Highly Stable Fe 2+ /Ti 3+ ‐Based Fluoride Cathode Enabling Low‐Cost and High‐Performance Na‐Ion Batteries

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, Na 2 TiFeF 7 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 Na 2 TiFeF 7 with 3D diffusion pathway delivers a specific capacity of ≈ 185 mAh g −1 at C/20 with an average operation voltage of ≈ 3.37 V (vs Na + /Na) including the high Fe 2+/3+ redox potential ( ≈ 3.75 V). Even at 5C, a specific capacity of ≈ 136 mAh g −1 is retained ( ≈ 73% of its theoretical capacity) owing to the low band gap energy ( ≈ 1.83 eV) and the low activation barrier energies ( ≈ 477.68 meV) required for facile Na + diffusion, indicating the excellent power‐capability. Moreover, Na 2 TiFeF 7 composed of three‐dimensionally interconnected (Fe, Ti)F 6 octahedra delivers an outstanding capacity retention of ≈ 71% after 600 cycles at 1 C owing to the small structural volume change ( ≈ 0.96%) during Na + de/intercalation. These findings provide insight into the development of fluoride‐based novel cathode materials for high‐performance SIBs.