Elucidating Mixed-Ion Conduction in Spinel Cathode Materials for Mg Batteries

Abstract

The magnesium battery (utilizing the reversible reactivity of Mg2+) offers the potential to surpass the Li-ion battery in terms of energy-density and cost, thereby enabling next-generation devices such as lower-cost and higher-range electric vehicles.1 A crucial component defining the energy density of the Mg battery is the cathode, which must display reversible Mg intercalation and redox chemistry at sufficiently high capacities and potentials. Of all known cathode materials, the Mg spinels MgM2O4 (where M = Cr, Mn, V) have conclusively demonstrated high capacity and high potentials vs Mg2+/Mg.2–5 The spinel structure possesses 3D networks of diffusion channels, with a combination of theoretical and experimental studies suggesting relatively low barriers for Mg2+ diffusion.5,6 However, reversible Mg2+ intercalation kinetics have proven to be extremely inhibitive in oxide cathode materials in general, with large overpotentials (and voltage hysteresis) observed on charge and discharge. While the origin of this effect is still a matter of debate, it has been suggested that sluggish bulk transport of Mg2+ can partially account for the hysteresis observed, although surprisingly little is known about the nature of ion transport (both electrons and Mg2+) in these systems.In this presentation, we discuss a combination of experimental and theoretical tools used to extract key ion and electron transport properties across a range of Mg-containing spinels, Mg(Cr1−x M)2O4, where M = Mn, V, Ti. We reveal the structure-composition-property relationships that emerge across this range of compounds and discuss the relative and cooperative role that ion and electron motion play in limiting conduction kinetics in these materials. This study represents a detailed treatment of bulk ion and electron transport in these systems, and provides analytical and theoretical tools to further study electrode materials for such next-generation energy storage devices.