Fabrication Of Ion-doped LiFePO₄/Carbon Composites Via The Glass-ceramic Route

LiFePO₄cathode materials have a high theoretical capacity of170mAh/g andare environmental benign, low raw materials cast, thermally stable in thefully-charged state, high energy density and safety, so it is regarded as a potentialcandidate of cathode materials for the next generation of rechargeable lithium ionbatteries. LiFePO₄has, however, poor electronic and ion conductivities, resulting inpoor rate performances and thus limiting commercial applications. So many effortsmust be made to improve the electronic and ion conductivities of LiFePO₄and theperformance of LiFePO₄cathode materials. Based on this trend and reviewing thedevelopment of cathode materials for lithium ion batteries, this dissertation focusingon olivine LiFePO₄cathode materials. This synthesis and modification of LiFePO₄cathode materials were prepared by melting method and the effects of calciningtemperature on physical performance and electrochemical behavior were studied inthis paper.In the aim of improving the performance of LiFePO₄materials, LiFePO₄/C weresynthesized. The microstructure, morphology of the samples and the structure ofcarbon were characterized and investigated by X-ray diffraction （XRD）, scanningelectron microscopy （SEM）. The results show that all the patterns can be identified tobe an orthorhombic olivine structure with a space group of Pmnb. Little impurity suchas Li3PO₄and others, which often appear in the LiFePO₄product synthesized bytradition routes, are observed. When the temperature increases from600℃to750℃, the peak shape increases and becomes sharp. It can be easily seen that thetemperature has a significant influence on the morphology of the prepared samples. Itis found that the particle size of the sample decreased first and then increased as thetemperature increases. For example, the primary diameter decreased from approximately0.5to2mm when the reaction temperature increases from600to700℃. In addition, the particle aggregation becomes especially serious for thesamples prepared at750℃, where most of the small particles connected to big onesand their particle shapes are relatively irregular. This serious agglomeration of theparticles would accordingly affect their electrochemical performance. The LiFePO₄cathode materials show the voltage of3.5V and capacity of92.1mAh/g. It isproposed that this new simple process has a potential for the fabrication ofLiFePO₄/carbon composites.LiFePO₄has a poor electronic and ion conductivity, resulting in the poor rateperformance and thus limiting commercial applications. LiFe_1-xVxPO₄/C （x=0.01,0.02,0.03,0.04）, LiFe_1-xCexPO₄/C （x=0.01,0.02,0.03,0.04）, Li_1-xVxFePO₄/C （x=0.01,0.02,0.03,0.04） and Li_1-xCexFePO₄/C （x=0.01,0.02,0.03,0.04） weresynthesized to investigate the influence of carbon doping and ion substitution on theelectrochemical properties of LiFePO₄/C composite. By doping of V⁵⁺, the electronicconductivity and diffusion rate of Li+were improved. The ion-doped LiFePO₄hadmuch more smaller-size particles. The reasons of improvement of electronicconductivity, one was excess electron conduction resulted from crystal defects causedby ion-doping, the other was hole conduction generated from the ion substitution forLi or Fe position. The Fe-sites doping and Li-sites doping LiFePO₄compound weresynthesized by melting method and the mechanism of the improvement of electronicconductivity was also discussed by four-probe method, powder microelectrode cyclicvoltammetry and electrochemical impedance spectroscopy measurements. The studyindicated, the Fe sites doping V⁵⁺and Ce⁴⁺did not only improve the electronicconductivity by3～4order of magnitude but also weakened the bound of oxygen tolithium，which is propitious to the transport of Li ions, the electronic conductivity ofdoping samples in Li sites by V⁵⁺and Ce⁴⁺was improved by4order of magnitude.