Carbon coating on lithium iron phosphate (LiFePO4): Comparison between continuous supercritical hydrothermal method and solid-state method

Carbon coating on lithium iron phosphate (LiFePO4) plays a crucial role in determining its electrochemical performance. This study investigates the effect of carbon coating on lithium iron phosphate particles synthesized using a continuous supercritical hydrothermal synthesis (SHS) method and a conventional solid-state (SS) method, with sucrose as a carbon precursor. The carbon content, carbon structure, morphology, electronic conductivity, and electrochemical performance of the carbon-coated LiFePO4 (C-LiFePO4) are characterized as a function of the following coating conditions: sucrose concentration, calcination temperature, and calcination time. The particles produced using supercritical water have a smaller size (400–1000 nm), larger BET surface area of 7.3 m2/g, and lower degree of particle aggregation compared with those produced via solid-state synthesis (particle size: 3–15 μm; BET surface area: 2.4 m2/g). The differences in the particle size and particle morphology of the LiFePO4 prepared using the two synthetic methods cause a significant difference in the uniformity of the carbon coating, carbon structure, and electronic conductivity. A more uniform carbon layer coating and greater amount of graphitic carbon are found in the LiFePO4 particles produced via the SS method. This leads to a higher discharge capacity of 147 mA h/g at a current density of 17 mA/g (0.1 C) after 30 cycles when compared with the C-LiFePO4 produced by the SHS method (135 mA h/g). No obvious capacity fading was observed. At a high current of 1700 mA/g (10 C), the delivered capacities of the C-LiFePO4 particles produced via the SS and the SHS methods are 55% and 52% of the theoretical value, respectively, at a carbon content of 6 wt.%. The carbon-coated samples prepared using the SHS and SS methods exhibit similar discharge capacity trends for the carbon content. As the carbon content increased to 6 wt.%, the discharge capacity increased, while a further increase in the carbon content to 10 wt.% resulted in a decrease in the discharge capacity. Thus, the carbon content and particle properties need to be carefully optimized to enhance the electrochemical performance of C-LiFePO4.

► Carbon coating on LiFePO4 via supercritical hydrothermal synthesis (SHS) first presented.
► Carbon coating on SHS’d LiFePO4 compared with solid-state synthesized LiFePO4.
► Non-uniform carbon coating and low graphitic carbon content observed on SHS’d LiFePO4.