Synthesis And Modification Of Lithium Vanadium Phosphate Anode Material For Lithium Ion Battery

Li3V2(PO4)3is attracting much interest due to its high theoretical capacity and its thermal stability that make it suitable for high power density and large scale applications. However, the low electronic conductivity and cyclability limit their wide application. This paper reviewed the research progress of monoclinic Li3V2(PO4)3. Li3V2(PO4)3/C and Li3V2-xFex(PO4)3/C composites were synthesized by the sol-gel method and P123-asisted Li3V2(PO4)3/C composites were synthesized by the rheological phase reaction method, and their synthesis conditions were optimized. Their physical properties and electrochemical performances were evaluated.Carbon-coated modification by sol-gel method is treated as feasibility work. Effects of different synthesis conditions(calcining temperature, calcining time) and electrode preparation process(mixing way) on Li3V2(PO4)3/C electrochemical properties were studied. Under Optimized conditions, effects of carbon resources on the crystal structure, morphology and electrochemical performance of the Li3V2(PO4)3/C were systematically investigated. The results show that the optimum synthesis of Li3V2(PO4)3/C for sol-gel method is using citric acid as carbon resource, calcining8h at800℃and mixing (PVDF+NMP)+(active substances(AB)) with electromagnetic stirring.The Fe-doped Li3V2(PO4)3/C cathode materials for Li-ion batteries were synthesized by sol-gel method, and the composite material was characterized by X-ray diffraction(XRD), scanning electron microscopy(SEM). The Fe-doping effects on the Li electrochemical extraction/insertion performance of Li3V2(PO4)3/C were investigated by galvanostatic charge-discharge test, cyclic voltammetery(CV) and electrochemical impedance spectroscopy(EIS). The optimal Fe-doping content x is0,0.01,0.02,0.05in Li3FexV2-x(PO4)3system. The Fe-doped Li3V2(PO4)3samples took on pure monoclinic crystal structure and showed a better cyclic ability between3.0and4.3V. For example, the discharge capacity of Li3Fe0.02V1.98(PO4)3/C was177mAh·g-1in the1st cycle and126mAh·g-1in the100th cycle. The retention rate of discharge capacity is about71%, much higher than58%of the undoped system. Moreover, it displayed good fast rate performance, which had discharge capacities of128.2,121.3and109.1mAh·g-1and capacity recantations after30times charge-discharge of99.1%,98.1%and94.1%at different rates of1C,5C and10C, respectively. The improved electrochemical performances of the Li3V2(PO4)3could be attributed to the increased electrical conductivity and structural stability deriving from the incorporation of the Fe3+ions which formed defeats in Li3V2(PO4)3/C to improve the degree of the disorder of crystal lattiee atoms.Monoclinic Li3V2(PO4)3/C cathode has been synthesized via a P123-asisted rheological phase reaction (RPR) method. The composite material was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrochemical performance has been testd by Galvanostatic charge-discharge test, cyclic voltammetery(CV), electrochemical impedance spectroscopy(EIS). The P123-assisted Li3V2(PO4)3/C takes on pure monoclinic crystal structure and exhibits high initial discharge capacity of128.9mAh·g-1and only decreases0.9%of initial discharge capacity after50cycles at0.1C between3.0and4.3V. Moreover, it displays good fast rate performance, which has discharge capacities of128.2,121.3and109.1mAh·g-1and capacity retetions after50times charge-discharge of99.1%,96.9%,90.7%at different rates of1C,10C and25C, respectively. Triblock copolymer surfactant P123introduced to the RPR system is attributed to the excellent electrochemical performance. It acts as surfactant as well as organic carbon source, then forms carbon network in particle surface and clearance which is helpful to improve the material conductive rate and reduce the charge transfer resistance and the electrode polarity effect during the charge-discharge process.