Recent Advances in Sodium Iron Sulfate Cathodes for Sodium-Ion Batteries: Crystal Structure, Synthesis, and Performance.

Sodium-ion batteries (SIBs) have shown promising application prospect in large-scale energy storage systems and low-speed electrical vehicle due to the rich abundance of sodium on earth, similar working principle with lithium-ion batteries, and potentially better low-temperature performance. Exploring low-cost and high-performance cathode materials is the key to promote the practical application of SIBs. Among various SIBs cathode materials, sodium iron sulfate (NFS) compounds stand out due to their stable three-dimensional (3D) framework structure, high operating voltage, cost-effectiveness, and environmental friendliness. They have received extensive attention from basic research to the industrialization of the results. This article systematically analyzes and discusses the crystal structure, sodium storage mechanisms, preparation methods, and corresponding optimization strategies of various types of NFS materials and reveals the structure-activity relationship between the chemical composition of NFS materials, structural modification and modulation, and performance enhancement. Meanwhile, the key challenges faced by NFS materials are discussed, and the problems to be solved and constructive suggestions are put forward in the process of large-scale industrialization, so as to provide theoretical and technological guidance for the development of low-cost and high-performance cathode materials and the industrialization of SIBs.