A Review of Revealing the Impact of Transition Metals in Polyanionic Cathode Material Na ₄ M ₃ (PO ₄ ) ₂ P ₂ O ₇ : Kinetic, Mechanisms and Optimization Strategies

ABSTRACT The growing demand for environmental protection and renewable energy will drive a continuous increase in the need for electrochemical energy storage systems. Sodium‐ion batteries (SIBs) have emerged as an ideal candidate for large‐scale energy storage technologies as a result of their advantages of abundant resources. Among the contenders, Na 4 M 3 (PO 4 ) 2 P 2 O 7 (M = Fe, Co, Mn, Ni) system materials have attracted extensive attention because of their robust three‐dimensional open framework and sodium storage performance. However, the performance differences among materials caused by variations in transition metals still need further summarization. Therefore, this review article provides a systematic overview of the research progress in mixed polyanion cathode materials Na 4 M 3 (PO 4 ) 2 P 2 O 7 (N(M)PP) for SIBs, focusing on their crystal structures, sodium ion migration mechanisms, and electrochemical performance optimization strategies. Notably, optimization strategies for iron‐based cathodes are emphasized, covering non‐stoichiometric design, metal/anion doping, defect engineering, and high‐entropy material approaches. Additionally, the review discusses the optimization directions for electrolytes suitable for high‐voltage cathode materials, including solvent selection, additive effects, and interface stability regulation. Finally, it summarizes the current research challenges and future development directions, providing theoretical guidance and technical references for the targeted design and optimization of N(M)PP materials.