Continuous synthesis of lithium iron phosphate nanoparticles in supercritical water: Effect of process parameters

This study investigates the effect of various process parameters during continuous synthesis in supercritical water on the physicochemical and electrochemical properties of lithium iron phosphate (LiFePO₄) for use in large-scale lithium 2nd battery applications. The process parameters include reaction temperature (300-400°C), precursor solution concentration (0.01-0.18M), precursor solution flow rate (1.5-3.0g/min), water flow rate (9.0-36.0g/min), and residence time (9-72s). Under subcritical water conditions, mixed Fe₃(PO₄)₂{dot operator}8H₂O, Fe₂O₃, and Fe₃O₄ particles formed; in contrast, under supercritical fluid conditions, well-crystallized LiFePO₄ particles with some Fe³⁺ impurities (i.e., Fe₂O₃ and Fe₃O₄) were obtained. In supercritical water, an increase in the precursor concentration leads to an increase in the Fe³⁺ impurity content. At a high water flow rate, a significant decrease in crystallinity and the formation of Fe₃(PO₄)₂{dot operator}8H₂O and Li₆P₆O₁₈{dot operator}9H₂O phases rather than LiFePO₄ were observed. Highly crystalline LiFePO₄ with good discharge capacity was obtained with high temperature, low precursor concentration, and low flow rate conditions. Depending on the synthetic conditions, bare LiFePO₄ particles exhibit discharge capacities of 55-85mAh/g at 0.1 C-rate after 30 cycles. After carbon coating, a marginal capacity decay from 141 to 135mAh/g was observed during the 30 charge-discharge cycles.