Synthesis And Characterization Of Cathode Materials LiFePO₄, Li₃V₂(PO₄)₃ And XLiFePO₄·yLi₃V₂(PO₄)₃ For Lithium Ion Battery

In this paper, the development of lithium ion batteries and cathode materials were reviewed in detail. The aims of the present study were focus on the preparation processes, the electrochemcial performances, physic properties, and the kinetics behaviors of LiFePO4, Li3V2(PO4)3 and composite materials xLiFePO4·yLi3V2(PO4)3.FePO4·xH2O precursor was synthesized from co-precipitation under various pH value. LiFePO4/C cathode material was then synthesized by carbothermal reduction using FePO4·xH2O. The effect of the solution pH on the performance of FePO4·xH2O was characterized. The results showed that there were no impurities existed in the FePO4·xH2O synthesized on the condition that the solution pH was between 2 and 5. LiFePO4 made from FePO4·xH2O precursor synthesized under the condition that the solution pH was about 2 showed excellent electrochemical performance, it exhibited a discharge capacity of 145mAh·g-1 at 0.1C, the capacity retention is about 98% after 50 cycles, and the tap density reaches 1.11 g·cm-3.It was the first time that LiFePO4 was synthesized by heating an amorphous LiFePO4, which obtained through lithiation of FePO4·xH2O by using oxalic acid as a novel reducing agent at room temperature. The effects of reaction temperature and time on the crystal structure, morphology and electrochemical properties of products were investigated. The results show that the sample synthesized at 600℃for 12h has fine particle sizes between 100-200nm, and with homogenous sizes distribution, the particles are wraped by 20nm thickness carbon webs. The synthesized LiFePO4 composites showed excellent discharge capacity of 165 mAh·g-1 and 95 mAh·g-1 at 0.1C and IOC rate. The cell retains 98.8% and 89.5% of its initial discharge capacity after 50 cycles.It was the first time that Li3V2(PO4)3 was synthesized by a room temperature reduction-low temperature calcining. XPS results showed V5+was reduced to V3+by oxalic acide, under ball milling at room temperature. Li3V2(PO4)3 particles are about 200nm in diameter and have porous structure, and coated with nano-carbon webs, the thickness of the nano-carbon webs are about 10-20nm, the specific surface area is as large as 25m2·g-1. Li3V2(PO4)3 synthesized at 650℃for 12h showed best electrochemical performance, it exhibited a capacity of 131.57mAh·g-1 at 0.1C, the capacity retention is about 95.23% after 100 cycles.In order to use the merits of LiFePO4 and Li3V2(PO4)3, composite materias xLiFePO4·yLi3V2(PO4)3 were synthesized through room temperature reduction-low temperature calcining. XRD results showed an olivine type LiFePO4 and a monoclinic component Li3V2(PO4)3 coexisted in the composite.5LiFePO4-Li3V2(PO4)3 synthesized at 650℃for 12h showed good electrochemical performance, it exibited 154.13mAh·g-1 at 1C rate. A capacity of 112.8mAh·g-1 can be obtained at IOC rate after 100 cycles, the capacity retention was 98.6%. The electrochemical performances of the composite material 5LiFePO4·Li3V2(PO4)3 are better than single phase LiFeP4 and Li3V2(PO4)3. Besides, the performances of the synthesized 5LiFePO4·Li3V2(PO4)3 are better than that of 5LiFePO4@Li3V2(PO4)3 obtained just mechanical mixed LiFePO4 and Li3V2(PO4)3 together. CV results of 5LiFePO4·Li3V2(PO4)3 electrodes showed that the oxidation and reduction peaks appeared in the way of the superimposition of the peaks of LiFePO4 and Li3V2(PO4)3. No other peaks can be found. In the same scanning velocity, the voltage intervel of the four pairs peaks of 5LiFePO4-Li3V2(PO4)3 are obviously smaller than that of single phase LiFePO4 and Li3V2(PO4)3, the lithium deintercalation/ intercalation kinetics behaviors of 5LiFePO4·Li3V2(PO4)3 are better than that single phase LiFePO4 and Li3V2(PO4)3. The coalescence machanism for the composite cathode material 5LiFePO4-Li3V2(PO4)3 were studied, the results showed some LiFePO4 and Li3V2(PO4)3 in the composite material were doped by V and Fe, respectively, and formed a solid solution. The valence of Fe and V after doping in Li3V2(PO4)3 and LiFePO4 was+2 and+3, respectively.The kinetics behaviors of LiFePO4, Li3V2(PO4)3 and 5LiFePO4-Li3V2(PO4)3 electrodes were studied by means of linear sweep voltammetry and cyclic voltammetry. It is found that the exchange current density was increased with the charge state increasing. The exchange current density is 38.64,64.95,44.77 and 76.18mA·g-1 for LiFePO4, Li3V2(PO4)3, mechanical mixied 5LiFePO4@Li3V2(PO4)3 and 5LiFePO4·Li3V2(PO4)3, respectively. The lithium diffusion coefficient around the oxidation peaks are larger than that of reduction peaks. And the lithium diffusion coefficients in the composite material 5LiFePO4·Li3V2(PO4)3 are larger than that in single phase LiFePO4 and Li3V2(PO4)3, indicated the lithium ion diffusion rate in 5LiFePO4·Li3V2(PO4)3 are faster than single LiFePO4 and Li3V2(PO4)3.Composite material LiFePO4-Li3V2(PO4)3 was synthesized using FeVO4·xH2O obtained by co-precipitation. XRD results showed that olivine type LiFePO4 and monoclinic Li3V2(PO4)3 coexisted in the composites. The composites synthesized at 700℃for 12h exibited good rate performance and stable cycle performance, and its discharge capacity is about 142.5 at 0.1C,138.4 at 1C and 123.4mAh·g-1 at 3C with satisfactory capacity retention.