Challenges and Strategies for Multi‐Electron Reactions in High‐Energy Phosphate‐Based Cathodes for Sodium‐Ion Batteries.

Sodium‐ion batteries (SIBs) have been considered as promising candidates for large‐scale energy storage systems and low‐speed electric vehicles due to abundant sodium resources and low cost. Phosphate‐based cathodes stand out for their high voltages, structural stability, superior safety, etc. However, their large molecular weight limits the overall capacity, compromising the energy density for practical applications. Recent advancements in multi‐electron reactions based on transition metal (TM) ions provide a promising pathway to achieve both high energy density and stability. This review discusses the fundamental principles behind the multi‐electron reactions of phosphate‐based cathodes from the perspectives of electrochemistry and materials science. The key factors, such as the conservation of matter and charge, thermodynamic, and kinetic feasibility, are addressed for activating and regulating the multi‐electron reactions, aiming for a high capacity exceeding 170 mAh g−1. The current progress in NASICON‐type phosphate cathodes is summarized, and the challenges associated with pyrophosphate and mixed phosphate cathodes are analyzed for multi‐electron reactions. Finally, the perspectives on the future development of high‐energy phosphate‐based cathodes are provided. [ABSTRACT FROM AUTHOR]