Electrochemical intercalation of anions into graphite: Fundamental aspects, material synthesis, and application to the cathode of dual-ion batteries.

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-1 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 AlCl4 -, Br- 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.

Factors Affecting the Electrochemical Behaviors of Graphene-Like Graphite as a Positive Electrode of a Dual-Ion Battery.

Charge-discharge behaviors of various graphene-like graphite (GLG) samples have been investigated, and factors affecting them were discussed. By changing the oxidation method of graphite to obtain the precursor material of graphite oxide and heat treatment temperature of it, GLG samples with various structural parameters were successfully prepared. The onset potential of intercalation changed mainly depending on the interlayer spacing and decreased with its increase. The oxygen content was also important especially for GLG with smaller interlayer spacings. The influence of oxygen became apparent when the intercalation of bis(fluorosulfonyl)amide ions was performed. The onset potential considerably decreased for GLG with larger oxygen contents. The capacity of GLG increased with the increase in oxygen content to reach a maximum value of 149 mAh g-1 and then slightly decreased.

Electrochemical Introduction/Extraction of Fluoride Ions into/from Graphene-like Graphite for Positive Electrode Materials of Fluoride-Ion Shuttle Batteries.

A cathode material, graphene-like graphite, was developed for all-solid-state-type fluoride-ion shuttle batteries (FSBs). Fluoride ions were electrochemically introduced/extracted into/from it, and covalent C-F bonds were formed upon electrochemical oxidation. The introduction of fluoride ions into it occurred at a lower voltage than that into graphite. While the layered structure of graphite was completely destroyed during charging, that of graphene-like graphite was still maintained to some extent. The discharge voltage was higher than 1 V versus Pb/PbF2, which was higher than that of most of the previously reported cathode materials. The first discharge capacity (161 mAh g-1) was larger than that of graphite (140 mAh g-1), and the Coulombic efficiency and cyclability were much higher. This work demonstrates that graphene-like graphite prepared by thermal reduction of graphene oxide at 300 °C, GLG300, is a promising material for positive electrodes of FSBs.