Stabilizing P2-Type Ni–Mn Oxides as High-Voltage Cathodes by a Doping-Integrated Coating Strategy Based on Zinc for Sodium-Ion Batteries

The key to development of high-voltage P2-type Na0.66Ni0.33Mn0.67O2 is the modification methods that can effectively improve its electrochemical reversibility. Herein, a doping-integrated coating strategy based on zinc element is proposed to modify P2-type Na0.66Ni0.33Mn0.67O2, which can be achieved by a facile one-step solid-state reaction. The formation mechanism of Na0.66Ni0.26Zn0.07Mn0.67O2@0.06ZnO (NNZM@0.06ZnO) is investigated, revealing that the spinel and P3 intermediate phases appear prior to the formation of the P2 phase. Ni2+ can be preferentially incorporated into the P2 structure in competition with Zn2+ at high temperature, resulting in a uniform enrichment of ZnO on the surface. A small amount of Zn2+ doping significantly suppresses the Na+/vacancy ordering effect and improves the structural reversibility. Furthermore, the electrolyte decomposition is effectively reduced because of the presence of the ZnO coating layer, leading to the formation of a thin cathode electrolyte interphase film that is favorable to fast Na+ diffusion. In virtue of the Zn2+ doping and in situ formed ZnO coating, NNZM@0.06ZnO exhibits excellent cycling stability with a capacity retention of 83.7% after 100 cycles at 100 mA g–1 and rate performance with a discharge capacity of 56.4 mAh g–1 at 2000 mA g–1, which significantly outperforms the uncoated Na0.66Ni0.26Zn0.07Mn0.67O2 and the Na0.66Ni0.26Zn0.07Mn0.67O2/0.06ZnO with the coating layer introduced by mechanical milling. This work provides a new strategy to design high-performance cathode materials for sodium-ion batteries.