Highly Stable Fe2+/Ti3+-Based Fluoride Cathode Enabling Low-Cost and High-Performance Na-Ion Batteries

Abstract

Grid-scale energy storage system is the need of batteries with low-cost, high-energy-density, and long cycle life. The requirement promotes the discovery of cathode materials enabling the storage of charge carrier ion within the open framework crystal structure having multi-dimensional diffusion path exhibiting small volume change. Herein, Na2TiFeF7 is reported as a promising fluoride-based cathode material for sodium-ion batteries (SIBs). Through combined studies using various experiments and first-principles calculations, it is confirmed that Na2TiFeF7 with 3D diffusion pathway delivers a specific capacity of approximate to 185 mAh g(-1) at C/20 with an average operation voltage of approximate to 3.37 V (vs Na+/Na) including the high Fe2+/3+ redox potential (approximate to 3.75 V). Even at 5C, a specific capacity of approximate to 136 mAh g(-1) is retained (approximate to 73% of its theoretical capacity) owing to the low band gap energy (approximate to 1.83 eV) and the low activation barrier energies (approximate to 477.68 meV) required for facile Na+ diffusion, indicating the excellent power-capability. Moreover, Na2TiFeF7 composed of three-dimensionally interconnected (Fe, Ti)F-6 octahedra delivers an outstanding capacity retention of approximate to 71% after 600 cycles at 1 C owing to the small structural volume change (approximate to 0.96%) during Na+ de/intercalation. These findings provide insight into the development of fluoride-based novel cathode materials for high-performance SIBs.