Ca/Li Synergetic‐Doped Na_0.67Ni_0.33Mn_0.67O₂ to Realize P2‐O2 Phase Transition Suppression for High‐Performance Sodium‐Ion Batteries

Abstract

P2-type layered transition metal oxide Na_0.67Ni_0.33Mn_0.67O₂ is considered as a promising cathode for advanced sodium-ion batteries due to its high theoretical specific capacity. However, the P2-type cathode suffers severe P2-O2 phase transition during cycling process, resulting unsatisfactory cyclic stability and rate capability. Herein, a Ca/Li co-doped P2-type Na_0.62Ca_0.05Ni_0.33Mn_0.57Li_0.10O₂ (NCNMLO) cathode material was synthesized through a simple sol-gel method. With the synergistic effect of Ca-doping at Na sites and Li substitution at transition metal (TM) sites, the cathode achieves an excellent electrochemical performance due to the inhibited P2-O2 phase transition and improved ion diffusion with Na⁺/vacancy disordering arrangement. The NCNMLO cathode exhibits a good cyclic stability with 70.8 % of capacity retention at 1 C after 200 cycles and excellent rate capability with 40.1 mAh g^-1 at 20 C. The dual sites doping strategy provides an effective and simple approach for designing high-performance layered oxide cathode materials for sodium-ion batteries.