Correlation between the Cation Disorders of Fe3+ and Li+ in P3-Type Na-0.67[Li-0.1(Fe0.5Mn0.5)(0.9)]O-2 for Sodium Ion Batteries

Abstract

Various Fe-based layered oxide materials have received attention as promising cathode materials for sodium ion batteries because of their low cost and high specific capacity. Only a few P3-type Fe-based oxide materials, however, have been examined as cathodes because the synthesis of highly crystalline P3-type Fe-based oxides is not facile. For this reason, the structural merits of the P3 structure are not yet fully understood. Herein, highly crystalline P3-type Na-0.67[Li-0.1(Fe0.5Mn0.5)(0.9)]O-2 heated at 900 degrees C is introduced to improve the electrochemical performance of Fe-based layered oxides. The structures, reaction mechanisms, and electrochemical performances of P3 Na-0.67[Li-0.1(Fe0.5Mn0.5)(0.9)]O-2, P2 Na-0.57[Li-0.1(Fe0.5Mn0.5)(0.9)]O-2, and P2 Na-0.67[Fe0.5Mn0.5]O-2 are compared to demonstrate the roles of Li+ doping in the improved electrochemical performance of P3 Na-0.67[Li-0.1(Fe0.5Mn0.5)(0.9)]O-2, such as stable capacity retention over 100 cycle s. P3 Na-0.67[Li-0.1(Fe0.5Mn0.5)(0.9)]O-2 significantly suppresses the migration of Fe3+ ions to tetrahedral sites in the Na layer during cycling because the cation disorder of Li+ is more favorable than that of Fe3+. As a result, P3 Na-0.67[Li-0.1(Fe0.5Mn0.5)(0.9)]O-2 shows better cycle performance than P2 Na-0.67[Fe0.5Mn0.5]O-2. P3 Na-0.67[Li-0.1(Fe0.5Mn0.5)(0.9)]O-2 also exhibits an improved rate performance compared to P2 Na-0.67[Fe0.5Mn0.5]O-2. This finding provides fundamental insights to improve the electrochemical performance of layered oxide cathode materials for sodium ion batteries.