Study On Performance Of LiFePO₄ Cathode Material Synthesized By Solid-State Reaction

LiFePO₄/C and LiFePO_4-xF_x/C samples were synthesized by carbothermal reduction method in this paper. The microstructures and morphologies of these composites were characterized by XRD, SEM, TEM, Raman and FTIR observations. The electrochemical performances were evaluated by galvanostatic charge-discharge test system. The effects of consumption amount of lithium salt, new organic carbon source and F-doping on the physical and electrochemical systematically investigated.The experimental results found that LiFePO₄/C with 8% excess of lithium salts, exhibited similar particle size and optimal electrochemical performance, whose initial discharge capacities could reach 156.6 and 143.5mAh/g at 0.2C and 1C rates, respectively, with a cycling capacity retention rate of 99.8% after 100 cycles at 2C rate.LiFePO₄/C samples were prepared with glucose, polyvinyl alcohol (PVA), polyethylene glycol (PEG) as carbon sources, respectively. The results showed that the graphitization degree of coated-carbon had little effect on the conductivities of LiFePO₄/C samples. Among which LiFePO₄/C with glucose as carbon source had a highest electronic conductivity of 5.70×10-2S/cm, while the sample with PEG as carbon source had a lowest electronic conductivity of 5.59×10-2S/cm. But the with PEG as carbon source showed the optimal electrochemical performance, whose initial discharge capacity could reach 155.9mAh/g at 0.2C rate, with a cycling capacity retention rate of 103% after 50 cycles at 3C rate.The orthogonal test L9(33) was designed for the LiFePO₄/C sample with PEG as carbon source and 8% excess of lithium salts to study the effects of calcination temperature, time, and C doping amount on the properties of samples using the initial discharge capacity at 0.2C rate as an indicator. The obtained optimal preparation conditions of LiFePO₄/C were as follows: calcination temperature 750℃, time 12h, FePO₄ : Li2 CO3 : C = 2: 1.08: 2. The sample prepared under the optimal conditions had perfect crystalline, small primary particle size (about 500nm), and uniform particle size distribution. The primary particles were connected with each other through coated-carbon. The LiFePO₄/C sample shwed a stable discharge platform and low polarization voltage (≤0.2V), whose initial discharge capacities could reach 157.6, 142.5, 134.3mAh/g at 0.2C, 1C and 2C rates, respectively. Meanwhile, based on the single factor experiment of calcination temperature, it was found that the increase of calcination temperature from 650℃to 750℃, led to the enhancement of electronic conductivity and discharge capacity. However, the further increase of temperature to 800℃, led to the decrease of discharge capacity from 157.6mAh/g (at 750℃) to 154.39mAh/g. At the same time, taking energy conservation into consideration, the optimal calcination temperature was set as 750℃.It is found that F-doping could effectively enhance the high-rate dischargeability and cycling performance of LiFePO₄/C by the study of the electrochemical properties of LiFePO_4-xF_x/C. The LiFePO3.96F0.04/C sample exhibited an initial discharge capacity of 109.0mAh/g at 5C rate higher than LiFePO₄/C, whose cycling capacity retention rate could reach 100.3% after 50 cycles at 3C rate.