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 LiFePO ₄ 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.