Effect of ZnO modification on the performance of LiNi0.5Co0.25Mn0.25O2 cathode material

ZnO was coated on LiNi0.5Co0.25Mn0.25O2 cathode (positive electrode) material for lithium ion battery via sol-gel method to improve the performance of LiNi0.5Co0.25Mn0.25O2. The X-ray diffraction (XRD) results indicated that the lattice structure of LiNi0.5Co0.25Mn0.25O2 was not changed distinctly after surface coating and part of Zn2+ might dope into the lattice of the material. Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) proved that ZnO existed on the surface of LiNi0.5Co0.25Mn0.25O2. Charge and discharge tests showed that the cycle performance and rate capability were improved by ZnO coating, however, the initial capacity decreased dramatically with increasing the amount of ZnO. Differential scanning calorimetry (DSC) results showed that thermal stability of the materials was improved. The XPS spectra after charge-discharge cycles showed that ZnO coated on LiNi0.5Co0.25Mn0.25O2 promoted the decomposition of the electrolyte at the early stage of charge-discharge cycle to form more stable SEI layer, and it also can scavenge the free acidic HF species from the electrolyte. The electrochemical impedance spectroscopy (EIS) results showed ZnO coating could suppress the augment of charge transfer resistance upon cycling. (C) 2009 Elsevier Ltd. All rights reserved.