Correlation between the Cation Disorders of Fe 3+ and Li + in P3-Type Na 0.67 [Li 0.1 (Fe 0.5 Mn 0.5 ) 0.9 ]O 2 for Sodium Ion Batteries.

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 (Fe 0.5 Mn 0.5 ) 0.9 ]O 2 heated at 900 °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 (Fe 0.5 Mn 0.5 ) 0.9 ]O 2 , P2 Na 0.57 [Li 0.1 (Fe 0.5 Mn 0.5 ) 0.9 ]O 2 , and P2 Na 0.67 [Fe 0.5 Mn 0.5 ]O 2 are compared to demonstrate the roles of Li + doping in the improved electrochemical performance of P3 Na 0.67 [Li 0.1 (Fe 0.5 Mn 0.5 ) 0.9 ]O 2 , such as stable capacity retention over 100 cycles. P3 Na 0.67 [Li 0.1 (Fe 0.5 Mn 0.5 ) 0.9 ]O 2 significantly suppresses the migration of Fe 3+ ions to tetrahedral sites in the Na layer during cycling because the cation disorder of Li + is more favorable than that of Fe 3+ . As a result, P3 Na 0.67 [Li 0.1 (Fe 0.5 Mn 0.5 ) 0.9 ]O 2 shows better cycle performance than P2 Na 0.67 [Fe 0.5 Mn 0.5 ]O 2 . P3 Na 0.67 [Li 0.1 (Fe 0.5 Mn 0.5 ) 0.9 ]O 2 also exhibits an improved rate performance compared to P2 Na 0.67 [Fe 0.5 Mn 0.5 ]O 2 . This finding provides fundamental insights to improve the electrochemical performance of layered oxide cathode materials for sodium ion batteries.