Synthesis And Characterization Of LiFePO₄ As A Cathode For Lithium Ion Batteries

Lithium iron phosphate (LiFePO₄), with its olivine structure, has attracted extensive interest as a potential cathode material for lithium ion batteries because of its numerous appealing features, such as high theoretical capacity, high safety, environmental benignity, good cycle stability, and low cost. However, due to the low intrinsic electronic conductivity and slow lithium ion diffusion across the LiFePO₄/FePO₄ boundary, it is difficult to utilize the full theoretical capacity at useful rates. The aim of this dissertation is to develop novel methods to synthesis LiFePO₄ with high performance, especially with excellent high-rate performance.The first method is solid-state reaction method. Solid-state reaction used a simple carbothermal reduction method. The synthesis process is green. These are in the interest of decreasing preparation cost and suitable for large scale industrialization. The synthesis conditions were optimized. In the method, glucose content, calcination temperature, and calcination time have important effects on the performance of LiFePO₄. The optimized conditions are heat-treatment at 700℃for 3h with 35% glucose. The sample synthesized at these conditions presents good kinetic performance and high rate performance, and cycle stability is also good. At 0.6, 5, 10 and 20C, the capacities are 120, 105, 90 and 75mAh/g, respectively.The second method is sol-gel combustion method. Samples were synthesized by a sol-gel and combustion process. The primary crystallite of LiFePO₄ is partially coated by a carbon layer and the carbon connects crystallites into a carbon matrix structure. The method can prepare LiFePO₄ with excellent performance, especially the superior high-rate performance. At 5 and 20C, the capacities are 133 and 111mAh/g. At 60C, the capacity is 82mAh/g, and there is still about 60% of the capacity derived from above 3.00V vs. Li+/Li at such a high C rate. Cyclic voltammetry (CV) shows the material has excellent kinetic performance. The excellent kinetic performance may attribute to the carbon network coating on and connecting particles. Li ion chemical diffusion coefficients (DLi) were measured by electrochemical impedance spectroscopy (EIS). From these results, we can conclude that the controlling factor in the kinetics is not the small DLi, but the charge transfer resistance of electrode/electrolyte, when the particle is small enough.The third is spray drying method. High performance LiFePO₄/C was synthesized by combining precipitation method, spray drying and grinding. Grinding or not and different adding way of sugar were studied. The as-prepared sample owns excellent performance. At 0.1, 1, 10, 40 and 60C, the capacities are 148, 141, 108, 72 and 60mAh/g. Both low and high rate performances are excellent.