New electrolyte solutions for rechargeable magnesium metal batteries were studied. The electrolyte solution composed of triethylene glycol dimethyl ether (triglyme) and butyl methyl triglyme as solvents and magnesium(II) bis(trifluoromethanesulfonyl)amide as a magnesium salt. The reversible deposition and dissolution of magnesium metal was achieved in these electrolyte solutions at ambient temperature. The electrochemical stability of triglyme-based electrolyte solutions showed high stability toward oxidation.

Graphene-like graphite (GLG), a carbon material synthesized by heat treatment of graphite oxide, can be charged at a high rate without deposition of lithium metal even when the potential of the GLG anode is stepped below the Li deposition potential. However, the factors contributing to this outstanding rapid chargeability of GLG have been unclear until now. In this study, to clarify the factors, the charge-transfer resistance and diffusion resistance of GLG were quantitatively evaluated by the electrochemical impedance method using GLG thin films as model electrodes. The activation energy of lithium-ion transfer at the GLG/electrolyte interface was found to be almost the same as that reported for graphite, and it was clear that this process was as slow as that in graphite. On the other hand, the lithium-ion diffusion coefficient in GLG calculated from the Warburg impedance was several orders higher than that of graphite, which clearly shows that this contributed greatly to the fast-charging characteristics of GLG. In addition, in the comparison among GLGs with different structural parameters, the lithium-ion diffusion coefficient was higher for those with less oxygen content and more pores formed in the graphene layer inside the GLG. Therefore, it was concluded that a high diffusion coefficient was ascribed to the pores in the GLG, which enabled lithium-ion diffusion in the c-axis direction.

For the development of lithium-ion batteries (LIBs) for electric vehicles, the reduction in the internal resistance of LIBs is strongly required. On the graphite negative electrodes, solid electrolyte interphase (SEI) is inevitably formed and causes partly the internal resistances. In addition, SEI covers the surface of graphite negative electrodes and affects the active sites for lithium-ion intercalation/deintercalation reactions. In this study, we investigated the influence of SEIs derived from vinylene carbonate (VC), fluoroethylene carbonate (FEC), and ethylene carbonate (EC) on the active sites for lithium-ion intercalation/deintercalation at highly oriented pyrolytic graphite (HOPG). We clarified the relation between the standard rate constant (k 0) of [Ru(NH 3) 6 ] 3+/2+ that is a parameter for the edge site of graphite and the interfacial lithium-ion transfer resistance (R ct) in various electrolyte solutions that deliver different SEIs. In the plots of k 0 vs. R ct −1 , there is a positive linear correlation between these two parameters, and the slopes increased in the order VC < EC < FEC. Additionally, the activation energy for the interfacial lithium-ion transfer remained unchanged despite the variation in SEIs. Based on these results, we conclude that SEIs affected the frequency factor of Arrhenius equation for the interfacial lithium-ion transfer on graphite.

The performance of the graphite anode of lithium-ion batteries is greatly affected by the solid electrolyte interphase (SEI) generated at the first charge. However, there are few studies on the kinetics of the lithium-ion intercalation/de-intercalation reaction in graphite to investigate the effect of SEI. In this study, the correlation between the interfacial lithium-ion transfer resistance (R ct ) and the double layer capacitance (C dl ) of graphite composite electrodes coated with various SEIs was investigated. It was found that the value of 1/R ct C dl was different for each SEI, that is, the frequency (or rate) of intercalation and de-intercalation of lithium ions into graphite was different for each SEI. The activation energy of R ct was almost the same for all the electrolyte solutions. These results indicate that the pre-exponential factor of the Arrhenius equation governing the rate of interfacial ion transfer in a practical graphite anode is dependent on the nature of SEI.

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.