Research progress of iron-based polyanionic cathode materials for sodium-ion batteries

Sodium ion batteries (SIBs), due to their abundant resources, low raw material costs, excellent performance in low-temperature conditions, and fast charging capabilities, offer promising prospects for power grid energy storage and low-speed transportation. They serve as a complementary alternative to lithium-ion batteries. The cathode material is crucial for overall battery performance, acting as a bottleneck for enhancing the specific energy of SIBs and a significant factor influencing costs. Low-cost iron-based polyanionic cathode materials have garnered attention in basic research and industrialization due to their inherent advantages: excellent structural stability, high safety levels, and minimal volume strain during charge-discharge cycles. These advantages are pivotal for practical implementations in electric vehicles, large-scale energy storage systems, portable electronics, and related applications. However, challenges such as capacity decay and structural stability during prolonged cycling may limit their industrial applicability. Therefore, enhancing material cycling life and battery system stability are critical concerns. Additionally, researchers are focused on discovering new iron-based polyanionic cathode materials with high specific capacity, operating voltage, and conductivity. This review comprehensively covers recent advancements in iron-based polyanionic cathode materials for SIBs, encompassing iron-based phosphates, fluorophosphates, pyrophosphates, sulfates, and mixed polyanionic compounds. The analysis systematically explores crystal structures, preparation methods, sodium storage mechanisms, and modification strategies for various iron-based polyanionic materials, elucidating the structure-activity relationship between chemical composition, structural regulation techniques, and performance enhancement. Moreover, the article discusses challenges encountered during the transition from laboratory-scale research to large-scale industrial applications of iron-based polyanionic cathode materials, along with corresponding solutions. These insights aim to offer theoretical and technical guidance for developing novel, low-cost cathode materials with high specific energy densities and advancing the industrialization of SIBs.