Study On Synthesis And Doping Modification Of LiFePO₄ Cathode Materials

LiFePO₄ is the most promising cathode material for rechargeable lithium batteries due to its abundant supply, environmentally benign, appreciable theoretical capacity and good thermal stability .In this thesis, LiFePO₄/C composite was synthesized by solid-state reaction. The micro-structure and morphologies were investigated by XRD, SEM. The electrochemical performances were evaluated by galvanostatic charge-discharge, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS). Technological conditions of this synthesis method were investigated systematically and optimized. The effects of cation doping (V⁵⁺, Ti⁴⁺) and F substitution were also studied.LiFePO₄/C composite was synthesized with a starting mixture of FePO₄.H₂O, LiOH.H₂O and PP. The LiFePO₄/C composite synthesized at 650 ℃ exhibited good electrochemical performance. The particle size was small (200³00nm) and homogeneous, large discharge capacity and good cycle stability were observed. Effect of heating rate on electrochemical performance of LiFePO₄/C composite was studied. Particle size of the product heated at 3℃/min increased considerably. The average size is larger than 2μm, it caused obvious polarization, the initial capacity at 0.5C is only 116 mAh/g. The effect of ball milling were also studied. It is found that the product was more pure and demonstrated better electrochemical performance if the raw material was ball milled. The more the content of C is, the larger discharge capacity was, the carbon uniformly distributing on the nanosized LiFePO₄ particles significantly improved the electrochemical performance, especially the LiFePO₄/C composite with 4.3 % carbon content demonstrated excellent charge-discharge abilities with stable capacity of 126mAh/g at 1C rate.LiFe_1-xPO₄/C(x=0.01, 0.02, 0.03, 0.04), LiFe_1-xTi_xPO₄/C(x=0.01, 0.02,.0.04) and Li_1-xFePO_4-xF_x/C (x=0, 0.01,0.03, 0.05) were synthesized to investigate the influence of cation doping and F subsitution on the electrochemical properties of LiFePO₄/C composite. By doping of V⁵⁺, electronic conductivity and diffusion rate of Li⁺ were improved and electrochemical polarization was obviously reduced which improved the high rate cycling property and cycle stability, the LiFe_0.97V_0.03PO₄/C showed better performance. Its initial capacity at 1C was 136 mAh/g. Discharge capacity inceased to 144 mAh/g after 50 cycles. It was found that dischare capacity was improved and cycle performance was enhanced with Ti⁴⁺ doped. When x is 0.04, the LiFe_1-xTi_xPO₄/C demonstrated better charge-dischare abilities. Its initial capacity at 0.5C and 1C are 135 mAh/g and 131 mAh/g respectively, the dischare capacity reached 145 mAh/g at 0.5C rate and 137 mAh/g at 1C rate after 50 cycles. Theinfluence of Fluorine subsitution on the electrochemical properties of LiFePCVC composite was also studied, electrochemical analyses indicated that doping of F reduced electrochemical polarization and electronic resistance, cyclic performance of LiFePCVC composite was improved, the stable capacity of Ii1.jcFePO4.JfyC (x=0.03) at 0.5C rate was 138mAh/g, capacity fading was not observed.