Synthesis, Structure And Electrochemical Performances Of Carbon Coated LiFePO₄ And Li₃V₂ (PO₄)₃ Cathodic Materials For Lithium-ion Batteries

This work aims to develop the olivine LiFePO₄ and monoclinic Li₃V₂(PO₄)₃ cathodic materials by improving the synthesis method, element doping or coating to enhance their electrochemical performances. The structure and morphology of these products were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). The electrochemical performances were evaluated by galvanostatic charge-discharge cycling, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS). The effects of the synthesis process and element doping or coating on the electrochemical performaces were investigated in detail.In Chapter 3, the LiFePO₄/C composite was prepared by one-step solid state carbon thermal reduction method using LiOH·H2O and FePO4·4H2O as the reagents and asphalt as both reductive agent and carbon source. Interesting carbon nano-interconnect structures were found between the LiFePO₄ grains. Based on the thermogravimetry (TG), differential scanning calorimetry (DSC), TEM and HRTEM results, a growth model was proposed to illustrate the formation of the carbon nano-interconnect between the LiFePO₄ grains. The effects of sintering temperature, preheating process, iron sources and carbon content on the electrochemical properties of materials were investigated. It was found that LiFePO₄/C composite prepared by pre-sintering showed good crystallization, narrow particle size distribution and excellent electrochemical properties. By analyzing the kinetic of the LiFePO₄/C composites, it was found that two-phase of Fe²⁺/Fe³⁺ were coexistent during the electrochemical reaction process. Experiments proved that the LiFePO₄/C composites with 3.16 wt% carbon had the initial discharge capacity of 150 mAh/g at 0.1 C rate in the voltage range of 2.5 to 4.2 V. And it showed good cycle performance with the capacity loss of 4.7% after 50 cycles. These LiFePO₄/C composites (3.16 wt% C) were synthesized with a heating rate of 5 oC/min, a pre-sintering temperature of 380 oC (1 h), and an intering temperature of 650 oC (8 h). The results indicated that appropriate amount of carbon could inhibit the growth of LiFePO₄ grain. The carbon nano-interconnect structures did contribute to the enhancement of the conductivity of the materials and the improvement of the discharge capacity and cycle stability.In Chapter 4, monoclinic Li₃V₂(PO₄)₃ powder was prepared by one-step solid state carbon thermal reduction method, in which the LiOH·H₂O, NH₄H₂PO₄ and NH₄VO₃ were used as the reagents, the asphalt as the carbon sources. The product showed high purity, good crystallinity, small particle sizes and homogenous size distribution. SEM, TEM and EDS resultes proved that amorphous carbon was coated on the surface of Li₃V₂(PO₄)₃. Abundant Li₃V₂(PO₄)₃ nanorods can be found. The effects of sintering temperature and lithium sources on the electrochemical properties of materials were investigated. The electrochemical performance and kinetic mechanism of the Li₃V₂(PO₄)₃ in different voltage rang were systematic investigated. The Li₃V₂(PO₄)₃/C composites that synthesized at 800oC exhibited good crystallinity, narrow size distribution and good electrochemical performances. The discharge capacity was about 141 mAh/g at 0.1 C rate in the voltage region (3.0-4.5 V). The capacity loss was 2.7% after 30 cycles. Kinetic mechanism of the Li₃V₂(PO₄)₃/C composites was also studied. The results indicated that the extraction and insertion the previous two lithium ion was two-phase reaction process. The the extraction and insertion of first lithium ion was corresponding to Li₃V₂(PO₄)₃←→Li₂V₂(PO₄)₃, the second lithium ion was corresponding to Li₂V₂(PO₄)₃←→LiV₂(PO₄)₃. Nevertheless, the inserting of the third lithium ion was solid solution reaction.