Electrochemical Characteristics of Anion-Redox Type Cathode Materials with High Specific Capacity
Authors: Okuda Daisuke , Kobayashi Hiroaki , Ishikawa Masashi 
Published: December 2020

1. Introduction
In recent years, high energy-density lithium-ion batteries have been used for power sources such as hybrid and electric vehicles and for home energy storage systems. However, with increasing performance of equipment, energy storage devices with higher energy density than conventional lithium-ion batteries are required. Co-doped Li2O (CDL), which utilizes the redox reaction of oxygen for charge compensation, has been investigated as such a material. In this material, Co plays a role in improving the electrochemical activity of oxygen; CDL has a high specific capacity of about 450 mAh g-1, but there is a problem of the active material decomposition when overcharged. In this work, Me-Li2O-MOX (Me: transition metal, M: Al and Si) was investigated to suppress the degradation reaction of active materials by simultaneously dissolving oxides such as Al2O3 without 3d orbitals and transition metal oxides with 3d orbitals in Li2O.
2. experimental
α and β-Li5AlO4(α and β-LA) were synthesized by a sol-gel method. Co-substituted α and β-Li5AlO4 (α and β-CSLA) were prepared via mechanical alloying of LiCoO2 (LCO) with α- and β-LA, respectively, in a 1:1 molar ratio as follows. The precursor mixture was sealed in a tungsten carbide (WC) pod with WC balls in an Ar-filled glove box (Pure box, Yamato Co., Ltd., Osaka, Japan), and then planetary ball milling (Premium Line P-7, Fritsch) was conducted at 300 rpm for 36 h to pulverize the oxides. The local structures of the obtained materials were measured by X-ray absorption spectroscopy (XAS) at SR center in Ritsumeikan University. The electrochemical characteristics were evaluated by charge–discharge cycling tests. The active material (α- or β-CSLA) was mixed with acetylene black (AB, Denka black, Denka Co., Ltd., Tokyo, Japan) and polyvinylidene difluoride (PVDF, #1100, Kureha Co., Ltd., Tokyo, Japan) in a 70:20:10 ratio by weight to form a paste, which was then applied to an Al foil using a doctor blade casting method and dried under vacuum at 60 °C for 12 h to obtain a working electrode. The working electrode, a Li metal sheet (the counter electrode), the electrolyte, and a polyolefin film separator (ND525, Asahi Kasei Co., Ltd) were assembled in a demountable coin-type cell (Hosen Corp., Osaka, Japan).
3. Results and Discussion
XAS measurements were performed to investigate the local structure of α and β-CSLA. We found a new bonding state of O, which was not observed in LiCoO2 and Li5AlO4 as raw materials. This is likely to be derived from the Co3+-O at tetrahedral coordination formed by Co-substitution to Li5AlO4 during the mechanical alloying. The pre-edge peak attributed to Co3+-O at the hexagonal coordination in the Co K-edge XANES spectra of α and β-CSLA suggests that α and β-CSLA contain unreacted LiCoO2.The charge-discharge curves of α and β-CSLA are shown in Figure 1. This shows that α and β-CSLA exhibit specific capacities of 200 and 250 mAh g-1 in the first cycle, respectively; the higher discharge capacities of β-CSLA by about 25% compared to α-CSLA may be due to the substitution of more Co in β-CSLA compared to α-CSLA.

Figure 1