Junichi Inamoto

<jats:title>Abstract</jats:title><jats:p>In this review, fundamental aspects of the electrochemical intercalation of anions into graphite have been first summarized, and then described the electrochemical preparation of covalent‐type GICs and application of graphite as the cathode of dual‐ion battery. Electrochemical overoxidation of anion GICs provides graphite oxide and covalent‐fluorine GICs, which are key functional materials for various applications including energy storage devices. The reaction conditions to obtain fully oxidized graphite has been mentioned. Concerning the application of graphite for the cathode of dual‐ion battery, it stably delivers about 110 mA h g<jats:sup>−1</jats:sup> of reversible capacity in usual organic electrolyte solutions. The combination of anion and solvent as well as the concentration of the anions in the electrolyte solutions greatly affect the performance of graphite cathode such as oxidation potential, rate capability, cycling properties, etc. The interfacial phenomenon is also important, and fundamental studies of charge transfer resistance, anion diffusion coefficient, and surface film formation behavior have also been summarized. The use of smaller anions, such as AlCl<jats:sub>4</jats:sub><jats:sup>−</jats:sup>, Br<jats:sup>−</jats:sup> can increase the capacity of graphite cathode. Several efforts on the structural modification of graphite and development of electrolyte solutions in which graphite cathode delivers higher capacity were also described.</jats:p>

It has been reported that lithium-rich cathode materials of LIB emit singlet oxygen during charging, which chemically oxidizes electrolyte solutions, and the decomposition products form surface film on the material. However, the detailed conditions and mechanism of the surface film formation and its effect on the electrochemical reaction at the electrode/electrolyte interface have not been clarified in detail. In this study, using 0.5LiCoO₂ • 0.5Li₂MnO₃ thin-film electrodes as the model electrodes of the lithium-rich cathode materials, the surface film formation behavior was investigated. After a constant current-constant voltage (CCCV) measurement to 4.8 V, passivation of the electrodes did not occur. On the other hand, the electrode after cyclic voltammetry (CV) up to 4.8 V showed complete passivation. The results of spectroscopic analyses revealed that decomposition products of the solvent formed thick surface film on the electrode after CV. From the results, it was concluded that the passivation surface film was formed by the simultaneous decomposition of the solvent via electrochemical oxidation at high potentials and chemical oxidation by singlet oxygen. Furthermore, the electrode with the surface film showed better cyclability than that without the surface film, indicating that it contributes to the suppression of side reactions at the electrode/electrolyte interface.

Dual carbon batteries have recently attracted significant attention because of their ecofriendliness and reliability. In this study, graphene-like graphite (GLG) was prepared by thermal reduction of graphite oxide to be used as a cathode material, and the electrochemical PF₆⁻ anion-intercalation reaction into GLG was investigated. Decreasing the heat-treatment temperature of GLGs from 900 °C to 600 °C resulted in increasing the reversible capacities and interlayer distances of GLG samples. Among them, GLG synthesized at 700 °C (GLG700) showed the largest discharge capacity of 137 mAh g⁻¹, which was much larger than that of graphite (52 mAh g⁻¹). Variations in the X-ray diffraction patterns and Raman spectra of GLG700 indicated that the stage number reached 1 at 4.8 V (vs Li⁺/Li) while that of graphite was 2 at the same potential. This indicates that GLG could store PF₆⁻ anion in every interlayer, which is probably one of the main causes of the larger capacity. The charge–discharge cycling test of GLG700 showed that the capacity gradually increased during cycling, and the coulombic efficiency was approximately 97% at every cycle after the 5th cycle. These results clearly demonstrate that GLG can be used as a cathode material with a large capacity for dual carbon batteries.