The Study Of Li-rich Layered Solid-Solution Cathode Materials For Lithium Ion Batteries

Li-ion batteries are extensively used in mobile communication equipments and electronic devices due to their merits like high energy density, low weight, long life, etc. Compared with commercialized LiCoO₂, solid solution materials, Li₂MnO₃-LiMO₂, have drawn researchers'attention for their relative high capacity and low cost. Nevertheless, this type of material tends to exhibit rapid capacity fade during cycles and poor performances when operated at a high current density. In order to overcome these problems, much effort has been done. In this thesis, the structural and electrochemical properties of Li-rich layered solid-solution system Li[Li_xNi_yMn_zCo_1-x-y-z]O₂ were studied, and their battery performances were improved by composition optimization together with surface modification.(1-x)Li1.17Ni0.25Mn0.58O₂·xLiNi_0.33Mn_0.33Co_0.33O₂ (x=0, 0.3, 0.5, 0.7) are synthesized by a mixed hydroxides method. Their initial charge/discharge capacities are closely related to their composition. Moreover, the sample whose composition is in the range of Mn>0.5 and 0<Co<0.1 would exhibit relatively high initial charge/discharge capacities.The surface of Li[Li_0.2Ni_0.18Mn_0.59Co_0.03]O₂ can be modified with LiCoPO4 through co-precipitation method. The resultant LiCoPO4 particles are in nana-scale and accumulate on the surface of the Li[Li0.2Ni0.18Mn0.59Co0.03]O₂ particles. The surface modification by LiCoPO4 is shown to significantly improve both the cyclic performance and the rate capability of Li[Li_0.2Ni_0.18Mn_0.59Co_0.03]O₂. After 40 cycles, the modified cathode materials'reversible capacity is 231 mAh·g-1, which is much higher than that of pristine Li[Li_0.2Ni_0.18Mn_0.59Co_0.03]O₂.