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

Abstract

The key to development of high-voltage P2-type Na_0.66Ni_0.33Mn_0.67O₂ 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 Na_0.66Ni_0.33Mn_0.67O₂, which can be achieved by a facile one-step solid-state reaction. The formation mechanism of Na_0.66Ni_0.26Zn_0.07Mn_0.67O₂@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 Na_0.66Ni_0.26Zn_0.07Mn_0.67O₂ and the Na_0.66Ni_0.26Zn_0.07Mn_0.67O₂/0.06ZnO with the coating layer introduced by mechanical milling. This work provides a new strategy to design high-performance cathode materials for sodium-ion batteries.