| The main subject of thus paper is to study the liquid phase reactionmethod (Sol-Gel) to produce Li3V2(PO4)3. The effect of complex, reactiontemperature, reaction time and stoichiometric ratio of Li and V wereinvestigated. The optimum process of synthesizing Li3V2(PO4)3 is listedas follows: the mixture using citric acid as the complex was calcined at700℃for 8 h, and the molar ratio of nLi:nV:np was 3.2:2:3. The firstdischarge capacity of the cathode was 129.81 mAh·g-1 under 0.1C rate.After charged/discharged at 0.1C rate for 100 cycles, the sample retaineda discharge specific capacity of 128mAh·g-1. The samples wereinvestigated by X-ray diffraction, which showed the temperature ofsynthesizing pure material by sol-gel procedure was lower than that ofsolid-state reaction. The shifts of the sample lattice parameters and crystalstructure were calculated by using least square method, which confirmedthe recurring process of the crystal structure.The compound of materials substituted with nonmetal (carbon), puremetal (silver) and transition metal ions (Mg2+, Cr3+, Ge4+ and Ti4+) weresynthesized by sol-gel method. And their charge-discharge properties,cycle performance and electrochemical impedance spectroscopy wereinvestigated.The electrochemical performance and Li+ deinsertion/insertion of pureLi3V2(PO4)3 were investigated. The average diffusion coefficient (DLi+)value for Li+ diffusing in solid state was calculated, which fluctuate in therange of 10-8 to 10-9 cm2·s-1. Electrochemical impedance spectroscopy(EIS) were conducted on a Li3V2(PO4)3 electrode, and two differentequivalent circuits were proposed for fitting the experimental results,consequently to evaluate the kinetic of Li-ion extraction/insertion atLi3V2(PO4)3 and its evolution of surface/interface.The electrochemical properties and structure features of substitutedLi3-2x(V1-xTix)2(PO4)3 were investigated. Compared with the pure materialof Li3V2(PO4)3, the substituted samples exhibited amalgamation of twoprevious plateaus and all plateaus slightly sloping. The DTA analysisindicated that aγphase product would be produced when the substitution ratios over x=0.1. The crystal structure was characterized by X-raydiffraction and Rietveld method. The Lithium dissolved in Ti-substitutedsample and all the lithium sites were partially occupied, which introducedadditional vacancies into the lithium sites. Such vacancies wouldcorrespond to the changes of its structure and electrochemical properties.The specific conductivities of Lix(V0.9Ge0.1)2(PO4)3(x=0.9～2.8) vs.temperatures were analyzed from blocking-electrodes by Wagner'spolarization method and the activation energy was calculated. Modelingthe EIS data with equivalent circuit approach reflects that the electrodereaction involves processes such as Li-ion migration in solution andthrough the surface film, charge-transfer process and solid-state Li-iondiffusion into the host material. The formation and nature of surface filmare shown to be the signature of the cell performance which in turnaffects the kinetics of electrode processes. The Li+ diffusion coefficient(DLi+) value was calculated, which fluctuate in the range of 10-8 to 10-7cm2·s-1. Therefore, thedoped material performs excellent chemicaldiffusion performance. |
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