Cathode Materials for Rechargeable Magnesium-Ion Batteries: A Review

Abstract: Using renewable energy sources such as wind, solar, and tidal power is one of the most effective ways to address the global energy crisis and the ensuing environmental issues. As essential complementary components to renewable energy, high-performance energy storage devices and systems are urgently required. Since the 1990s, the global battery market has been dominated by lithium-ion batteries (LIBs) owing to their high energy density and long cycle life. They have been widely used in portable electronics, and more recently, in electric vehicles. However, lithium resources are limited and unevenly distributed; therefore, the manufacturing costs of LIBs are still high. There is also increasing concern about their operational safety. Thus, it is crucial to develop next-generation battery technologies with lower costs and higher safety. In recent years, magnesium-ion batteries (MIBs) have attracted increasing attention as one of the most promising multivalent ion batteries. The use of magnesium is encouraged owing to its good air stability, lower reduction potential (−2.356 V vs. standard hydrogen electrode), higher volumetric specific capacity (3833 mAh∙cm−3), and dendrite-free deposition upon cycling. Moreover, magnesium reserves (2.3%) are 1045 times more than those of lithium (0.0022%), because of which, MIBs are considerably less expensive than LIBs. The development of MIBs has, however, encountered a few challenges arising from the comprising cathodes, electrolytes, and anodes. Mg2+ ions with smaller radii and higher charge densities have strong Coulomb interactions with electrode materials, which leads to sluggish kinetics and high diffusion barriers during de-/intercalation. Contemporary electrolytes generally have poor chemical compatibility with cathodes of MIBs, narrow electrochemical windows, and high deposition overpotential, which limits the development of high-voltage MIBs. Moreover, Mg tends to react with organic solvents (especially carbonates and nitriles), forming passivation layers on the surfaces, which increase the interfacial resistance and lead to battery irreversibility. Therefore, material design and technological innovation are crucial for developing commercially viable MIBs. This review focuses on recent advances on MIB cathode materials. First, we present a brief description of the characteristics of MIBs and discuss their strengths and drawbacks. Then, we overview three types of cathode materials, namely, intercalation-type cathodes, conversion-type cathodes, and organic cathodes, followed by a summary of their limitations and recent efforts for addressing the above-mentioned challenges. We conclude with perspectives for future research directions.