Synthesis and characterization of carbon-coated lithium transition metal phosphates LiMPO4 (M = Fe, Mn, Co, Ni) prepared via a nonaqueous sol-gel route

An organic solvent-based sol-gel method has been utilized for the synthesis of lithium transition metal phosphates. With this simple and versatile method, particles with sub-mu m size and uniform size distribution are obtained for all the LiMPO4 (M=Fe, Mn, Co, Ni) materials investigated. Homogeneous in situ carbon coating of a few weight percent is achieved with all of them, except LiCoPO4, where the in situ carbon coating is only 0.6 wt %. The best-performing as-prepared LiFePO4, with in situ surface carbon coating of 1.8 wt %, achieves an electronic conductivity on the order of 10(-2) S/cm and exhibits highly promising electrochemical performance, with only slight dependence on the carbon content of the composite electrode in certain range. Carbon-coated LiFePO4 samples prepared at lower temperatures exhibit lower electronic conductivity, evidently due to lower specific conductivity of the surface carbon. The dependence of the morphology and electrochemical performance of the synthesized LiFePO4 on the precursor concentration and the aging time of the gel is investigated. The results also yield information for understanding of the mechanisms of the sol-gel synthesis process, which are discussed. Electrochemical performance of the carbon-coated LiMnPO4 and LiCoPO4 was tested and discussed.