Structure optimization and the structural factors for the discharge rate performance of LiFePO 4/C cathode materials

Abstract

Carbon coating and iron phosphides of high electron conductivity were introduced into the LiFePO 4 materials which were derived via a sol–gel method in order to improve the high discharge rate performance. The start constituents were FeC 2O 4·2H 2O, LiOH·H 2O, NH 4H 2PO 4 and ethylene glycol. Effects of the calcination temperature and the ethylene glycol on the structure and the electrochemical performance of the LiFePO 4 materials were investigated. Structure analyses showed that the addition of ethylene glycol caused an obvious decrease in the particle size of LiFePO 4. Calcination temperature and ethylene glycol jointly affected the formation of iron phosphides. Combining the electrochemical testing, it was found that, at low discharge rate, small particle size and high content of LiFePO 4 were much important for the capacity rather than the iron phosphides, and relative high content of Fe 2P (e.g. 8 wt.%) even worsened the capacity. However, with the increase of the discharge rate, the high electron conductive iron phosphides turned to play important role on the capacity. Fe 2P effectively increased both the reaction and diffusion kinetics and hence enhanced the utilization efficiency of the LiFePO 4 at high discharge rate. Combining relative small particle size, even 2 wt.% of iron phosphides could improve the high rate performance of LiFePO 4/C significantly.