Small capacity decay of lithium iron phosphate (LiFePO4) synthesized continuously in supercritical water: Comparison with solid-state method

Nanosize lithium iron phosphate (LiFePO4) particles are synthesized using a continuous supercritical hydrothermal synthesis method at 25 MPa and 400 °C under various flow rates. The properties of LiFePO4 synthesized in supercritical water including purity, crystallinity, atomic composition, particle size, surface area and thermal stability are compared with those of particles synthesized using a conventional solid-state method. Smaller size particles ranging 200-800 nm, higher BET surface area ranging 6.3-15.9 m2 g−1 and higher crystallinity are produced in supercritical water compared to those of the solid-state synthesized particles (3-15 μm; 2.4 m2 g−1). LiFePO4 synthesized in supercritical water exhibit higher discharge capacity of 70-80 mAh g−1 at 0.1 C after 30 cycles than that of the solid-state synthesized LiFePO4 (60 mAh g−1), which is attributed to the smaller size particles and the higher crystallinity. Smaller capacity decay at from 135 to 125 mAh g−1 is observed during the 30 cycles in carbon-coated LiFePO4 synthesized using supercritical water while rapid capacity decay from 158 to 140 mAh g−1 is observed in the carbon-coated LiFePO4 synthesized using the solid-state method.