Theoretical Insights into Na5M(PO4)2F2 (M = Cr, V): A Fluorophosphate-Based High-Performance Cathode System for Sodium-Ion Batteries

Sodium-ion batteries are gradually leveling up with lithium-ion battery benchmarks in terms of voltage deliverance and electrochemical cyclic stability. The versatility in sodium chemistry makes it possible to investigate diverse polyanionic frameworks as cathodes, by regulating both the cationic and anionic components for enhanced performance. Fluorophosphates are known to be efficient owing to their high energy densities as well as preservation of structural stability in charge–discharge cycles. However, limitations in Na extraction and large diffusion barriers still need to be overcome. In this work, we probe into a transition-metal fluorophosphate framework Na5M(PO4)2F2 (M = V, Cr) and provide a theoretical model for its electrochemistry based on its crystallography and electronic structure. The ability to extract a high content of Na leads to a high voltage. These systems also show superionic Na conduction with very low hopping barriers.