P2-Type Na0.67Ni0.23Fe0.1Mn0.67O2 Cathode Material with Suppressed P2-O2 Phase Transition for Sodium-Ion Batteries

Na0.67Ni0.23Fe0.1Mn0.67O2 (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 Fe3+ ions substituted Na0.67Ni0.23Fe0.1Mn0.67O2 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).