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

Abstract

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?0.18 M), precursor solution flow rate (1.5?3.0 g/min), water flow rate (9.0?36.0 g/min), and residence time (9?72 s). Under subcritical water conditions, mixed Fe3(PO4)2?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?8H2O and Li6P6O18?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?85 mA h/g at 0.1 C-rate after 30 cycles. After carbon coating, a marginal capacity decay from 141 to 135 mA h/g was observed during the 30 charge?discharge cycles.