The Study Of Preparation Of LiFePO₄/C Composite Cathode Material For Lithium Ion Batteries Using Different Raw Materials

With the merits of abundant raw materials, non-toxicity, good thermal stability, excellent charge/discharge cycle ability and high theoretical capacity (170mAh·g^-1), olivine-structured LiFePO₄ can meet the needs of both demand of new energy-consuming information society and environmental protection. LiFePO₄ has become one of the most promising contenders among the new generation of commercial cathode materials. Its intrinsic crystal structure and elemental composition, however, result in properties of low tap density, poor electronic conductivity and electrochemical inertness, which urgently needed to be improved.In the paper, the impacts of different iron sources and carbon precursors on the properties of LiFePO₄/C material were studied systematically: the tap density of LiFePO₄/C was enhanced with improved technological conditions; new raw materials and new techniques were explored for synthesis of LiFePO₄/C in order to bypass the monopoly of foreign patents; doping effects of representative ions on both Li and Fe sites on the crystal structure and electrochemical properties of LiFePO₄ were investigated; the rate performance of LiFePO₄/C electrode was further improved by coating the electrode surface with carbon; CV and EIS were used to investigate the electrochemical behavior of LiFePO₄/C electrode at higher working temperatures, which provided a theoretical reference for opening up a method to further improve material's electrochemical performance.Using Fe₂O₃ as the iron source, the influences of synthesis temperature on the crystal property, carbon content, tap density and electrochemical performance of LiFePO₄/C were investigated in detail. LiFePO₄/C material of high tap density and excellent electrochemical performance was prepared when using Fe₂O₃, compared to FeC₂O₄·2H₂O as an iron source. The effects of different ball-milling dispersive media on charge/discharge capacity and tap density of LiFePO₄/C were studied. Compared to ethanol and water, using acetone as dispersive medium, LiFePO₄/C cathode material of better charge/discharge capacity and plateau, better cycling and rate performance and higher tap density was synthesized.In the paper, one-step solid state reaction was for the first time put forward to prepare Fe₂P₂O₇ using Fe₂O₃ and NH₄H₂PO₄ as raw materials. Fe₂P₂O₇ and Li₂CO₃ were employed to synthesize LiFePO₄ which exhibited good particle morphology and even size distribution. Fe³⁺→Fe²⁺ was realized with no extra addition of reducing agent: Fe₂O₃→Fe₂P₂O₇→LiFePO₄. LiFePO₄/C of good property was synthesized by using acetone instead of ethanol as dispersive medium.Reduced iron, LiH₂PO₄ and glucose were mechanically activated by ball-milling and LiFePO₄/C cathode material was prepared at high temperature. 700℃was the optimum synthesis temperature. With nano-sized primary articles, LiFePO₄/C powder material showed a good electrochemical performance.Most carbon from oxygen free pyrolysis of organic precursors exhibited a fluey morphology and formed an effective conducting connection between LiFePO₄/C particles. LiFePO₄/C of good electrochemical properties could be prepared using pyromellitic anhydride, citric acid and sucrose as carbon precursor respectively. Little improvement in electrochemical performance could be obtained by modifying LiFePO₄ with using only graphite or acetylene black.The doping effects of Li and Fe site on the electrochemical properties of LiFePO₄ were investigated. Electrochemical performance of LiFePO₄ could be enhanced to different extent by Mg²⁺ and Ca²⁺ doping on Li site with a certain concentration. The influences of Fe site doping by Co²⁺, Ni²⁺ and Mn²⁺ depended on doping concentration. The electrochemical activity at lower potential of Co³⁺/Co²⁺ and Ni³⁺/Ni²⁺ redox couples in orthophosphate was for the first time discovered in the paper. 4V redox peaks for Co³⁺/Co²⁺ and Ni³⁺/Ni²⁺ were revealed on the CV curves for LiFe_0.94Co_0.06PO₄/C and LiFe_0.94Ni_0.06PO₄/C electrode.Electrochemical properties of LiFePO₄/C electrode were improved by carbon coating modification: discharge capacity and rate ability weregreatly enhanced; oxidation and reduction voltages for Fe³⁺ /Fe²⁺ couple were decreased and increased respectively; charge transfer impedance was greatly reduced.The electrochemical properties of LiFePO₄/C at higher temperatures were examined with CV and EIS. Results indicated diffusion coefficient for Li⁺ and irreversibility for LiFePO₄ electrode were raised by increasing working temperature. Higher temperature could benefit exertion of charge/discharge capacity and raise the depth of charge and discharge.