The effect of carbon coating thickness on the capacity of LiFePO4/C composite cathodes

Two types of carbon source and precursor mixing pellets were employed simultaneously to prepare the LiFePO4/C composite materials: Type I using the LiFePO4 precursor with 20 wt.% polystyrene (PS) as a primary carbon source, and Type II using the LiFePO4 precursor with 50 wt.% malonic acid as a secondary carbon vapor source. During final sintering, a Type I pellet was placed down-stream and Type II precursor pellet(s) was(were) placed upstream next to a Type I precursor pellet in a quartz-tube furnace. The carbon-coated product of the sintered Type I precursor pellet was obtained by using both PS and malonic acid as carbon sources. When two Type II pellets were used as a carbon vapor source (defined as Product-2), a more uniform film between 4 and 8 nm was formed, as shown in the TEM images. In the absence of a secondary carbon source (defined as Product-0), the discharge capacity of Product-0 was 137 mAh g(-1) with 100 cycles at a 0.2C-rate, but Product-2 demonstrated a high capacity of 151 mAh g(-1) with 400 cycles. Our results indicate that electrochemical properties of LiFePO4 are correlated to the amount of carbon and its coating thickness and uniformity. (C) 2008 Elsevier B.V. All rights reserved.