Charge storage mechanisms of cathode materials in rechargeable aluminum batteries

Rechargeable aluminum batteries (RABs) have attracted great interest as one of the most promising candidates for large-scale energy storage because of their high volumetric capacity, low cost, high safety and the abundance of aluminum. However, compared with the aluminum anodes, the cathode materials face more problems including low specific capacity, relatively sluggish kinetics in most host structures and/or limited cycle lifespan, which pose the major challenge for RABs in further practical applications. During the past years, intensive efforts have been devoted to developing new cathode materials and/or designing engineered nanostructures to greatly improve RABs' electrochemical performances. In addition to nanotechnology-based electrode structure designs, the intrinsic chemical structures and charge storage mechanisms of cathode materials play an equally crucial role, if not more, in revolutionizing the battery performances. This review, here, focuses on current understandings into the charge storage mechanisms of cathode materials in RABs from a chemical reaction point of view. First, the fundamental chemistry, charge storage mechanisms and design principles of RAB cathode materials are highlighted. Based on different ion charge carriers, the current cathode materials are classified into four groups, including Al3+-hosting, AlCl4--hosting, AlCl2+/AlCl2+-hosting, and Cl--hosting cathode materials. Next, the respective typical electrode structures, optimization strategies, electrochemical performances and charge storage mechanisms are discussed in detail to establish their chemistry-structure-property relationships. This review on current understandings of the cathode charge storage mechanisms will lay the ground and hopefully set new directions into the rational design of high-performance cathode materials in RABs, and open up new opportunities for designing new electrolyte systems with respect to the targeted cathode systems.