Continuous synthesis of lithium iron phosphate (LiFePO4) nanoparticles in supercritical water: Effect of mixing tee

Continuous supercritical hydrothermal synthesis of olivine (LiFePO4) nanoparticles was carried out using mixing tees of three different geometries; a 90° tee (a conventional Swagelok® T-union), a 50° tee, and a swirling tee. The effects of mixing tee geometry and flow rates on the properties of the synthesized LiFePO4, including particle size, surface area, crystalline structure, morphology, and electrochemical performance, were examined. It was found that, when the flow rate increased, the particle size decreased; however, the discharge capacity of the particles synthesized at the high flow rate was lower due to the enhanced formation of Fe3+ impurities. The use of a swirling tee led to smaller-sized LiFePO4 particles with fewer impurities. As a result, a higher discharge capacity was observed with particles synthesized with a swirling tee when compared with discharge capacities of those synthesized using the 90° and 50° tees. After carbon coating, the order of initial discharge capacity of LiFePO4 at a current density of 17 mA/g (0.1C) and at 25 °C was swirling tee (149 mAh/g) > 50° tee (141 mAh/g) > 90° tee (135 mAh/g). The carbon-coated LiFePO4 synthesized using the swirling tee delivered 85 mAh/g at 20C-rate and at 55 °C.