Intercalation of Solvated Mg2+ into Graphite As a Negative Electrode
Authors: Yamamoto Keito , Nakahigashi Atsuhito , Arai Susumu , Shimizu Masahiro 
Published: December 2024

In the research on rechargeable Mg-ion batteries1−7 which are not limited by resource constrains, the development of negative electrode materials has been dominant on Mg metal due to its large theorical capacity per not only volume but weight. Apart from Mg metal, the reversible capacities of oxides are relatively small, despite their excellent cycling stabilities. On the other hand, althouth Sn and Sb based on alloy/dealloy reaction mechanim deliver large theoretical capacities, the large volumetric changes associated with alloying reactions with Mg disintegrate the electrodes. Meanwhile, little importance has been paid to graphite which has been used as a negative electrode material for Li-ion batteries. Mg ions are less likely to intercalate into the interlayer alone. The strong Lewis acidity hinders desolvation of the solvent molecules arround Mg ions, and thereby results in obstacles for the formation of a binary Mg-graphite intercalation compound (GIC), unlike Li-GIC. In this study, we reconsider the potential of graphite as a host material for Mg ions, including in view from ternary graphite interlayer compounds (t-GICs).8

Because the overpotential of deposition/stripping of Mg metal is relatively high, the active material could not be properly evaluated in electrochemical tests using a two-electrode cell, depending on the electrolyte used, and the potential of graphite as a negative material might have been overlooked. First, the structural changes in graphite during the application of reductive currents were evaluated by using ex-situ XRD. The electrolyte used in this study is 0.5 M Mg(TFSA)2/DMF. The formation of ternary graphite interlayer compounds (GICs) with a stage structure was demonstrated by the continual application of a constant current without considering the cut-off voltage to eliminate the influence of the counter Mg electrode. Considering the extended distance between interlayers (ca. 11 Å), the intercalated species were not Mg2+ alone but solvated Mg2+/[Mg(DMF)
x
]2+ (Figure 1a). The GICs formed at the charge state turned blue tint just like the GICs synthesized by a vapor method. Galvanostatic charge/discharge tests with a current density of 7.44 mA g–1 showed a reversible capacity of approximately 200 mA h g–1 (Figure 1b). Although the challenge of large polarization during deintercalation of solvated Mg2+ has remained, the result expands the potential of graphite as a negative electrode material for rechargeable Mg-ion batteries.
Acknowledgement
This work was supported by a Grant-in-Aid for Scientific Research (B) (20H02840) from the Japan Society for the Promotion of Science (JSPS).
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Figure 1