Abstract: Rechargeable aluminum batteries have become one of the important candidates in the next generation of electrochemical energy storage because of the advantages including high specific capacity, wide distribution of resources, and high safety. There are diverse mechanisms for the cathodes of rechargeable aluminum batteries. The charge carriers include ions such as Al³⁺, AlCl₄^-, AlCl²⁺, AlCl₂⁺, and Cl^-, and the electrochemical reaction mechanisms include intercalation type, conversion type, and coordination type, etc. Various mechanisms endow the cathodes with distinct performances. The multi-electron reactions bring about high theoretical capacity for cathodes, while the mono-electron reactions provide cathodes with high rate performance. Otherwise, the materials already used as cathodes for rechargeable aluminum batteries widely include metal oxides and chalcogenides, carbon, chalcogens, organics, and others. Consequently, there are multiple potential choices for novel materials. However, cathode materials of different mechanisms and types also face different challenges. For the cathodes operating based on reactions of Al³⁺, a critical challenge of the sluggish reaction kinetics comes from the high charge density of the ion. For AlCl₄^- and other charge carriers with low charge density, few charges are transferred through the reaction of the ions, limiting the specific capacity of the cathodes. Mechanisms with phase transition might deteriorate the structural stability of cathodes, which is especially severe for conversion-type cathodes. Meanwhile, the performances of cathodes are restricted by the intrinsic inferiorities, such as poor electronic conductivity, large volume expansion with ions inserted, and high dissolubility in the electrolyte. Therefore, a comprehensive review of the mechanisms, challenges, and efficient modifications of cathodes for rechargeable aluminum batteries is of great necessity. This review starts from the energy storage mechanisms of the cathode materials for rechargeable aluminum batteries. The reaction steps and classical material types of different energy storage mechanisms are cataloged, and the performance characteristics of the materials are specifically analyzed. This review further summarizes the main problems of cathode materials and current corresponding strategies, based on the analysis of the energy storage mechanisms and the classification of ion storage states and other effects, including dissolution and other side reactions. The directions of the design and modification of cathode materials are also forecasted. The developing tendency could be concluded as follows: (a) rapid reaction kinetics for high rate performance; (b) enriched active sites and redox centers for high specific capacity; (c) strengthened structure for stable cycling performance; (d) rational-designed composition and structure for ideal discharging voltage. This work aims to provide systematic guidance for the solution toward common challenges faced by rechargeable aluminum batteries.

Key words: Rechargeable aluminum battery, Cathode material, Energy storage mechanism, Specific capacity, Structural stability, Modification strategy