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Scale-up synthesis, Structure Characterization and Electrochemical Characteristics of C-LiFePO4 Nanocomposites for Lithium Ion Rechargeable Batteries

Carbon-coated LiFePO4 (C-LiFePO4) nanocomposites particles have been scale-up synthesized by a direct and economic solid-state reaction process. A variety of analytical techniques such as X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM, TEM, HRTEM, and HAADF), and selected area electron diffraction (SAED) are applied to investigate particles morphologies and phase structures on the nanometer scale. Single crystal and an olivine structure of the rough spherical LiFePO4 are confirmed by XRD pattern, HRTEM images and SAED patterns. The details of the coating including carbon content, thickness, and structure are particularly studied by energy filtered EF-TEM imaging, electron energy-loss spectroscopy (EELS) analysis, and X-ray photoelectron spectroscopy (XPS) analysis. The size distribution is estimated at 50-200 nm from XRD analysis and TEM images. An average 4.2% carbon content is measured and a homogenous 3 nm carbon thick layer on the particles surfaces is clearly revealed by HRTEM and EF-TEM imaging. An amorphous carbon structure was further confirmed by both EELS and XPS valence analysis. The characteristics of these nanostructures and the amorphous carbon-coating has been demonstrated to improve the electronic conductivity and cell performance by reducing the path of both electron transfer and lithium ions diffusion while the C-LiFePO4 cathode is used in the battery cell. Electrochemical performance has been evaluated by cyclic voltammetry (CV), and galvanostatic charge/discharge cycling, and AC impedance spectroscopy (EIS). The C-LiFePO4 particles exhibited improved electric conductivity, good rate capability, capacity retention, and cycling performance and superior discharge capacity with delivery of almost 99% of its theoretical discharge capacity of 168 mAh/g at a C/10 rate with a high coulomb efficiency. The CV profiles show that lithium ions and electrons are quite active during two-phase kinetic reaction, which could be attributed to the smaller particles and carbon-coating layer that facilitated extraction and insertion of lithium ions and electrons transfers, thereby is beneficial to the kinetic behavior and improving electrochemical performance during charge-discharge processes. This investigation showed that the low cost and direct solid-state manufacturing process can produce the active LiFePO4 cathode for the development of high-power Li-ion batteries for hybrid electric vehicles.