Polyanionic cathodes for sodium-ion batteries: Materials, working mechanism, and applications

Amidst the global energy matrix transformation and escalating sustainability imperatives, sodium-ion batteries (SIBs) have attracted significant attention in the energy storage field, driven by their resource sustainability and cost competitiveness. As pivotal determinants of electrochemical performance, cathode materials govern essential metrics including specific energy density, cyclability, and operational safety in SIBs. Among these, polyanionic cathode materials have emerged as a focal research domain, distinguished by their excellent thermal and structural stability. This review systematically categorizes the types of polyanionic cathode materials and analyzes their intrinsic merits and challenges as Na hosts. To address intrinsic constrains in electronic conductivity and energy density, modification strategies encompassing lattice doping, surface coating, and nanoengineering are elucidated. Furthermore, the storage and transport mechanisms of Na⁺ in polyanionic compounds are revealed with the support of theoretical calculations, which provide theoretical guidance for material design. In addition, the application-specific evaluation of polyanionic cathode materials is conducted, particularly emphasizing suitability for stationary energy storage and low-speed electric vehicles. We further identify critical technical barriers and the future development directions of polyanionic cathodes for SIBs. Through a comparative study of polyanionic cathode materials, this review aims to provide a viable guide for advancing the development paradigm for cost-effective SIB technology.