Influence of Li substitution on the structure and electrochemical performance of P2-type Na_(0.67)Ni_(0.2)Fe_(0.15)Mn_(0.65)O_2 cathode materials for sodium ion batteries

摘要

A series of layered Na0.67Lix(Ni0.2Fe0.15Mn0.65)(1-x)O2+delta (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)(0.8)O2+delta 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 (15mAg(-1)). When tested at 5C (750 mAg(-1)) rate, the electrode exhibits a discharge capacity of 68mAh 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.