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.