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∕CLiFePO4∕C composites by using ferric iron as iron precursor. Effects of reductive conditions including the amounts of organic additive of citric acid and the H2H2 content in the sintering atmosphere of N2N2 on the microstructural characteristics and electrochemical properties of LiFePO4∕CLiFePO4∕C are investigated. The LiFePO4∕CLiFePO4∕C sample prepared with citric acid and cations in a ratio of 1:2 under sintering atmosphere of 10% H2+N2H2+N2 shows a highest discharge capacity of 135mAh∕g135mAh∕g at the rate of 0.1C0.1C (1C=170mA∕g)(1C=170mA∕g) . A carbon content of around 4wt%4wt% provides a satisfactory discharge capacity of LiFePO4LiFePO4 . Citric acid can act as a reducing reagent; hydrogen in a range of 10–20vol%10–20vol% in the sintering atmosphere can also reduce Fe3+Fe3+ to Fe2+Fe2+ completely and a further suitable amount of electro-conductive Fe2PFe2P phase could possibly form. Iron phosphides play an important role in the improvement of the high rate capacity of LiFePO4∕CLiFePO4∕C and the reaction kinetics of lithium ion. At discharge rates higher than 1C1C , suitably increasing the amount of Fe2PFe2P to a range of 6–11wt%6–11wt% in the LiFePO4∕CLiFePO4∕C composite favors the rate performance of LiFePO4∕CLiFePO4∕C .