Na2Fe2F7: a fluoride-based cathode for high power and long life Na-ion batteries

Abstract

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 (Na₂Fe₂F₇) with three-dimensionally interconnected FeF₆ octahedra and three-dimensional Na⁺ diffusion pathways. Through combined studies using first-principles calculations and experiments, we confirm that Na₂Fe₂F₇ 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.