Influence of Li substitution on the structure and electrochemical performance of P2-type Na0.67Ni0.2Fe0.15Mn0.65O2 cathode materials for sodium ion batteries

A series of layered Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2 and 0.3) compounds is prepared via a facile oxalate coprecipitation method and a solid-state reaction process, and investigated as the promising positive electrode materials for Na-ion batteries. As the Li content increases, O3 phase and Li2MnO3 gradually grow at the expense of the P2 phase and the particles become smaller and more agglomerated based on X-ray diffraction, scanning electron microscopy and transmission electron microscope results. The optimal Na0.67Li0.2(Ni0.2Fe0.15Mn0.65)0.8O2+δ shows the improved electrochemical performance with a high specific capacity of 151 mAh g−1 and 78% capacity retention over 50 cycles at 0.1 C (15 mA g−1). When tested at 5 C (750 mA g−1) rate, the electrode exhibits a discharge capacity of 68 mAh g−1. The results of electrochemical impedance spectroscopic and ex-situ X-ray diffraction measurements demonstrate that the improved cyclability and rate capability of the Li-substituted cathode can be ascribed to the decreased resistance and the enhanced structure stability in the high voltage of 4.3 V.