Towards high-performance phosphate-based polyanion-type materials for sodium-ion batteries

Efficient energy storage techniques are prerequisites for the utilization of sustainable energy. During the recent decades, the emergence of lithium-ion batteries (LIBs) has greatly promoted the development of portable electronic equipment and electrical vehicles, yet, there is still a large difference between supply and demand, especially for large-scale energy storage. Sodium-ion batteries (SIBs) are regarded as promising alternatives for LIBs based on the abundant sodium resources and the reaction pattern that is comparable with that of LIBs. Among these cathode materials, phosphate-based polyanion-type materials have attracted much attention due to their inherent advantages such as uniquely stable structures and high operating potentials. Moreover, the small structural variations that occur during the cycling process within phosphate-based materials facilitate excellent rate performance and cycling stability. However, there still exist challenging issues within phosphate-based polyanion-type materials for further application due to their intrinsically low electronic conductivity and limited energy density. Many efforts will be needed before they can be widely used as commercial SIB cathode materials. In this review, the recent progress of phosphate-based polyanion-type electrode materials is briefly summarized based on compositional structure, reaction mechanism, fundamental issues, and remaining difficulties for phosphates, pyrophosphates, NASICON-type, and fluoride phosphate types of materials. Furthermore, structural-induced high performance and the flexible manipulation of material structure toward rational design of phosphate-based polyanion-type electrodes have also been discussed. We hope this review can provide some insights into the working mechanisms of and potential improvements of phosphate-based polyanion-type electrode materials and inspires further design enhancements of SIB materials.