charging voltage increased to 3.2 V at 0–150 mAh g–1, and reached a 3.2 V plateau region at 150–400 mAh g–1. Pressure started increasing at around 320 mAh g–1, indicating that an irreversible O2 evolution reaction occurred. During the following discharge, the voltage curve contained a plateau region at 2.7–3.1 V and a slope region at 1.7–2.7 V, and showed capacity of 410 mAh g–1. This capacity was higher than that of the preceding first charge, during which the irreversible O2 evolution reaction occurred. This is because the capacity by the reduction from Co3+ to Co2+ was greater than the irreversible capacity due to the O2 evolution. The capacity 410 mAh g–1 was much higher than our previous reported value 270 mAh g–1, which is due to the pretreatment with VC. Although the mechanism enhancing the specific capacity by the VC treatment has not been clarified, we confirmed from separately conducted experiments that VC was more effective as a pretreatment agent than an additive into an electrolyte.
After the first cycle, the charge/discharge was carried out with the charge depth reset to 410 mAh g–1, which was the capacity of the first discharge. Fig. 1(b) exhibits second charge/discharge profiles and pressure in the cell during second charge. The second charge curve differed from the first one; a slope region at around 2.3 V due to oxidation of Co2+ appeared. No change in pressure was observed in the whole region, indicating that the evolved O2 gas during the second charge was below the detection limit (5×102 Pa). The following discharge curve was similar to that of the first discharge. The coulombic efficiency (= discharge capacity/charge capacity) was 98.8%. The voltage curves for a multi-cycle test are shown in Fig. 1(c). While the charge/discharge curves present little changes in shape, the discharge capacity gradually decreased with cycles to be 400 mAh g–1at 10th cycle.
Acknowledgments
A part of this work was conducted with the support of JSPS Grant-in-Aid for Scientific Researches (B) Grant Number 26289371 and the support of JSPS Grant-in-Aid for Young Scientists (B) Grant Number 15K18326.
References
1. S. Okuoka et al., Sci. Rep., 4, 5684 (2014).
2. H. Kobayashi et al., J. Power Sources, 306, 567 (2016).

Figure 1