Effects on Structure and Electrochemical Performance of Li[Li0.2Ni0.15Mn0.55Co0.1]O2 by Surface Modification with MnO2

Lithium-rich Li[Li0.2Ni0.15Mn0.55Co0.1]O(2 )cathode materials prepared by a sol-gel method were coated by various content of MnO2(1%, 3%, 5% in mass) via high temperature sintering. The effects of surface modification with MnO2 on crystal structure and morphology have been characterized by X-ray diffraction (XRD) and scanning electron microscope(SEM), which reveals that the bulk structures of all samples maintain a typical alpha-NaFeO2 layered oxide structure and the surface becomes relatively rough after modification. Electrochemical performances were investigated by charge/discharge cycling tests performed at room temperature and 55 degrees C, respectively. The 3wt.% MnO2-coated material delivers discharge capacity of 253.8 mAh.g(-1) and 312.2 mAh.g(-1) with first columbic efficiency of 82% and 91% at room temperature and 55 degrees C, respectively, which are much higher than those of the pristine material, 236.7mAh.g(-l) with first columbic efficiency of 69% at 25 degrees C and 298.2 mAh.g(-1) with first columbic efficiency of 83% at 55 degrees C. After 50 cycles, the 3wt.% coated material exhibits a capacity retention of 94%, compared to a capacity retention of 83% for the pristine one. The cyclic voltammetry measurement confirms that the MnO2 modification facilitates the structural stability of the materials. The suppression of side reaction between active materials and electrolyte and the changes of the structure of MnO2 coating layer during cycling, as well as more oxygen vacancies retention caused by MnO2 would be responsible for the improvement of the electrochemical properties.