Kinetic Mechanism Studies On The Synthesis Of Lithium-ion Battery Cathode Material Li₃V₂（PO₄）₃Via Solid-state Reaction

With the high-speed development of society and the continuous progress ofmodern industrialization, human demand for energy varieties and quantity isincreasing. It is imperative to develop clean, efficient, safe and renewable solar energy,tidal energy, wind energy, nuclear energy, biomass energy, chemical, power and othernew energy. In recent years, Lithium-ion batteries has aroused more and moreinterests for its light quality, small size, high specific energy, no memory effect, longcycle life, no pollution, etc. The cathode material is an important composite of Li-ionbattery. Among the cathode materials, monoclinic Li3V2(PO4)3has received greatattention because of its stable framework, relative high operative voltage, higherlithium ion transport ability, higher theoretic capacity and high safety. However, themain drawback of pristine Li3V2(PO4)3is its intrinsic low electronic conductivity andLi+diffusion coefficient, which results in poor electrochemical performance. In orderto get the optimized physical and chemical properties of Li3V2(PO4)3/C composites,some features of a solid-state reaction, such as whether or not reacting betweensolid-state reactants, the reaction degree and the reaction controlling step directlyaffecting the structure and performances of the final material, must be investigated.In this paper, to evaluate the optimum condition in the synthesis of Li3V2(PO4)3/Ccomposites, the mechanism and kinetics of solid-phase reactions of Li3V2(PO4)3through the precursor route was studied by TG-DTA, and analyzed by both the FWOand FRL methods. The data obtained by thermogravimetry (TG), differential thermalanalysis (DTA) and X-ray diffraction (XRD) measurements of the stoichiometricmixture of LiNO3, NH4VO3, and NH4H2PO4were analyzed by both the FWO andFRL methods. The nucleation of LiVP2O7is a determining step of the whole reactionprocess, for which the mechanism of random nucleation and subsequent growth,described by the Avrami-Erofeev equation (n=4) G(α)=[-ln(1-α)]4, f(α)=(1/4)(1－α)[－ln(1－α)]-3, was assumed. Moreover, the apparent activation energy E was205.30kJ·mol-1, the pre-exponential factors A was5.55×1018min-1, and the Gibbs free energy of activation ΔG*, enthalpy of activation ΔH*and entropy of activationΔS*at maximum temperature of the peak for the determining step were99.24kJ·mol-1,199.97kJ·mol-1and136.30J·mol-1·K-1, respectively.On basis of the results of the mechanism and kinetics of solid-phase reactions ofLi3V2(PO4)3, the structural, morphological and electrochemical properties of thecathode materials prepared under an argon atmosphere at different sinteringtemperature and time were characterized by XRD, scanning electron microscopy(SEM) and electrochemical test. The results show that both the phases of Li3V2(PO4)3and LiVP2O7existed in the samples sintered at700°C and750°C. However, whenthe calcination temperature is raised to800°C, the LiVP2O7is transformed to wellcrystallized Li3V2(PO4)3. The initial specifc charge capacity of the cathode materialsprepared at700°C,750°C and800°C for12h at0.2C,3.0~4.8V were158.2,161.8and165.8mAh·g-1, respectively, and the corresponding discharge capacity were139.9,145.6and151.7mAh·g-1, respectively, and the corresponding dischargecapacity after50cycles were109.3,118.1and140.8mAh·g-1, respectively. Moreover,the initial specifc discharge capacity of the cathode materials prepared at800°C for4h and8h were146.9mAh·g-1and149.6mAh·g-1, respectively.The structural, morphological and electrochemical properties of the cathodematerials prepared under the carbon covered protective condition had been alsoanalyzed. The results show that both the phases of LiVP2O7and LiVOPO4existed inthe samples perpared at800~1100°C for10h. The initial specifc discharge capacityof the cathode materials3prepared at800,900,1000,1100°C for10h were96.1,108.3,119.2and125.4mAh·g-1, respectively, and the corresponding dischargecapacity after50cycles were44.8,63.6,90.6and113.5mAh·g-1, respectively.Moreover, the initial specifc discharge capacity of the cathode materials prepared at1100°C for3h and6h were100.4mAh·g-1and116.2mAh·g-1, respectively, and thecorresponding discharge capacity after50cycles were61.5mAh·g-1and100.3mAh·g-1, respectively.