Fluorophosphates and fluorosulfates cathode materials: Progress towards high energy density sodium-ion battery

The rapid diffusion of renewable energy boosts the wide deployment of large-scale energy storage system. With the low cost and high crustal abundance, sodium-ion battery (SIB) technology is expected to become a dominant technology in that area in the future. Toward the practical application, novel cathode materials are urged to develop that show high energy density without sacrificing their cost and benignity to the environment. While the years of many studies, this still remains a huge challenge to battery scientists. In this review, we discuss recent breakthroughs in SIB cathode materials with high energy density, namely fluorphosphates and fluorosulfates. The design of materials, the crystal structure, the electrochemical performance, and the underlaying intercalation mechanism are systematically reviewed. Useful strategies and research directions are also provided to advance future high-energy, low-cost, and ecofriendly cathode materials for next generation SIB.