Preparation And Anion-doping Modification Of LiFePO₄/C Cathode Materials For Lithium Ion Batteries

Olivine lithium iron phosphate (LiFePO₄) with the advantages of environmentally benign, low cost, good thermal stability, non-hygroscopic, excellent cycling performance and improved safety has been recognized as one of the most promising cathode materials candidates for rechargeable lithium ion batteries. However, the low ionic and electronic conductivity has greatly inhibited its commercial applications.On the base of reviewing the development of lithium ion battery cathode material,this research takes LiFePO₄ as research object, focusing on anion–doping modification of LiFePO₄. The effect of F–doping on the electrochemical behavior of LiFePO₄ by optimal solid–state reaction and hydrothermal reaction was studied in details. The morphology of the samples was observed by scanning electron microscopy (SEM). The crystallographic structure of the samples was characterized by X–ray diffraction (XRD). The cycling performance and charge-discharge curves were evaluated by battery tester. Electrochemical impedance spectroscope (EIS) and cyclic voltammetry (CV) curves were adopted to analyse electrochemical reaction mechanism.The detailed contents of this thesis are as following:The LiFePO₄/C materials were synthesized by solid-state reaction, using Li₂CO₃, FeC₂O₄·2H₂O and NH₄H₂PO₄ as starting materials, glucose as reducing agent and carbon source. The effect of heating time and heating temperature on electrochemical properties of materials was studied. The optimum synthesis conditions for LiFePO₄/C were heat-treatment at 700 oC for 12 h. The obtained materials fitted the olivine structure and showed a discharge capacity of 152.03 mAh/g at 0.1 C, after 30 cycles the capacity retention is 97%.The F-doped LiFePO₄/C materials were obtained by optimal solid-state reaction, and the relationship between microstructure and electrochemical behavior of materials was investigated by XRD, SEM and EIS. The LiFe(PO₄)_1-xF_3x/C (x=0, 0.01, 0.03, 0.05) samples belong to olivine structure, and delivered a discharge capacity of 153.4 mAh/g, 159.72 mAh/g, 165.06 mAh/g and 162.47 mAh/g at 0.1 C, respectively.The other methods of modification were discussed. F-doped LiFePO₄/C was prepared via a hydrothermal process, and the effect of F-doping on crystallographic structure and electrochemical properties of LiFePO₄ materials was investigated. It can be confirmed that the samples belong to the olivine structure by XRD and XPS analysis. This proves that fluorine was successfully introduced into LiFePO₄ matrix structure and the incorporation of F^– did not alter the LiFePO₄ structure but slightly increased the lattice parameters. When doping amounts x=0.03, the material showed obviously better high rate performance and stable cycle ability.