Stable Electrochemical Properties of Magnesium-Doped Co-Free Layered P2-Type Na0.67Ni0.33Mn0.67O2 Cathode Material for Sodium Ion Batteries

P2-type layered structure manganese-based materials have been reported as the most promising candidate for practical applications of sodium ion batteries because of their high capacity, facile fabrication, low cost, and environmental friendliness. In this work, a novel Cobalt-free layered P2-type Na0.67Ni0.33Mn0.67O2 cathode material was designed using cation potential, and the cathode material was successfully synthesized by solid-state reaction method. We present an in-depth investigation of the effect of Mg doping on the electrochemical performance and structural stability of Na0.67Ni0.33Mn0.67O2 by comparing series compositions Na0.67Ni0.33–xMgxMn0.67O2 (x = 0, 0.05, 0.1, 0.15, 0.2). The Mg-doping sample Na0.67Ni0.18Mg0.15Mn0.67O2 delivered an initial discharge capacity of 123 mAh·g–1 at 0.1 C in the potential range from 2.0 to 4.3 V. The capacity of the material remained at 92% after 100 cycles at a rate of 0.1 C. Mg ion as an electrochemical inert element can effectively reduce interlayer slip, inhibit phase transition, and stabilize the layered structure.