Research progress of layered P2-Na2/3Ni1/3Mn2/3O2 cathode material for sodium ion batteries

P2 layered oxide cathode materials for sodium ion batteries are favored by many scholars because of their advantages such as high energy density, abundant type, simple synthesis way and low environmental pollution. Among them, the most representative material is P2-Na2/3Ni1/3Mn2/3O2 (P2-NaNMO). However, the following issues can potentially affect how the material is used in practice: The Na+/vacancy ordering increases the diffusion barrier of Na+, which has a significant impact on the electrochemical performance; Mn3+ induces Jahn-Teller distortion, which disrupts the structural stability of the oxygen octahedron, resulting in structural degradation and decreased performance; an unexpected P2-O2 phase transition will occur when the cut-off voltage is >4.2 V. Therefore, the preparation of P2-NaNMO with high stability under high operating voltage has become the primary problem at present. Firstly, the research progress of layered transition metal oxides is introduced. Secondly, the structure and advantages of P2-NaNMO materials are analyzed. At the same time, in order to inhibit the phase transition of P2-NaNMO during the charging and discharging process, the modification method of the material was proposed. Finally, the influences of doping elements at various positions, surface modification, different binders on the performances of P2-NaNMO are systematically discussed, and their future development is prospected. •P2-Na2/3Ni1/3Mn2/3O2 is a prospective positive material for SIBs with high theoretical capacity and operating voltage.•The Na+/vacancy ordering makes the diffusion barrier of Na+ larger, which seriously affects the electrochemical performance.•Ion doping tactics can inhibit the P2-O2 transition and convert it to more reversible OP4 or Z phase.