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 NaNiMnO 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 NaNiMnO, which can be achieved by a facile one-step solid-state reaction. The formation mechanism of NaNiZnMnO@0.06ZnO (NNZM@0.06ZnO) is investigated, revealing that the spinel and P3 intermediate phases appear prior to the formation of the P2 phase. Ni can be preferentially incorporated into the P2 structure in competition with Zn at high temperature, resulting in a uniform enrichment of ZnO on the surface. A small amount of Zn 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 Zn 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 and rate performance with a discharge capacity of 56.4 mAh g at 2000 mA g, which significantly outperforms the uncoated NaNiZnMnO and the NaNiZnMnO/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.