Organic cathodes for rechargeable magnesium metal batteries: materials, mechanisms, and prospects

Rechargeable magnesium metal batteries (RMBs) represent a promising alternative to lithium-ion batteries due to the high abundance, safety, and theoretical capacity of magnesium. However, the development of efficient cathode materials remains a critical challenge. In contrast to the rigid crystalline structures of inorganic cathodes, organic cathode materials offer unique advantages for Mg²⁺ storage, including weak intermolecular interactions that facilitate flexible ion diffusion pathways and non-dense structures that further reduce migration barriers. Moreover, organic materials are renewable, structurally tunable, and environmentally sustainable, making them highly attractive for next-generation RMBs. Organic cathodes typically operate via conversion-type redox reactions, with carbonyl (CO) or imine (CN) groups serving as active centers for reversible Mg²⁺ storage. This review systematically categorizes organic cathode materials into small molecules, small molecular salts, and polymers, with the latter further classified into polyanthraquinones, polyimides, covalent organic frameworks (COFs), and metal organic frameworks (MOFs). Small molecules such as 2,5-dimethoxy-1,4-benzoquinone (DMBQ) and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) demonstrate high specific capacities but suffer from dissolution issues, necessitating strategies like electrolyte engineering and separator modification to mitigate shuttling effects. Polymer-based cathodes, including polyanthraquinonyl sulfide (PAQS) and polyimides (PIs), exhibit enhanced cycling stability due to their low solubility and robust structures. Networked polymers, including COFs and MOFs, further improve performance through ordered porous structures and efficient ion transport pathways. This review comprehensively discusses the redox mechanisms, electrochemical performance, and degradation pathways of various organic cathodes, highlighting structure-property relationships and innovative design strategies. Future perspectives are provided to guide the development of high-performance, sustainable organic cathodes for RMBs.