Structure optimization and the structural factors for the discharge rate performance of LiFePO4/C cathode materials

Carbon coating and iron phosphides of high electron conductivity were introduced into the LiFePO₄ materials which were derived via a sol-gel method in order to improve the high discharge rate performance. The start constituents were FeC₂O₄·2H₂O, LiOH·H₂O, NH₄H₂PO₄ and ethylene glycol. Effects of the calcination temperature and the ethylene glycol on the structure and the electrochemical performance of the LiFePO₄ materials were investigated. Structure analyses showed that the addition of ethylene glycol caused an obvious decrease in the particle size of LiFePO₄. 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 LiFePO₄ were much important for the capacity rather than the iron phosphides, and relative high content of Fe₂P (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. Fe₂P effectively increased both the reaction and diffusion kinetics and hence enhanced the utilization efficiency of the LiFePO₄ at high discharge rate. Combining relative small particle size, even 2 wt.% of iron phosphides could improve the high rate performance of LiFePO₄/C significantly.