Development of cathode materials for rechargeable magnesium batteries: From intercalation to enolization

In the past decade, magnesium batteries have been pursued as potentially low-cost, high-energy, and safe alternatives to Li-ion batteries due to the use of earth-abundant, high-capacity, and dendrite-free Mg anodes. However, strong interactions of Mg2+ with electrolyte solutions and cathode materials lead to sluggish ion dissociation and diffusion in cathode materials, resulting in low capacity and sluggish kinetics in intercalation cathode materials. In this article, we review the development of cathode materials for Mg batteries, guided by a better understanding of electrolyte-electrode interactions and the strong influence of electrolyte solutions on cathode storage mechanisms. We first examine the challenges of Mg2+-storage cathodes based on intercalation chemistry. We then discuss cathode materials that store complex ions due to complications arising from electrolyte solutions. We further review conversion-type and enolization-type cathode materials, which bypass cation dissociation and solid-state ion diffusion altogether. The goal of this work is to provide readers with an overview of cathode material design strategies that have enabled genuine Mg2+ storage with competitive performance.