Preparation And Electrochemical Performance Study Of Lithium Vanadium Phosphate And Its Composite With Lithium Iron Phosphate

To face the challenge of energy crisis and environmental pollution, differentcountries all over the world have paid great attention to the development of positivematerials of power lithium ion battery for electric vehicles. In current status,LiFePO₄material is the new hope for power battery, because of its good safety andlong cycle life, and it is friendly to environment. But its rate performance andlow-temperature performance are poor. Therefore the charge and dischargeperformance at high current density and low temperature of Li₃V₂（PO₄）₃materialand xLiFePO₄-yLi₃V₂（PO₄）₃composite material was studied. Moreover, based onthe study in laboratory, the key technology of pilot scale test and scale production ofLi₃V₂（PO₄）₃material were studied.The Cu-doping effects and Mg-doping effects on Li₃V₂（PO₄）₃material werestudied. XRD refinement study indicated that Cu doping changed the dischargebehavior of Li₃V₂（PO₄）₃material with the emergence of a new voltage plateau at4.05V via changing the bond length of Li-O. It opens a new window to find the wayto enhance the cycle performance of Li₃V₂（PO₄）₃material in the range of3.0–4.8V.Mg doping decreased the cell volume of Li₃V₂（PO₄）₃material, and lengthens theLi-O bond of Li1and Li2ion in the structure, which decreases the constraint of Liion in the structure, and is beneficial to the reversible intercalation/deintercalationof lithium ion. X-ray absorption near edge structure study revealed that Mg dopingimproved the symmetry of VO6octahedron. At10C rate, the discharge capacity ofLi₃(V_0.9Mg_0.1)₂（PO₄）3material reaches100mAh/g in the range of3.0-4.3V, which ishigher than the undoped Li₃V₂（PO₄）₃material. This indicates that the Mg dopingexerts the significant positive effect on the electrochemical performance ofLi₃V₂（PO₄）₃material, and makes the Li₃V₂（PO₄）₃material more suitable to thepractical application in the power battery field.The calcination temperature and time for carbothermal synthesis of7LiFePO₄-Li₃V₂（PO₄）₃composite material were optimized. The optimized synthesistemperature is770℃, and the optimized synthesis time is12h. After the synthesistechnique was optimized, the physical properties of7LiFePO₄-Li₃V₂（PO₄）₃ composite material was studied. The XRD study indicates that the cell volumes ofLiFePO₄and Li₃V₂（PO₄）₃in the composite are decreased. The XANES studyreveals that the valence state of Fe ion in LiFePO₄material is slightly increased,while the valence state of V ion in Li₃V₂（PO₄）₃material is slightly decreased. Inaddition, the symmetry of VO6octahedron is enhanced, and the stability of thestructure is improved. It reveals that in the composite, Fe substituts V inLi₃V₂（PO₄）₃material, V substituts Fe in the LiFePO₄material. The study onelectrochemical properties of the7LiFePO₄-Li₃V₂（PO₄）₃composite material revealsthat the charge and discharge performance of the composite at high current densitiesis much better than that of LiFePO₄material. When charged and discharged at thecurrent density of1500mA/g, the discharge capacity of the composite was89mAh/g, which is much higher than that of original LiFePO₄material （70mAh/g）,and the over potential of the composite was lower than that of original LiFePO₄material. The study on Mg doping of7LiFePO₄-Li₃V₂（PO₄）₃composite materialindicates that the electrochemical performance of the composite is enhanced. Thecharge and discharge test indicates that the discharge capacity of Mg-dopedcomposite material is higher than the undoped composite. The cyclicvoltammograms at different scan rates reveals that Mg doping increases thelithium-ion diffusion coefficient of the LiFePO₄in the composite.The low-temperature study of Li₃V₂（PO₄）₃material and7LiFePO₄-Li₃V₂（PO₄）₃composite material reveals that the main reason for the decrease of the dischargecapacity of Li₃V₂（PO₄）₃material at low temperatures is the increase of the chargetransfer resistance. Mg doping can significantly increase the discharge capacity ofLi₃V₂（PO₄）₃material and7LiFePO₄-Li₃V₂（PO₄）₃composite material at lowtemperatures. At-30℃, the discharge capacity of Mg-doped7LiFePO₄-Li₃V₂（PO₄）₃composite material is89mAh/g, which is39mAh/g higher than that of originalLiFePO₄material （50mAh/g）, and17mAh/g higher than that of the undopedcomposite （72mAh/g）. Based on the symmetry cell study, it is found that Mgdoping can decrease the charge transfer resistance Rctof Li₃V₂（PO₄）₃material and7LiFePO₄-Li₃V₂（PO₄）₃composite material, enhance the exchange current density,and improve the electrochemical reactivity.In this paper, key technology of the pilot scale test and scale production ofLi₃V₂（PO₄）₃/C material was studied. Using water as the ball-milling medium changed the reaction process of the synthesis of Li₃V₂（PO₄）₃/C material. However,pure Li₃V₂（PO₄）₃/C material was still obtained. The particles of the obtainedproduct in pilot scale test were hollow spheres with large size, and theelectrochemical performance was poor. To circumvent the hollow-sphere particleproblem, a twice ball-milling procedure was employed. The particles of theobtained Li₃V₂（PO₄）₃/C material were porous solid spheres，which were good forthe infiltration of the electrolyte and the increase of tap density. Thus, theelectrochemical performance of the material was enhanced. Based on the pilot scaletest, an industrial scale production study of Li₃V₂（PO₄）₃/C material was carried out,and the electrochemical performance of the as-prepared material was excellent. At10C charge and discharge rate, the discharge capacity of the material is79mAh/g.The excellent performance indicates that a well prepared Li₃V₂（PO₄）₃/C materialwith application potential is achieved in the scale production.