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

Layered Li_1+xV₃O₈become one of the most potential cathode materials for lithium ionbatteries for over three decades because of its advantages of high capacity, compatible operationvoltage, facile preparation，excellent stability in air and low cost. However, its shortcomingssuch as structural changes and slow rate of Li+diffusion during charge/discharge processes lendto poor cycle performance, which is unfavorable for its development and practical applications.In order to solve the above-mentioned disadvantages, the structural stability and rate of Li+diffusion of Li_1+xV₃O₈have been improved by doping Na⁺and coating LaF₃in this paper, andhence to enhance the electrochemical performance of Li_1+xV₃O₈. The prepared cathodematerials were characterized by X-ray diffraction （XRD）, scanning electron microscopy （SEM）,energy dispersive spectrometer （EDS）, transmission electron microscopy （TEM）, cyclicvoltammetry （CV）, galvanostatic intermittent titration technique （GITT）, electrochemicalimpedance spectroscopy （EIS）and galvanostatic charge/discharge measurements. The effects ofcontents of lithium, Na⁺dopant and LaF₃coating on the physical and electrochemicalperformances of Li_1+xV₃O₈were investigated and the preparation conditions were optimized.Li_1+xV₃O₈（x=0,0.1,0.2,0.3） cathode materials were prepared by a sol-gel method usingNH₄VO₃, LiC2H3O2·2H2O and citric acid as raw materials and the effects of Li contents on theelectrochemical performance of Li_1+xV₃O₈were investigated. The results show that Li_1.2V₃O₈exhibits best electrochemical performance among all Li_1+xV₃O₈（x=0,0.1,0.2,0.3） samples. Inthe voltage range of2.0⁴.0V, Li_1.2V₃O₈displays initial discharge capacities of305, and173mAh g^-1at0.1C and2C, respectively. It remains discharge capacities of266，and113.2mAh g^-1at0.1C and2C, respectively, and the corresponding capacity retentions are80%and49.6%,respectively. Additionally, Li+diffusion coefficients of Li_1.2V₃O₈and LiV₃O₈were determinedby GITT measurement, and the value for Li_1.2V₃O₈is higher than that of LiV₃O₈, which furtherelucidates that the higher Li+diffusion coefficient favors improving the electrochemicalperformance of LiV₃O₈.LaF₃coated Li_1.2V₃O₈cathode materials were synthesized by a co-deposition methodusing NH₄F and La（NO₃）₃and the prepared Li_1.2V₃O₈samples as raw materials, and thecontents of LaF₃coating on the electrochemical performance were explored. Theelectrochemical tests indicate that the optimal LaF₃coating is3wt%. This optimal LaF₃coatedLi_1.2V₃O₈shows initial discharge capacities of298and169.8mAh g^-1at0.1C and2C in thevoltage range of2.0⁴.0V, respectively, and the corresponding capacity retentions are as high as93%and81%, respectively, after50cycles.Li_1.2-xNa_xV₃O₈（x=0,0.03,0.05,0.07） cathode materials were prepared by a sol-gel methodusing NH₄VO₃,LiC2H3O2·2H2O,NaNO₃and citric acid as raw materials, and the effects of Nacontents on the electrochemical performance of the Na-doped cathode materials were studied.The results indicate that the Na doping is favorable to improve the electrochemical performanceof Li_1.2V₃O₈and Li_1.15Na_0.05V₃O₈is of best electrochemical performance among all theNa-doped samples. The results showed that compared with other samples, Li_1.15Na_0.05V₃O₈hasthe best electrochemical performance. In the voltage range of2.0-4.0, the initial dischargecapacity of Li_1.15Na_0.05V₃O₈was as high as312,182mAh g^-1at0.1C and2C, respectively. After50cycles its discharge capacities still remain280.8and136.5mAh g^-1,corresponding to thecapacity retentions are90%and75%, respectively.