Preparation And Characterization Of Li₃V₂（PO₄）₃and LiFePO₄Cathode Materials For Lithium Ion Batteries

As typical polyanion cathode materials, monoclinic lithium vanadiumphosphate (Li3V2(PO4)3) and olivine-type lithium iron phosphate (LiFePO4) areconsidered the most promising cathode materials, because they have highcapacities and operation voltages, stable structure, excellent cycle performance,besides, they are safety and nontoxic. However, their low electronicconductivity and lithium ion diffusion coefficient hinder them to becommercialized. Recently, the research emphasis has been put on how toimprove the electronic conductivity and lithium ion diffusion coefficient. In thispaper, Li3V2(PO4)3/C and Li3WxV2-x(PO4)3/C composites were respectivelysynthesized by sol-gel method. LiFePO4was synthesized by microwavehydrothermal method. XRD, SEM, TEM, XPS and EDS were used tocharacterize the phase composition, crystal structure and morphology,respectively. Galvanostatic current charge-discharge, cyclic voltammetry (CV),electrochemical impedance spectroscopy (EIS) were used to investigate theelectrochemical performance and calculate the lithium ion diffusion coefficient.The effects of preparation conditions, doping and surface modification on thephysical properties and electrochemical performance were studied.Li3V2(PO4)3/C composite was prepared by sol-gel method, using NH4VO3,LiOH·H2O and H3PO4as raw materials, citric acid as complexing agent. Theas-synthesized Li3V2(PO4)3has monoclinic structure, and the carbon that comefrom the pyrolysis of citric acid at high temperature is amorphous. Theinfluence of sintering temperature and time on the composition, structure,morphology and electrochemical performance of Li3V2(PO4)3/C wasinvestigated. The Li3V2(PO4)3/C composites show low purity and poorelectrochemical performance at too low sintering temperature or sintered for tooshort time. The Li3V2(PO4)3/C composites also show poor electrochemicalperformance at too high sintering temperature or sintered for too long time,because the composites aggregate severely, that is not benefit for lithium ion diffusion. In this study, the Li3V2(PO4)3/C calcined at750℃for6h is wellcrystallized, pure and small in particle size, with better electrochemicalperformance. In the voltage range of3.0~4.6V, the initial discharge capacity at0.5C is135.1mAh/g (79.5%of theoretical capacity)，the capacity retention is87.7%after50cycles. The sol-gel preparation activation energy of Li3V2(PO4)3,92.4kJ/mol, is calculated from Kissinger equation.Monoclinic Li3WxV2-x(PO4)3/C（x=0，0.03，0.05，0.07、0.10and0.15）composites were prepared by sol-gel method, using NH4VO3, LiOH·H2O,(NH4)10W12O41and H3PO4as raw materials, citric acid as complexing agent.When the doping content is less than0.15, there are not impurities in theproducts, indicating W ions enter the lattice. XPS results show that Tungstenions with+5valence substitute partial V3+ions in Li3V2(PO4)3. As the dopingW5+ions increases, Li3V2-xWx(PO4)3/C displays bigger lattice volume andsmaller particle size, that is benefit for lithium ion diffusion. Theelectrochemical performance of Li3V2-xWx(PO4)3/C is better than that of thepristine Li3V2(PO4)3. Li3V1.93W0.07(PO4)3/C presents better electrochemicalperformance than all the other samples. In the voltage range of3.0~4.6V, theinitial discharge capacity at0.5C is160.3mAh/g (94.3%of theoreticalcapacity)，the capacity retention is92.2%after50cycles. The initial dischargecapacity at5C is92.6mAh/g，the capacity retention is75.9%after50cycles.The effect of W5+ions doping on the electrochemical kinetics ofLi3V2(PO4)3/C was investigated by cyclic voltammetry and electrochemicalimpedance spectroscopy. CV and EIS results also show that the lithium iondiffusion coefficient of Li3V2(PO4)3/C increase after W5+doping. Comparingwith Li3V2(PO4)3/C and Li3V1.90V0.10(PO4)3/C, Li3V1.93W0.07(PO4)3/C showshigher lithium ion diffusion coefficient, smaller ohmic resistance andcharge-transfer resistance. The kinetics results is in general agreement with thetrend of the electrochemical performance. The lithium ion diffusion coefficientcorresponding to different insertion or desertion step for Li3V2(PO4)3/C andLi3V1.93W0.07(PO4)3/C was studied by cyclic voltammetry method. The CVresults show the diffusion coefficients for the second lithium ion desertionduring charging and the first lithium ion insertion during discharging are thesmallest, respectively. Orthorhombic LiFePO4cathode materials with olivine structure wereprepared by microwave hydrothermal method at160~220℃, usingFeSO4·7H2O, LiOH·H2O and H3PO4as raw materials. The influence of pHvalue of the precursors, reaction temperature and time on the composition,structure, morphology and electrochemical performance of LiFePO4wasinvestigated. LiFePO4synthesized at200℃for60min with the pH value of9.26has high purity, and is well crystallized. The LiFePO4particles are1~2μmin size with rhombic shape. LiFePO4synthesized at200℃for60min showsbetter electrochemical performance than other samples. In the voltage range of2.0~4.2V, the initial discharge capacity is88.1mAh/g at0.1C, with columbicefficiency of93.2%, and the capacity retention is95.2%after20cycles.LiFePO4/C was synthesized with glucose as the modifying agent. Comparedwith LiFePO4, LiFePO4/C shows smaller particle sizes, lithium ion diffusioncoefficient, ohmic resistance and charge-transfer resistance with thicker coatinglayer, as well as enhanced electrochemical performance. In the voltage range of2.0~4.2V, LiFePO4/C shows the initial discharge capacity of125.6mAh/g at0.1C. Even at a rate of1C, it still can deliver a discharge capacity of106.2mAh/g in the voltage range of2.0~4.2V, the capacity retention is91.3%after30cycles.