Effect of particle size and purity on the low temperature electrochemical performance of LiFePO4/C cathode material

The effect of particle size and purity of LiFePO4/C material on the low temperature performance of LiFePO4/C material prepared with FePO4 center dot 2H(2)O as precursor are systematically investigated. It is found that reducing particle size of LiFePO4/C material can improve the low temperature performance, and the purity of LiFePO4/C material has more important influence on the low temperature performance. Compared to the high purity LiFePO4/C material, the low purity LiFePO4/C material exhibits poor low temperature performance. At -20 degrees C under 0.2C, the specific capacity of low purity LiFePO4/C material is 52.4 mAh g(-1) lower than that of high purity LiFePO4/C material which is 105.6 mAh g(-1). The 18650-1400 mAh cylindrical batteries confirm the opinion. At the operation temperature of 0, -20 and -40 degrees C under 0.2C, the capacity retentions of low purity LiFePO4/C material are 40.9%, 30.3% and 10.9% much lower than those of high purity LiFePO4/C material which are 81.5%, 70.2% and 46.6% compared with those of at 25 degrees C, respectively. The studies on the mechanism indicate that low purity LiFePO4/C material contains impurities which could incorporated into LiFePO4 lattice by high-temperature diffusion process and cause the lattice distortion, blocking the Li+ ions diffusion. It have calculated that the D-Li value of low purity LiFePO4/C material is 8.28 x 10(-15) cm(2) s(-1) lower two orders than that of high purity LiFePO4/C material which is 2.73 x 10(-13) cm(2) s(-1), leading to the sluggish Li+ ions diffusion kinetics at low temperature. Therefore, the particle size and purity of LiFePO4/C material are the mainly factors influencing the low temperature performance of LiFePO4/C material, of which removing the impurities in the raw materials and improving the purity of the iron source to synthesize FePO4 center dot 2H(2)O material is a deterministic way to improve the low temperature performance of LiFePO4/C material. (C) 2016 Elsevier B.V. All rights reserved.