Advancing iron-based polyanionic cathodes for sodium-ion batteries: unravelling Fe-induced structural failure and targeted mitigation

Sodium-ion batteries (SIBs) have emerged as an attractive technology for grid-scale energy storage, owing to their long cycle life, high power capability, and excellent low-temperature performance. Among the diverse cathode candidates for SIBs, iron-based polyanionic materials, particularly phosphates, mixed phosphates, and sulfates, stand out as the most promising systems by virtue of their low cost and intrinsic safety. Taking triphylite NaFePO 4 , Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 , and Na 2 Fe 2 (SO 4 ) 3 as representative examples, this review focuses on the Fe 2+ /Fe 3+ redox-induced structural failures and corresponding targeted modification strategies. The failure origins are firstly analyzed, in which we propose Fe O covalency and Fe Fe distance as quantitative descriptors that capture the intrinsic failure tendencies of the three cathodes. Subsequently, we classify the existing modification strategies into a three-level framework, namely source suppression, process intervention, and consequence buffering, and further sort out strategy combinations tailored for materials with differentiated degradation pathways. Finally, we discuss the remaining challenges in this field, such as advanced in-situ characterization and extreme-condition adaptability, and outline future research directions including atomic-level structural design and energy density enhancement. This work provides a rational design roadmap for accelerating the deployment of high-performance iron-based polyanionic cathodes for SIBs.