Low-temperature electrochemical performances of LiFePO4 cathode materials for lithium ion batteries

The key factors influencing the low-temperature electrochemical performances of LiFePO4 cathode materials are systematically investigated by measuring and comparing the amount of surface carbon, particle size, and conductivities of LiFePO4, LiFePO4/C and electrolyte in the temperatures range of -20 to 40 degrees C. It is found that the low-temperature electrochemical performance of the material is improved obviously by coating the surface of LiFePO4 with carbon. At -20 degrees C, the discharge specific capacity of LiFePO4/C with 4 wt% surface carbon is 81.4 mAh/g, while the uncoated LiFePO4 only delivers the discharge specific capacity of 27.7 mAh/g. The low-temperature electrochemical performance of LiFePO4/C has no significant change as increasing the carbon content, but is further improved by reducing the particle size. In order to understand the difference of low-temperature electrochemical performance between the carbon-coated and uncoated LiFePO4 cathodes, the ionic conductivity of electrolyte, the electronic/ionic conductivities of LiFePO4 and LiFePO4/C at various temperatures were measured. The electronic conductivity of LiFePO4 with 4.9 x 10(-11) S/cm at -20 degrees C is found to be responsible for its poor low-temperature electrochemical performance. However, for the LiFePO4/C, the low-temperature electrochemical performance is mainly determined by its ionic conductivity, therefore, the decrease of the particle size can increase the low-temperature electrochemical performance. (C) 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.