Polyanion-type electrode materials for sodium-ion batteries

Sodium-ion batteries, representative members of the post-lithium-battery club, are very attractive and promising for large-scale energy storage applications. The increasing technological improvements in sodium-ion batteries (Na-ion batteries) are being driven by the demand for Na-based electrode materials that are resource-abundant, cost-effective, and long lasting. Polyanion-type compounds are among the most promising electrode materials for Na-ion batteries due to their stability, safety, and suitable operating voltages. The most representative polyanion-type electrode materials are Na₃V₂(PO₄)₃ and NaTi₂(PO₄)₃ for Na-based cathode and anode materials, respectively. Both show superior electrochemical properties and attractive prospects in terms of their development and application in Na-ion batteries. Carbonophosphate Na₃MnCO₃PO₄ and amorphous FePO₄ have also recently emerged and are contributing to further developing the research scope of polyanion-type Na-ion batteries. However, the typical low conductivity and relatively low capacity performance of such materials still restrict their development. This paper presents a brief review of the research progress of polyanion-type electrode materials for Na-ion batteries, summarizing recent accomplishments, highlighting emerging strategies, and discussing the remaining challenges of such systems.