Ca/Li Synergetic‐Doped Na0.67Ni0.33Mn0.67O2 to Realize P2‐O2 Phase Transition Suppression for High‐Performance Sodium‐Ion Batteries

P2‐type layered transition metal oxide Na0.67Ni0.33Mn0.67O2 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 Na0.62Ca0.05Ni0.33Mn0.57Li0.10O2 (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.