Study on the effect of Na content on the structure and properties of Na x Ni 0.23 Mn 0.6 Cu 0.05 Mg 0.07 Ti 0.01 O 2 cathode materials for high performance Na-ion batteries

Sodium content in layered oxide cathodes for sodium-ion batteries (SIBs) has a noticeable impact on phase structure and cyclic rate performance. In this study, a series of samples NaxNi0.23Mn0.6Cu0.05Mg0.07Ti0.01O2 (x = 0.66, 0.71, 0.76, 0.81 and 0.86), with a P2/O3 biphasic phases structure arising from Na content, is optimally synthesized aiming to achieve the synergetic combination of structural stability offered by P2 phase and high specific capacity offered by O3 phase. The P2/O3 coexisting phase and lattice structure are studied by X-ray diffraction (XRD) and Rietveld refinement, high-angle annular dark field scanning transmission election microscopy (HAADF-STEM) images, in situ synchronous XRD, etc. Based on various electrochemical evaluation, Na0.76Ni0.23Mn0.6Cu0.05Mg0.07Ti0.01O2 (NM-76) delivers the initial capacity of 146.29 mA h g-1 (0.1 C rate) and reaches 80.4 % capacity-retention after 400 cycles (57.46 mA h g-1, 10 C rate) when being cycled in a voltage range of 2.0-4.3 V. P2/O3 biphasic structure constructed by Na content effectively hinders the phase evolution during the (de-)sodiation processes, thus leads to a high structural stability of NM-76. This work puts forward a novel route for the design of biphasic metal oxide cathode materials for high performance SIBs.