Study On Preparation And Electrochemistry Performance Of LiVPO₄F As Cathode Materials For Lithium-ion Battery

Recently, lithium vanadium fluorophosphates, LiVPO₄F, has been considered as a potential cathode material for lithium ion batteries due to its excellent thermal stability, high discharge voltage and high theoretic capacity of 156 mAh·g^-1. However, the poor conductivity of LiVPO₄F material results in poor high rate capability and limits its applications in lithium ion batteries.The study in this thesis mainly focuses on the improvement of electrochemical performance of LiVPO₄F by improving its poor conductivity. The conductivity of LiVPO₄F was enhanced by improving the synthesis method and LiV_0.95Al_0.05PO₄F/C with excellent performance was prepared by sol-gel method. The effects of preparation conditions and the content of dopped Al on the structure and electrochemical performance of LiVPO₄F were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), cyclic voltammetry (CV) and charge-discharge tests.LiVPO₄F/C composites were prepared by a traditional solid-state method and a modified solid-state method, respectively, using V₂O₅, NH₄H₂PO₄, glucose and LiF as raw materials. Defect of uneven mixing of raw materials resulted from traditional solid-state method was overcomed by modified solid-state method through the improved mixing procedure. Comparative study of physical and electrochemical properties of LiVPO₄F/C composites prepared by traditional and modified solid-state methods, respectively, was carried out. Results suggest that the two synthesis methods are effective to prepare composite cathode materials consisting of triclinic LiVPO₄F and carbon. But the discharge capacity and cyclability of LiVPO₄F/C composite synthesized by modified solid-state method are better than that composite prepared by traditional solid-state method. The discharge capacities of LiVPO₄F/C composite prepared by modified solid-state method at 0.2, 0.5 and 1C are 133.7, 124.9 and 118.7 mAh·g^-1, respectively, while the LiVPO₄F/C composite prepared by traditional solid-state method exhibits 131.2, 121.4 and 104.9 mAh·g^-1 at the corresponding above-mentioned current rates, respectively.LiVPO₄F/C composite cathode materials were prepared by a sol-gel method using V₂O₅, H₃PO₄, citric acid and LiF as raw materials and the effects of preparation conditions on the electrochemical performance of composites were investigated. The results reveal the optimum preparation conditions as follows: mole ratios of V₂O₅, H₃PO₄ and citric acid are 1:2:2.4; VPO₄/C was prepared by calcination of dried gel at 700℃for 6h; preparation of LiVPO₄F/C by sintering mixture of VPO₄/C and LiF at 750℃for1 h. The discharge capacity and cyclability of LiVPO₄F/C composite prepared by sol-gel method at optimum conditions are better than that of LiVPO₄F/C composite prepared by solid-state method. The discharge capacities of LiVPO₄F/C composite prepared by sol-gel method are 123.7 and 104.4 mAh·g^-1 at 1C and 5C, respectively.Based on the preparation of LiVPO₄F/C composite by sol-gel method, LiV_1-xAl_xPO₄F/C (x=0.03, 0.05 and 0.07) composites were synthesized by sol-gel method using V₂O₅, H₃PO₄, citric acid, Al(NO₃)₃·9H₂O and LiF as raw materials, and the electrochemical performance was investigated. The discharge voltage plateau and cyclability of LiV_1-xAl_xPO₄F/C are improved with the increase in doped-Al content, while the discharge capacity decreases with the increase of doped-Al content and LiV_0.95Al_0.05PO₄F/C exhibits highest energy density among the above-mentioned LiV_1-xAl_xPO₄F/C composites. The high rate discharge capacity of LiV_0.95Al_0.05PO₄F/C is higher than that of LiVPO₄F/C composite and displays high capacity of 110 mAh·g^-1 at 5C.