Study On Preparation And Modification Of Lithium Vanadium Phosphate Cathode Material For Lithium-ion Batteries

Monoclinic Li3V2(PO4)3(LVP) has been recognized as the most promising cathodematerial for lithium-ion batteries due to its high theoretical capacity, high diffusion rate oflithium ion, excellent thermostability and cyclability. However, the low intrinsicconductivity (2.0×10-8S cm-1) of LVP has been considered as the fateful shortcoming andimpedes the rapid development of LVP. Therefore, improvement of electronic and ionicmobilities is considered as an effective strategy to enhance the electric conductivity andhence to improve the electrochemical performance of LVP. In this thesis, to overcome theshortcomings of LVP, the improved LVP/C cathode materials were prepared by modifiedsynthesis methods, and Li3-2xMgxV2(PO4)3/C and Li3-3xMgxNaxV2(PO4)3/C with betterelectrochemical performance than that of LVP/C were synthesized. The physical andelectrochemical properties of the above-mentioned composites were investigated byscanning electron microscopy (SEM), X-ray diffraction (XRD) and electron diffractionspectrum (EDS), electrochemical impedance spectroscopy (EIS), cyclic voltammograms(CV) and galvanostatic charge/discharge tests. The effects of synthesis conditions, dopantof Mg2+and co-dopant of Mg2+and Na+on the electrochemical performance of pristineLi3V2(PO4)3/C and doped Li3V2(PO4)3/C were studied. The optimal synthsis conditions andthe dopant contents of Mg2+and Na+were obtained.The Li3V2(PO4)3/C composite was prepared by a sol-gel method and characterized bya combination of XRD, SEM, CV, EIS and galvanostatic charge/discharge tests. Theinfluence of calcination temperature, carbon sources and vanadium sources on the physicaland electrochemical properties of Li3V2(PO4)3/C composite was evaluated, and the optimalsynthesis conditions were found to be as follows: calcination temperature of800oC,calcination time of8h, citric acid as carbon sources and V2O5as vanadium sources.Li3-2xMgxV2(PO4)3/C（x=0.01, x=0.03and x=0.05）composites were synthesized by asol-gel method using LiAc, NH4H2PO4, Mg(NO3)2·6H2O,30%(v/v) H2O2and citric acid asraw materials. The effects of dopant contents of Mg2+in Li sites of monoclinic Li3V2(PO4)3on the physical and electrochemical properties of active cathode materials were investigated.The XRD results reveal that Li3V2(PO4)3with dopant contents of Mg2+in the range of0.01≤x≤0.05still remains its monoclinic structure but the doped Li3V2(PO4)3samplespresent somewhat larger cell volumes than pristine Li3V2(PO4)3. All the Mg dopedLi3V2(PO4)3/C composites exhibit better electrochemical performance than pristine Li3V2(PO4)3/C and Li2.94Mg0.03V2(PO4)3/C shows the best electrochemical performanceamong the all above-mentioned Li3-2xMgxV2(PO4)3/C composites. Compared with the lowintial discharge capacity of170mAh g-1at0.1C for Li3V2(PO4)3/C in voltage range of3.0-4.8V, Li2.94Mg0.03V2(PO4)3/C displays a much higher discharge capacity of192mAhg-1in the same charge/discharge conditions and remains discharge capacity of170mAh g-1after30cycles, corresponding to the capacity retention of89%, while the correspondingcapacity retention of Li3V2(PO4)3/C is85%. The lithium-ion diffusion coefficients ofLi3V2(PO4)3/C and Li2.94Mg0.03V2(PO4)3/C were determined by a CV method and the resultsshow that the doping of Mg2+can significantly improve the lithium-ion diffusioncoefficients. The larger lithium-ion diffusion coefficients are in favorable to improve thediffusion rate of Li+in electroactive particles and hence to improve the electrochemicalperformance.Li3-3xMgxNaxV2(PO4)3/C（x=0.01, x=0.03, x=0.05and x=0.07）composites weresynthesized by a sol-gel method using LiAc, NH4H2PO4, Mg(NO3)2·6H2O, NaNO3,30%(v/v) H2O2and citric acid as raw materials, and the effects of co-doping of Mg2+and Na+inLi sites on the electrochemical performance of Li3V2(PO4)3/C were studied. The resultsshow that the electrochemical performance of all doped samples is better than that ofpristine Li3V2(PO4)3/C and Li2.85Mg0.05Na0.05V2(PO4)3/C presents the best electrochemicalperformance among the all doped samples. In the voltage range of3.0-4.8V, the pristineLi3V2(PO4)3/C composite exhibits the initial discharge capacity of170mAh g-1at0.1C andcapacity retention of85%after30cycles, while Li2.85Mg0.05Na0.05V2(PO4)3/C displays themuch higher initial discharge capacity of190mAh g-1and capacity retention of88%. Theanalysis of lithium-ion diffusion coefficients by a CV method indicates that co-doping ofMg2+and Na+in Li sites of Li3V2(PO4)3can greatly enhance the diffusion rate of Li+inelectroactive particles, and the optimal co-doping may be responsible for the improvedelectrochemical performance.