Effects of Reductive Conditions on the Microstructure and Electrochemical Properties of Sol-Gel Derived LiFePO[sub 4]∕C

Abstract

A sol-gel method was used to prepare LiFePO4 C composites by using ferric iron as iron precursor. Effects of reductive conditions including the amounts of organic additive of citric acid and the H2 content in the sintering atmosphere of N2 on the microstructural characteristics and electrochemical properties of LiFePO4 C are investigated. The LiFePO4 C sample prepared with citric acid and cations in a ratio of 1:2 under sintering atmosphere of 10% H2 + N2 shows a highest discharge capacity of 135 mAhg at the rate of 0.1 C (1 C=170 mAg). A carbon content of around 4 wt % provides a satisfactory discharge capacity of LiFePO4. Citric acid can act as a reducing reagent; hydrogen in a range of 10-20 vol % in the sintering atmosphere can also reduce Fe3+ to Fe2+ completely and a further suitable amount of electro-conductive Fe2 P phase could possibly form. Iron phosphides play an important role in the improvement of the high rate capacity of LiFePO4 C and the reaction kinetics of lithium ion. At discharge rates higher than 1 C, suitably increasing the amount of Fe2 P to a range of 6-11 wt % in the LiFePO4 C composite favors the rate performance of LiFePO4 C. © 2007 The Electrochemical Society.