Tuning Anionic Chemistry To Improve Kinetics of Mg Intercalation

Rechargeable Mg batteries (RMBs) hold great promise for high energy density in consumer electronics because of the high theoretical volumetric capacity and dendrite-free stripping/plating of Mg. Their development, however, is severely limited by the sluggish diffusion of Mg2+ in inorganic cathodes, mainly induced by strong interaction between Mg2+ and host anions. In this paper, for the first time, we systematically investigate how anionic chemistry (O, S, and Se) affects Mg2+ migration in layered MX2 (M = Ti and V; X = O, S, and Se), by combining theoretical density functional theory (DFT) calculations with electrochemical characterizations. At room temperature, TiSe2 and VSe2 achieve much better electrochemical performance than TiS2 and VS2, due to the faster migration of Mg2+ in selenide than in sulfide and oxide, as demonstrated by electrochemical kinetic characterizations and DFT calculations. The improved kinetics of selenide can be attributed to three criteria: (i) larger diffusion channel, (ii) weaker interaction between Mg2+ and anion lattice, and (iii) higher electronic conductivity. Finally, the three criteria might not only be applicable to layered materials but also be generalizable to materials with other structures, like Chevrel phase, spinel, olivine, etc., which paves the way for the future design and modification of intercalation materials for RMBs.