Preparation and optimization of ZrO2 modified P2-type Na2/3Ni1/6Co1/6Mn2/3O2 with enhanced electrochemical performance as cathode for sodium ion batteries

Surface stabilization is necessary for cathode materials to gain a long-term cycling stability because of unfavorable side reactions and exfoliation caused by corrosive environment. To improve the cyclic stability of P2-type ternary cathode Na2/3Ni1/6Co1/6Mn2/3O2 for sodium ion batteries, we prepare a ZrO2-coated Na2/3Ni1/6Co1/6Mn2/3O2 through a simple wet chemical method. The coating layer is distributed homogeneously on the surface, and the fraction of ZrO2 (1 wt-%, 2 wt-%, 3 wt-%, 4 wt-%, 5 wt-%) helps control the thickness of the coating layer. It turns out that all the materials exhibit pure P2 structure without any impurities. The material with a 2 wt-% ZrO2 coating exhibits the best electrochemical performance in rate capability and long-term cyclic stability. It delivers a superior initial discharge capacity of 140 mA h·g−1 between 2 and 4.5 V at 20 mA g−1. Even cycles at high current density (100 mA g−1), it shows 106 mA h·g−1 reversible discharge capacity with 88% capacity retention after 300 cycles. The improvement in electrochemical performance is attributed to the segregation of cathode materials from the corrosive electrolyte by the nano-sized ZrO2 layer. The EIS results confirm that a thin ZrO2 coating layer can effectively protect the electrode from dissolution and stabilize the SEI film. This study can be used to develop the electrochemical performance of cathode materials for sodium ion batteries by surface modification via ZrO2.