Synthesis And Modification Of Lithium Vanadium Phosphate Cathode Materials

As a higher working voltage cathode material, monoclinic lithium vanadiumphosphate has many advantages, such as outstanding cycle performance and secure.However, lithium vanadium phosphate material displays low electronic conductivity,and the discharge performance under a large magnification is not very good. Theseproblems seriously restrict its wide application. First, two kinds of Li₃V₂（PO₄）₃/Csamples were prepared by sol-gel and solid-state ball-milling methods, respectively.And then, the Li₃V₂-xCo_2x/3（PO₄）₃/C and Li₃V_2-4x/3Ti_x（PO₄）₃/C compsites with singlecation doping, and Li₃V_1.93Co_0.05Ti_0.03（PO₄）₃/C composites with co-doping wereobtained through sol-gel method. The structure of the obtained compsites wasinvestigated by scanning electron microscopy （SEM） and X-ray diffraction （XRD）.The electrochemical properties were detected by cyclic voltametry （CV）,galvanostatic charge-discharge measurements and electrochemical impedancespectroscopy （EIS）, respectively.In the first place, we prepared three carbon coated lithium vanadium phosphate（LVP） materials by different carbon sources （PVA, Tartaric acid and Glycine） and theproducts were labeled as P-LVP, T-LVP and G-LVP, respectively. The initial dischargecapacity of G-LVP was128.4mAh g^-1at0.1C between3.0-4.3V while the retentionrate of capacity was95.5%after30cycles. We also studied the effect of syntheticmethod on the structure and performance of Li₃V₂（PO₄）₃/C materials prepared by thesol-gel and solid-state ball-milling methods. XRD results suggested that both sampleswere with single crystal structure. From SEM images, it could be seen that theparticles of ball-milling sample were appeared with agglomeration phenomenon. Theinitial discharge capacity of LVP prepared by sol-gel method was157.7mAh g^-1at0.5C between3.0-4.8V and the retention rate of capacity was85.1%after50cycles.The results suggested that the Li₃V₂（PO₄）₃/C which synthesized by sol-gel methodhad the better electrochemical performance.Then, the effects of Co²⁺and Ti⁴⁺doping were also studied in the paper. TheLi₃V₂-xCo_2x/3（PO₄）₃/C and Li₃V_2-4x/3Ti_x（PO₄）₃/C were synthesized by sol-gel methodusing cobalt acetate as the cobalt source and tetrabutyl titanate as the titanium source.The structure, morphology and electrochemical performance of theLi₃V₂-xCo_2x/3（PO₄）₃/C and Li₃V_2-4x/3Ti_x（PO₄）₃/C were characterized. The resultsshowed that a certain amount of Co²⁺or Ti⁴⁺doping did not change the crystal structure of Li₃V₂（PO₄）₃, while the discharge specific capacity and cyclic stability ofthe composites were enhanced after doping. When x=0.05, Li3V1.97Co0.05（PO4）3/Chas the best electrochemical performance. The initial discharge capacity ofLi3V1.97Co0.05（PO4）3/C is164.59mAh g^-1at0.5C rate and the retention rate ofcapacity is86.92%after50cycles. When x=0.03, Li3V1.96Ti（PO4）3/C has the bestelectrochemical performance. The initial discharge capacity of Li3V1.96Ti（PO4）3/C is164.59mAh g^-1at0.5C rate and the retention rate of capacity is83.91%after50cycles.Finally, based on the optimal doping ratio, Li₃V_1.93Co_0.05Ti_0.03（PO₄）₃/C wassynthesized by sol-gel method. In a different voltage range, the results proved that theelectrochemical performances of the Li₃V_1.93Co_0.05Ti_0.03（PO₄）₃/C were both enhanced.Between the voltage of3.0～4.8V, the initial discharge specific capacity ofLi₃V_1.93Co_0.05Ti_0.03（PO₄）₃/C at the rate of0.5C and1C were169.10and146.06mAhg^-1, even at the high rate of10C, the capacity of Li₃V_1.93Co_0.05Ti_0.03（PO₄）₃/C was110mAh g^-1.