Inhibition of P2-O2 Phase Transition for P2-Na2/3Ni1/3Mn2/3O2 as Cathode of Sodium-ion Battery via Synergetic Substitution of Cu and Mg

The research of sodium-ion batteries (SIBs) is of great significance for development of new energy and energy storage methods. As cathode material, P2-type layered oxide material Na2/3Ni1/3Mn2/3O2 has attracted wide attention due to its excellent capacity and high working voltage. However, it suffers from undesired P2-O2 phase transition, which leads to a drastic change in volume and rapid capacity decay. Here, a P2-Na0.67Ni0.18Cu0.10Mg0.05Mn0.67O2 (NCMM-10-05) cathode was synthesized through solid-state method with synergetic substitution of Cu and Mg. The results indicated that the incorporation of Cu and Mg suppressed irreversible P2-O2 phase transition when charging to high voltage and initialized OP4 phase formation, which improved reversible stability of structure. Thus the as-obtained material exhibited excellent electrochemical performance, which delivered an initial discharge capacity of 113 mAh.g(-1) in the voltage range of 2.00-4.35 V (vs. Na+/Na), a reversible capacity of 64.1 mAh.g(-1) at 8C (1C=100 mA.g(-1)), and a capacity retention of 88.9% after 200 cycles at 1C. The effect of Cu and Mg synergetic substitution on the structure and electrochemical properties of P2-type layered oxides was explored, and the specific roles played by Cu and Mg in the structural evolution were further investigated by in situ X-ray diffraction (XRD) analysis and density functional theory (DFT) calculations. This work provides a new insight into the rational design of highly stable cathode materials with rapid Na+ transport capability for SIBs.