Synthesis, Structure And Performance Of LiFePO₄ /C By Mechanical Activation Combined With Carbothermal Reduction Method

Olivine-type LiFePO4 have been known as an interesting cathode material for lithium ion batteries due to its flat potential plateau, high thermal and chemical stability and high specific capability, as well as good cycle performance, non-toxicity and low cost. However, there are three main things to do to put LiFePO4 into commercially used. The first one is to find a low-cost preparation method which could be large-scale produced. The second one is to find an effective way to improve its ion and electrochemical conductivity. The third one is to synthesize fine and homogeneous particle sizes of LiFePO4 and improve its tap-density. Compared with other synthesis techniques, CTR (carbothermal reduction method) is the most promising method for industry producing with its low cost and simple process. However, the hard-controlled particle size during the calcination at high temperatures makes it an obstacle for its practical applications. In this paper, LiFePO4/C composite cathode materials have been synthesized by mechanical activation combined with carbothermal reduction method. The influences of parameters of mechanical activation (ball-to-power weight ratio, milling media, the atmosphere, temperature and time of ball-milling) on the structure and performance of the material were investigated. The structure, morphology and electrochemical properties of LiFePO4/C prepared at different temperature, time, proportion ratio and heating rate were also studied. The main results and conclusions were listed as following:(1) Studied the processing conditions of the precursor including ball-to-power weight ratio, milling media, the atmosphere, temperature and time of ball-milling on the performance of the prepared LiFePO4/C. The result indicated that, LiFePO4/C having an average particle size of about 500nm can be prepared by ball-milling the forerunner with ball-to-powder ration at 6:1, and the powder exhibits a capacity of 134mAh/g at 1C rate. The cycling retention rate of it after 20 cycles was about 99.2% of its maximum capacity. LiFePO4/C material prepared by ball-milling the precursor in agate tank are spherical with average particle size of about 500-600nm. The discharge capacity of it reaches 134mAh/g at 1C rate and the capacity retention after 20 cycles is 99%. LiFePO4/C material prepared by ball-milling the precursor in Ar atmosphere with the highest specific surface area of 30.452m2/g and carbon content of 5.27% has an excellent discharge capacity of 132mAh/g at 1C rate. LiFePO4/C material prepared by ball-milling the precursor at 25℃for 6h can deliver 136mAh/g of reversible capacity at 1C rate with a low capacity fading after 20 cycles.(2) Studied the allocated proportion of the precursor (FePO4 and Li2CO3), calcining temperature, the calcine time and heating rate on the performance of the prepared LiFePO4/C. The result indicated that increasing the sintering temperature and extending sintering time resulted in higher crystallinity but a larger particle size. In the range of 600～750℃and 6～24h, 650℃and 12h are the optimum synthetic temperature and sintering time for the LiFePO4/C with small particle sizes and perfect crystal. The specific discharge capacity reaches as high as 138mAh/g at 1C rate. When the raw material mole ratio (nLi:nFe:nP) is 1.02:1:1, the LiFePO4/C showed the highest potential plateau at 3.36V and the best capacity of 137mAh/g at 1C rate with capacity retention of 99% after 20 cycles. When the heating rate is 2℃/min, LiFePO4/C reaches the best specific discharge capacity and Coulomb's efficiency with 134.6mAh/g at 1C rate In the present case, LiFePO4/C synthesized at 650℃for 12h with raw material mole ratio (nLi:nFe:nP) of 1.02:1:1 and heating rate of 2℃/min shows best electrochemical performances. Under this condition, the obtained LiFePO4/C shows an initial discharge capacity of 158.7 mAh/g,150.9.2 mAh/g,143.2mAh/g and 135.9mAh/g at 0.1C, 0.2C,0.5C and 1.0C rate, respectively. After 20 cycles, the composite cathode retains 99.6% and 98.3% of the first cycle discharge capacity at 1.0C and 2C, respectively.