Study On Synthesis And Performance Of Borate Compounds As Cathode Materials For Lithium-ion Battery

Borate cathode material attracted widely attention due to their high theoretical capacity(220 m Ah·g-1), good structure stability, low cost, and environmental friendliness. However, their low electronic conductivity, low discharge potential has affected its electrochemical properties, and then prohibited their large-scale application. By studying on new synthesis methods, carbon coating, reducing crystal size and ion doping to improve the electrochemical properties of borate materials.In the synthesis and modification of Li Mn BO3: the Li Mn BO3/C was synthesized by microwave solid state method, this method was convenient and greatly reduced the synthesis time. The m-Li Mn BO3/C was calcined at microwave oven at the microwave power 240 W for 7 min. The electrochemical property test showed that the initial discharge capacity of m-Li Mn BO3/C reached 156.3 m Ah·g-1 at the rate of 0.05 C. Until the 50 th cycle, the discharge capacity remained at 148.2 m Ah·g-1, showed a higher electrochemical capacity. The crystal structure of Li Mn BO3 was monoclinic, mixed phase and hexagonal when clacined at 500, 600 and 700℃ respectively. The monoclinic Li Mn BO3/C showed higher capacity than the hexagonal Li Mn BO3/C. It determined the best carbon source was glucose to synthesis Li Mn BO3/C by solid-state method, and the optimum carbon content was 10wt%. Doping Ag+, Mg2+ and In3+ into Li Mn BO3 to investigate the best doping element and the suitable amount of doping element. The result of electrochemical property test showed that the discharge capacity of all materials was improved after doping. In addition, using ethanol as solvent by a sol-gel method obtained m-Li Mn BO3/C. When replaced the solvent to acetic acid, h-Li Mn BO3/C can be obtained. This method is benefit for reducing the particle size, shortening the diffusion path of Li+ to improve the electrochemical properties.In the synthesis and electrochemical property test of Li Fe BO3: the carbon coated Li Fe BO3/C was synthesized by solid-state method. The result showed that when using glucose as the carbon source, Li Fe BO3 showed good crystallinity, uniform particle size and good electrochemical characterization. The initial capacity of Li Fe BO3/C was 153.6 m Ah·g-1 at 0.05 C, after 30 cycles, it remained at 148.2 m Ah·g-1. It used a new sol-gel method with N, N-dimethylformamide as solvent and citric acid as carbon source to synthesize Li Fe BO3/C with the size range 40～60 nm. The initial specific discharge capacity was 205.8 m Ah·g-1 at current density of 10 m A·g-1. While the material also showed relatively good rate stability, indicating that nanostructure is benefit for the ion diffusion.In the design, synthesis and electrochemical property test of composite materials: the Li MnxFe1-xBO3 composite materials were synthesized by solid-state method, with the increase of the amount of iron, the structural stability improved, the polarization decreased and the discharge capacity increased. The h-Li Mn0.9V0.1BO3 composite materials were synthesized by sol-gel method. Despite the initial capacity of h-Li Mn0.9V0.1BO3 composites was not high, the discharge capacity increased into 85.7m Ah·g-1 until 30 th cycle, which has been significantly improved. Fe and V were added to improve the conductivity and structural stability of the material, in order to improve the electrochemical properties of materials.