Advanced P2-Na2/3Ni1/3Mn7/12Fe1/12O2 Cathode Material with Suppressed P2–O2 Phase Transition toward High-Performance Sodium-Ion Battery

As a promising cathode material of sodium-ion battery, P2-type Na2/3Ni1/3Mn2/3O2 (NNMO) possesses a theoretically high capacity and working voltage to realize high energy storage density. However, it still suffers from poor cycling stability mainly incurred by the undesirable P2–O2 phase transition. Herein, the electrochemically active Fe3+ ions are introduced into the lattice of NNMO, forming Na2/3Ni1/3Mn2/3–xFexO2 (x = 0, 1/24, 1/12, 1/8, 1/6) to effectively stabilize the P2-type crystalline structure. In such Fe-substituted materials, both Ni2+/Ni4+ and Fe3+/Fe4+ couples take part in the redox reactions, and the P2–O2 phase transition is well restrained during cycling, as verified by ex situ X-ray diffraction. As a result, the optimized Na2/3Ni1/3Mn7/12Fe1/12O2 (1/12-NNMF) has a long-term cycling stability with the fading rate of 0.05% per cycle over 300 cycles at 5 C. Furthermore, the 1/12-NNMF delivers excellent rate capabilities (65 mA h g–1 at 25 C) and superior low-temperature performance (the capacity retention of 94% at −25 °C after 80 cycles) owing to the enhanced Na diffusion upon Fe doping, which is deduced by the studies of electrode kinetics. More significantly, the 1/12-NNMF also displays remarkable sodium-ion full-cell properties when merged with an LS–Sb@G anode, thus implying the possibility of their practical application.