Utilization of Delithiation-Induced Amorphous Oxide as a Cathode for Rechargeable Magnesium Batteries

Realizing rechargeable magnesium batteries (RMBs) requires cathode materials, into and from which Mg cations can be inserted and extracted near room temperature. Oxide materials are advantageous due to their high electrode potential. However, activating oxide cathodes near room temperature is challenging due to the sluggish Mg diffusion in oxides, which is attributed to the inherently strong affinity between Mg and other constituent elements. Herein, we show that an amorphous oxide cathode containing Ti and Mo enables reversible Mg insertion/extraction near room temperature (60 °C). Li extraction from Li2Ti1/3Mo2/3O3 induces a phase transformation from a layered rock-salt structure into a composite of a major amorphous oxide with a minor disordered rock-salt oxide. This amorphous oxide yields a reversible capacity of up to ∼160 mAh g–1 at a current density of 5 mA g–1. Multimodal analyses reveal that the major amorphous phase accommodates Mg by means of the redox reaction of the constituent Mo4+/6+. This amorphous phase can be further classified into Mg-poor and Mg-rich amorphous phases, whose phase fractions change during charging and discharging, i.e., the two-phase reaction between the distinct amorphous phases. The large free volume introduced by delithiation, i.e., the remnants of the Li vacancies, in the amorphous structure is considered to flatten the energy landscape for Mg hopping, facilitating Mg diffusion and eventually enabling operation at a relatively low temperature of 60 °C. The present study provides a new opportunity for utilizing amorphous phases for designing novel cathode materials for RMBs.