Advanced P2-Na_2/3Ni_1/3Mn_7/12Fe_1/12O₂ 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 Na_2/3Ni_1/3Mn_2/3O₂ (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 Na_2/3Ni_1/3Mn_2/3-xFe_xO₂ (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 Na_2/3Ni_1/3Mn_7/12Fe_1/12O₂ (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.