Preparation And Performance Modification Research Of LiFePO_ 4/C For Cathode Material Of Lithium-Ion Battery

The olivine LiFePO₄ is the most promising cathode material for lithium ion batteries. However, the extremely low electronic conductivity and lithium-ion diffusion rate limit the high-rate properties and hinder its commercial application. In this thesis, a series of LiFePO₄/C materials were synthesized bycarbothermal reduction method. The effects of processing parameters such as ball-milling way, sintering temperature and time, surface modification by LiCoO₂ and La₂O₃ and addition of Li₃V₂(PO₄)₃ were studied. Phase composition and micro-morphologies of these materials were investigated by X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The electrochemical performances were evaluated by galvanostatic charge-discharge, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).XRD results showed that the LiFePO₄/C cathode materials synthesized has olivine structure, which belongs to space group of Pmnb. The optimizing technological conditions on preparing LiFePO₄/C were determined. The raw materials are dry ball-milled. The sintering temperature and time are 700℃and 12 h, respectively. The size of LiFePO₄/C particle synthesized at the optimized condition is about 2μm and a layer of carbon film with about 10 nm thickness surrounding the particle can be observed. The initial discharge specific capacity reaches 135.5 mA·h/g at 0.2C rate.LiFePO₄/C particles were surface-modified by LiCoO₂ and La₂O₃ via sol-gel process. XRD patterns of modified LiFePO₄/C revealed that the crystal structure of material is unchanged. The rate performance of LiFePO₄/C is remarkably improved by 2 wt.% LiCoO₂. At 1C rate, the initial discharge specific capacity of surface-treated LiFePO₄/C is 109.3 mA·h/g. After La₂O₃ modification, the dissolution of Fe²⁺ into LiPF₆ electrolyte is greatly suppressed. The cycle performance of LiFePO₄/C is remarkably improved by 1 wt.% La₂O₃. At 1C rate, the capacity retention rate of stabilized LiFePO₄/C is 91.4% of initial capacity after 30 cycles.LiFePO₄-based LiFePO₄·Li₃V₂(PO₄)₃ composite cathode materials were prepared by carbothermal reduction method. The results indicated LiFePO₄ and Li3_V2(PO₄)₃ are part-doped by V and Fe, respectively. The LiFePO₄·Li3_V2(PO₄)₃ powder containes well-mixed LiFePO₄ and Li3_V2(PO₄)₃ phases, which adhere together. At 1C rate, as mass ratio of LiFePO₄ and Li3_V2(PO₄)₃ reaches to 9:1, the initial discharge specific capacity of as-synthesized composite material is 138.6 mA·h/g and the capacity retention rate is 91.8% of initial capacity after 30 cycles.