P2‐Type Na _0.67 Ni _0.23 Fe _0.1 Mn _0.67 O ₂ Cathode Material with Suppressed P2−O2 Phase Transition for Sodium‐Ion Batteries

Abstract Na 0.67 Ni 0.33 Mn 0.67 O 2 (NNMO) is one of the most promising cathodes for Sodium‐ion batteries. However, due to undesired P2−O2 phase transition at high voltage during Na + extraction, its cyclic stability and coulomb efficiency are greatly adversely affected. Herein, electrochemically active Fe 3+ ions substituted Na 0.67 Ni 0.23 Fe 0.1 Mn 0.67 O 2 samples were prepared by both sol‐gel method (NNFMO‐SG) and solid phase method (NNFMO‐SP). It has been found that Fe substitution can suppress P2−O2 phase transition with a stable P2 phase during charging and discharging. Additionally, it also suppresses Na + /vacancy ordering, expands the slab‐spacing of the structure and improves the reversibility of the redox of anions. Owing to the combined advantages, NNFMO‐SG material exhibits the enhanced rate capability (70.1 mAh/g at 1000 mA/g) and excellent cycling stability (75.3 % capacity retention after 100 cycles at 200 mA/g).