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

• Published in: Advanced functional materials Vol. 32; no. 29
• Main Authors: Kang, Jungmin, Ahn, Jinho, Park, Hyunyoung, Ko, Wonseok, Lee, Yongseok, Lee, Seokjin, Lee, Sangyeop, Jung, Sung‐Kyun, Kim, Jongsoon
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc 01.07.2022
• Subjects: Cathodes; Crystal structure; Current carriers; Diffusion barriers; Electrode materials; Energy gap; Energy storage; Ferrous ions; first‐principle calculations; Fluorides; fluoride‐based cathode materials; high operation voltages; Ion charge; large energy densities; Materials science; Na‐ion batteries; Rechargeable batteries; Sodium; Sodium-ion batteries; Storage batteries
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.202201816

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 ≈185 mAh g−1 at C/20 with an average operation voltage of ≈3.37 V (vs Na+/Na) including the high Fe2+/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, Na2TiFeF7 composed of three‐dimensionally interconnected (Fe, Ti)F6 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. In Na2TiFeF7, high structural stability and energy density are formed by integrating the advantages of layered oxide and polyanion‐type cathode. Moreover, by introducing Fe2+/Ti3+ ions in the structure of Na2TiFeF7, three‐dimensional pathways for facile Na+ diffusion and low band‐gap energy (≈1.83 eV) are formed, which result in excellent power‐capability and high‐energy density of 623 Wh kg−1 despite the fluoride‐based cathodes.