An investigation of silicon-doped LiCoO 2 as cathode in lithium-ion secondary batteries

Abstract

Powders of layered–structured LiCo (1− x) Si x O 2 ( x = 0, 0.01, 0.05, 0.10, 0.35) were synthesized by a co-precipitation method followed by a 950 °C sintering. Their structures were analyzed by the X-ray diffraction (XRD) and scanning electron microscopy (SEM). Iodine titration method was also employed to measure the average valence of cobalt ions. With these powders as the active materials of positive electrodes versus lithium, the electrochemical behaviors of the cells were investigated using charge–discharge cycling, AC impedance spectroscopy and DC resistance measurement. It is found that the Si-doping results in the decrease of cobalt valence and the crystallite size. When the Si-content is less than 10%, pure LiCo 1− x Si x O 2 phases are obtained. A second phase Li 2CoSiO 4 is also obtained when the Si-content is 35%. Among the five compositions, the non-doped LiCoO 2 exhibits a high initial specific capacity (about 150 and 185 mA h/g at a current density of 0.4 mA/cm 2 from 2.8 to 4.2 and from 2.8 to 4.5 V, respectively), but degrades in the following cycles; while the Si-doped LiCo 1− x Si x O 2 electrodes especially LiCo 0.99Si 0.01O 2 show the best performance of long-life cycling. And all of the doped powders have better stability of the 3.6 V-plateau efficiency due to the improved stability effect on the cell impedance.