Sythesis And Modification Of Polyanionic Cathode Materials For Lithium-ion Batteries

Polyanionic LiFePO₄ and Li₃V₂(PO₄)₃ have been considered as one of the most promising cathode materials for lithium ion batteries because of their low cost, high capacity, stable structure and thermal stability. However, their low electronic conductivity induces poor rate capability, and impedes their commercialization. In this paper, the structure, electrochemical properties and electrode kinetics of the two kind of materials were investigated by X-ray diffraction (XRD),scanning electron microscope (SEM),cyclic voltammetry (CV),electrochemical impedance spectroscopy (EIS) and charge-discharge tests. Some measures such as improving synthesis methods, optimization preparation conditions, coating or doping modification were taken to improve the electrochemical performances of them.The effects of ration of raw materials, hydrothermal temperature and time on the structure, morphology and electrochemical properties of LiFePO₄ were investigated in detail. The optimized conditions were Li:Fe:P=3:1:1, 170?C for 10h. EIS was applied to study the electrode kinetics of LiFePO₄. It was shown that the electrochemical control process was charge transfer in high frequence region, and diffusion in low frequence region during Li⁺ intercalation/deintercalation prophase and telophase. In order to improve the performances of LiFePO₄, multi-walled carbon nanotubes (MWCNTs) and PbTe alloy coating were studied, respectively. XRD results showed the MWCNTs or PbTe coated materials kept the same structure as LiFePO₄. SEM tests showed that LiFePO₄ particles were coated by well-dispersed MWCNTs or little PbTe particles respectively. Charge-discharge tests showed the rate and cyclic performances of LiFePO₄ were improved significantly and 5mass% coated materials presented the best electrochemical properties. The results of CV,EIS and conductivity tests indicated that the improvement of electrochemical properties by MWCNTs or PbTe coating could be ascribed to higher reversibility of Li⁺ intercalation/deintercalation reaction, higher conductivity and lower ohm resistance Re and charge transfer resistance Rct.LiFePO₄/C was synthesized by sol-gel method and the effects of the surcose amount, sintering temperature and time on the structure, morphology and electrochemical performances were investigated. The optimized conditions were 60mass% sucrose, 700?C for 10h. In order to further improve the performances of LiFePO₄/C, the effects of Al doping on the structure, morphology and electrochemical properties were studied. XRD results showed that the lattice volume of doped materials shrank,and the formation of olivine structure increased. SEM tests showed the uniformity of particles size was improved. Charge-discharge tests showed the rate and cyclic performances of LiFePO₄/C were improved obviously and the doped material of x=0.01 had the best electrochemical properties. The results of CV, EIS and conductivity tests indicated that the improvement of electrochemical performances by Al doping could be ascribed to higher reversibility of Li⁺ intercalation/deintercalation reaction, higer conductivity and lower Re and Rct. Elecronic structure analysis indicated Al doping increased the conductivity of LiFePO₄ through the band gap reduction and the doping material exihibited local metal property.Li₃V₂(PO₄)₃/C was synthesized by sol-gel method and the effects of the Vc amount, sintering temperature and time on the structure, morphology and electrochemical performances were investigated. The optimized conditions were 20mass% Vc, 800℃for 8h. EIS technology was applied to study the electrode kinetics of Li₃V₂(PO₄)₃/C. It was shown that the electrochemical control process was charge transfer in high frequence region, and diffusion in low frequence region during Li⁺ intercalation/deintercalation prophase. But during telophase, the electrochemical control process was charge transfer. In order to further improve the properties of Li₃V₂(PO₄)₃/C, the effects of Zr doping on the structure, morphology and electrochemical properties were studied. XRD results showed that the formation of moliclinic structure increased. SEM tests showed the uniformity of particles size was improved. Charge-discharge tests showed the rate and cyclic performances of Li₃V₂(PO₄)₃/C were improved obviously and the doped material of x=0.1 had the best electrochemical properties. The results of CV, EIS and conductivity tests indicated that the improvement of electrochemical performances by Zr doping could be ascribed to higher reversibility of Li⁺ intercalation/deintercalation reaction, higer conductivity and lower membrance resistance Rsf and charge transfer resistance Rct. Elecronic structure analysis indicated Zr doping increased the conductivity of Li₃V₂(PO₄)₃ through the band gap reduction.