Research on Sodium Storage Performance of Cu and Mg Doped P2 Type Layered Oxide Cathode Materials

Abstract

The P2-type Ni–Mn-based oxide cathode materials have drawbacks such as the Jahn-Teller effect and crystal phase transition under high pressure. In this study, Cu and Mg modifications were introduced to inhibit the P2–O2 phase transition and increase the lattice spacing, thereby reducing the resistance of sodium ion de-embedding and intercalation to improve the overall electrochemical performance of the battery. A series of P2-type Na0.67Ni0.33−xMn0.67CuxO2 and Na0.67Ni0.33−xMn0.67MgxO2 cathode materials were synthesized through solid-state reaction. Cu substitution significantly alters the structural stability and electrochemical properties. In the voltage range from 1.5 V to 4.2 V and at 0.1 C, the initial discharge specific capacity of Na0.67Ni0.18Mn0.67Cu0.15O2 was 167 mAh·g−1, with 86 mAh·g−1 remaining after 100 cycles at 1 C. Additionally, a high discharge specific capacity of 207 mAh·g−1 was achieved with 0.075 Mg doping, suggesting that part of O2− participated in the internal electrochemical reaction. Furthermore, the rate performance of Na0.67Ni0.28Mn0.67Mg0.05O2 was found to be the best. This phenomenon was attributed to the inactive Mg2+ retaining more Na+ in the interlayer, inhibiting crystal structure transformation and Jahn-Teller distortion. The larger radius of Mg2+ increased the layer spacing, widened the Na+ de-embedding channels, and increased the diffusion coefficient.