Continuous flame aerosol synthesis of carbon-coated nano-LiFePO 4 for Li-ion batteries

Abstract

Core–shell, nano-sized LiFePO 4-carbon particles were made in one step by scalable flame aerosol technology at 7 g/h. Core LiFePO 4 particles were made in an enclosed flame spray pyrolysis (FSP) unit and were coated in-situ downstream by auto thermal carbonization (pyrolysis) of swirl-fed C 2H 2 in an O 2-controlled atmosphere. The formation of acetylene carbon black (ACB) shell was investigated as a function of the process fuel-oxidant equivalence ratio (EQR). The core–shell morphology was obtained at slightly fuel-rich conditions (1.0<EQR<1.07) whereas segregated ACB and LiFePO 4 particles were formed at fuel-lean conditions (0.8<EQR<1). Post-annealing of core–shell particles in reducing environment (5 vol% H 2 in argon) at 700 °C for up to 4 h established phase pure, monocrystalline LiFePO 4 with a crystal size of 65 nm and 30 wt% ACB content. Uncoated LiFePO 4 or segregated LiFePO 4–ACB grew to 250 nm at these conditions. Annealing at 800 °C induced carbothermal reduction of LiFePO 4 to Fe 2P by ACB shell consumption that resulted in cavities between carbon shell and core LiFePO 4 and even slight LiFePO 4 crystal growth but better electrochemical performance. The present carbon-coated LiFePO 4 showed superior cycle stability and higher rate capability than the benchmark, commercially available LiFePO 4.