Synthesis and properties of optimized LiFePO4/C by a CVD-assisted two-step coating method

Abstract

LiFePO4/C composite was synthesized by a chemical vapor deposition (CVD)-assisted two-step coating method using sucrose and methane as the carbon sources. Sucrose and methane were introduced in the presintering and the CVD-assisted sintering process, respectively. The structure and morphology of LiFePO4/C composite were characterized by X-ray diffraction (XRD), scanning electron microscope, and transmission electron microscope (TEM). The electrochemical properties of the composite were evaluated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge cycling techniques. The results showed that the synthesized LiFePO4/C composite had an average particle size of about 200 nm and exhibited superior properties. XRD indicated that the olivine-type LiFePO4 without any detectable impurities was obtained. TEM showed not only nano-sized carbon webs connecting the LiFePO4 particles were obtained, but also a uniform carbon layer between 3 and 4 nm was coated on the surface of LiFePO4 particles at a low carbon content of 1.35 %. The as-prepared LiFePO4/C composite exhibited a high initial coulomb efficiency of 99.6 % and an excellent rate performance with discharge capacity of 118.8 mAh g−1 at 10 C. The remarkable electrochemical improvement was attributed to the increase of both electric and ionic conductivity due to the small particle size and the homogeneous conductive network. Compared with the traditional solid-state and CVD method, a pure LiFePO4 with homogeneous conductive network at lower carbon content was obtained.