Na 2 Fe 2 F 7 : a fluoride-based cathode for high power and long life Na-ion batteries

Despite the high energy density of layered-type cathode materials for Na-ion batteries, their two-dimensional crystal structure suffers a large volume change and phase transition during Na + de/intercalation, which often results in their poor cycling performances. Thus, a robust three-dimensional framework with minimal structural change is required for stable electrochemical sodium storage. Here, we introduce an earth-abundant element-based trigonal-type Na–Fe–F compound (Na 2 Fe 2 F 7 ) with three-dimensionally interconnected FeF 6 octahedra and three-dimensional Na + diffusion pathways. Through combined studies using first-principles calculations and experiments, we confirm that Na 2 Fe 2 F 7 delivers excellent power-capability due to large three-dimensional Na + diffusion pathways as well as ultra-long cycling performance due to negligible structural change during Na + de/intercalation. These results will guide new insights for material discovery for high performance rechargeable batteries.