A critical review of inorganic cathode materials for rechargeable magnesium ion batteries

Rechargeable magnesium ion batteries (RMIBs) have received growing attention due to their advantages of abundant resources, low price, environmental friendliness, and high volumetric capacities. However, the highly polarized Mg2+ ions tend to interact with cathode materials, resulting in the sluggish diffusion kinetics. Therefore, design and synthesis high-performance cathode materials are crucial to the development of RMIBs. Most organic cathode materials for RMIBs not only face the problems of poor intrinsic conductivity and low density of the active material, but also are easily soluble in organic electrolytes and have complex redox mechanisms. At present, most of the research on cathodes for RMIBs is inorganic materials. In recent years, researchers have devoted great effort to the design of efficient cathode materials for RMIBs and the reported cathode materials mainly include transition metal oxides, transition metal sulfides, transition metal selenides, polyanionic compounds, and so on. In this review, we provide an extensive overview of different types of inorganic cathodes for RMIBs, with emphasis on describing the structural characteristics, the charge storage mechanisms, electrochemical properties and optimization ideas of different types of cathodes, highlighting some common strategies currently used to improve the electrochemical magnesium storage performance of cathodes. The aim of this widely covered review is to present important research works on different types of inorganic materials when used as cathodes for RMIBs, to summarize the challenges they face and the future directions. This is a departure from articles with an in-depth study of a particular branch. We hope to promote more research efforts on RMIBs in the future.