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Magnesium bis(trifluoromethanesulfonyl)amide (Mg(TFSA)2)/glyme solutions are promising electrolytes for rechargeable magnesium batteries, but the passivation of the Mg electrode because of the decomposition of [TFSA]− anion from the coordination state is a critical issue in practical use. In the present study, functionalized nanofiber gel electrolytes swollen in 0.5 M Mg(TFSA)2/triglyme solution were prepared using the silica nanofibers modified with oxymagunesium chloride (–OMgCl) groups (f-SiO2NFs) and used them as a gelator. The f-SiO2NF gel electrolyte improved the Mg plating/stripping reaction compared to the SiO2NF gel without –OMgCl groups.

The electrochemical N(CF3SO2)2− (TFSA−) anion intercalation into graphite was studied, aiming at the use for Mg secondary batteries. In order to achieve both the Mg plating/stripping reaction and anion intercalation, the electrolyte solution consisting of Mg2+/glyme = 1 (mol) with hydrofluoroether (HFE) as the diluent was used. The Raman spectrum indicated that all glyme solvated Mg2+-ion, which led the improvement of anti-oxidation durability up to 4.5 V (vs. Mg2+/Mg). The TFSA− anion intercalation took place in the solution with almost 100% of coulombic efficiency. Besides, the charge-discharge test exhibited the discharge plateau over 3.4 V (vs. Mg2+/Mg) with the capacity of ca. 70 mA h g−1. The results indicated the anion intercalation into graphite will be the attractive candidate for Mg secondary batteries.

Mg plating/stripping reaction in Mg[N(CF3SO2)2]2/glyme based solution is studied by electrochemical quartz crystal microbalance method. During the cyclic voltammetry, the apparent mass decrease is observed in spite of the negative scan. The irregular response also appears in the Mg plating reaction with low constant current density apply. In the cases, Mg plating takes place locally and the size of each plating is relatively large of about 50 µm. The cross-sectional image of the plated Mg is tree-like structure, i.e., the large Mg crystal connects to the substrate with small contact area. From the results, we conclude that the specific Mg morphology causes the restoring force to the quartz substrate, resulting in the apparent mass decrease.

Electrochemical anion intercalation/de-intercalation into the graphite electrode generally takes place at high electrode potential of over 4.5 V (Li+/Li), so it is attractive as the positive electrode reaction for the post lithium-ion batteries. In this study, the kinetic behavior of the anion intercalation reaction in the organic solution is studied. The interfacial activation energy of anion species is as low as about 25 kJ mol−1, which is much lower than that of Li+-ion. The result means the anion intercalation reaction is essentially fast and therefore, it is suitable for the high power batteries. In addition, the activation energy is not influenced by the solvation ability in the solution and from the result, we conclude that the interfacial activation barrier will exist on the graphite electrode side, contrary to the Li+-ion case.