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 Mg 2+ in inorganic cathodes, mainly induced by strong interaction between Mg 2+ and host anions. In this paper, for the first time, we systematically investigate how anionic chemistry (O, S, and Se) affects Mg 2+ migration in layered MX 2 (M = Ti and V; X = O, S, and Se), by combining theoretical density functional theory (DFT) calculations with electrochemical characterizations. At room temperature, TiSe 2 and VSe 2 achieve much better electrochemical performance than TiS 2 and VS 2 , due to the faster migration of Mg 2+ 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 Mg 2+ 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.