Study On Synthesis And Performance Of Cathode Materials LiFePO₄

Since Padhi et al found lithium iron phosphate (LiFePO4) could be used as cathode material for the lithium ion batteries, many research groups have been devoted to study the performance of this material. They think that LiFePO4 is one of the most promising cathode materials for the lithium ion batteries because it is abundant, environmentally benign, stable and safe. The disadvantage of LiFePO4 is the low electrical conductivity and Li-ion diffusivity. Numerous approaches directed at overcoming these problems have been described in the literature, including adopting low temperature liquid-phase process to control particle size and morphology, coating conductive material by carbon or fine metal particles to enhance the electrical conductivity, and attempting at doping with supervalent cations to improve the stability of material structure. So we decide to adopt sol-gel method to control particle size and morphology and adopt in-situ polymerization method to coat polyaniline on the surface of LiFePO4 particle to improve the electrochemical performance of LiFePO4. The main contents of this paper are given as following.Firstly, we retrospect the development of cathode material for lithium ion batteries, focusing on the synthesis and improvement of properties of LiFePO4. The problems in application and researching direction in the future are analyzed and pointed out. Then the LiFePO4 has been synthesized by solid-state method and sol-gel method, and the synthetic conditions of sol-gel method were optimized. The crystalline structure, morphology of particles and crystalline size of the LiFePO4 samples prepared at this optimized condition were investigated by X-ray diffraction and scanning electron microscopy. The charge-discharge test results of the optimal LiFePO4 sample show that the first discharge capacity is 131.3 mAh/g, the charge-discharge efficiency is 91.5% and the capacity retain ratio is 95.5% after 10 cycles.Polyaniline (PAn), one of conductive polymers, is synthesized from aniline (An) by chemical oxidation polymerization method in chapter four. The effects of several factors (including oxidant concentration, reaction temperature, reaction time, acid concentration) were studied. The optimized parameter of synthesis is that the mole ratio of ammonium persulfate (APS) and An is 1:1, reaction temperature is 5℃, reaction time is 6 h, and acid concentration is 1mol/L, respectively. A series of polyaniline-LiFePO4 (PAn-LiFePO4) composites were synthesized by in-situ polymerization in chapter five. Research results showed that PAn-LiFePO4 composites had higher discharge capacity and better cycling performance. The PAn-LiFePO4 composite with 25 wt% polyaniline showed the best electrochemical performance.Its first specific discharge capacity was 140.3 mAh/g at 0.1 C and its specific discharge capacity was 118.7 mAh/g after 50 cycles at 1 C.