Polyanion-type electrode materials for advanced sodium-ion batteries

Electrical energy storage (EES) exploiting secondary battery technologies is ideal for large-scale energy storage needs due to the rapid growth in proliferation of renewable energy sources and the emerging markets of grid-scale battery applications. Sodium-ion batteries (SIBs), a more sustainable EES option alternative to lithium-ion batteries (LIBs), have attracted intensive interests over the past decade, because of the natural abundance, evenly geographical-distributed Na-source, significantly low cost, and environmental benignity. Polyanionic compounds offer an appealing combination of rich structural diversity, superior ionic conductivity, high structural and thermal stability, and convenient storage/handling, making them competitive and attractive candidate electrode materials for next generation energy storage systems. In this review, recent advancements made regarding polyanion-type electrodes used for SIBs are summarized, elaborating their intrinsic structural characteristics, electrochemical properties, and corresponding sodium-storage mechanisms. Furthermore, we highlight the recent achievements in the lab-scale Na-ion full-cell prototypes that consist of polyanion-type compounds as one or both working electrodes. Last but not least, challenges and outlooks on further optimization of the structure and the electrochemical performance of polyanion-type electrode materials for SIBs are also presented to provide some insights to facilitate the practical realization of sodium-ion technologies.