Alluaudite Na2Fe2(SO4)3 and NASICON‐Type Na4Fe3(PO4)2(P2O7) as Promising Cathode Materials in Sodium‐Ion Batteries

Abstract Sodium‐ion batteries (SIBs) are expected to be widely used in low‐speed vehicles and distributed energy storage devices due to their merits of abundance in natural resources, wide distribution, and low toxicity of raw materials. However, the performance of the state‐of‐the‐art SIBs is restricted by the lack of ideal cathode materials. Iron‐based polyanionic compounds have attracted much attention as potential SIBs cathode due to their low cost, excellent cycling stability, safety, and environmental friendliness, among which Na 2 Fe 2 (SO 4 ) 3 and Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) are the most commercializable representatives. Nevertheless, both academic and industrial communities hold a wait‐and‐see attitude. Herein, each of the aspects of crystal structure, synthesis methods, and electrochemical properties are first introduced. Then, the optimized schemes to improve electrochemistry, hygroscopicity, and conductivity for both are summarized. Finally, according to the current practical demand for iron‐based polyanionic compounds, their application prospects and the corresponding bottleneck issues are comprehensively analyzed.