Stabilizing Na0.7MnO2 cathode for Na-ion battery via a single-step surface coating and doping process

P2-type Na0.7MnO2, with high capacity and excellent Na ion conductivity, is a promising cathode material for Na-ion batteries. However, its rapid capacity decay upon repeated cycles restricts its practical application. In this study, we demonstrate a facile method to surface coat and dope P2-Na0.7MnO2 with a P2-Na0.7Ni0.33Mn0.67O2 layer in a single step to enhance its cycle stability. The coating suppresses the dissolution of Mn ions into electrolyte, and the Ni dopant suppresses orthorhombic distortion, inhibits Na+/vacancy ordering and improves structural stability upon cycling. As a consequence, the coating enhances capacity retention from 62.2% (Na0.7MnO2) to 87.7% (Na0.7MnO2/20 wt% Na0.7Ni0.33Mn0.67O2) over 50 cycles, and from 20.7% (Na0.7MnO2) to 68.9% (Na0.7MnO2/20 wt% Na0.7Ni0.33Mn0.67O2) over 100 cycles without sacrificing the initial discharge capacity. In addition, the air-stable Na0.7Ni0.33Mn0.67O2 surface layer protects Na0.7MnO2 against air exposure.