| As two important members of phosphate system of lithium ion batterycathode materials, the olivine-type lithium ion phosphate(LiFePO4) andmonoclinic lithium vanadium phosphate(Li3V2(PO4)3) have been a focus forresearchers due to the advantages of stable structure, high safety, environmentalfriendly and excellent cycling performance and so on. However, their lowelectronic conductivity and Li+diffusion rate are the main obstacles to thepracticle applications. So improving the electronic conductivity and Li+diffusion rate are the focus of studying the two materials.In this study, the LiH2PO4, FeC2O4·2H2O, NH4VO3and C6H8O7·H2O are usedas the raw materials. The outstanding performance of LiFePO4and Li3V2(PO4)3arecombined organically to prapare xLiFePO4·yLi3V2(PO4)3/C composites via solidreaction method and sol-gel method respectively which exhibit excellentelectrochemical performance. The influence of the proportion, calcinationtemperature, calcination time and the dosage of citric acid on the phase composition,microstructure and the electrochemical performance of the composite cathodematerials is discussed. And the composite mechanism is also discussed. And then, thecomposite cathode material is doped and the influence of Mg2+doping amount on theperformance of xLiFePO4·yLi3V2(PO4)3/C composites is discussed. The mainconclusions are as follows:1. xLiFePO4·yLi3V2(PO4)3/C composites are synthesized by solid method. In theoptimized synthetic conditions(the molar ratio of LiFePO4and Li3V2(PO4)3is7:1, thecalcination temperature is700oC, calcination time is8h, the mole of citric acid isequivalent to that of V3+), the7LiFePO4·Li3V2(PO4)3/C composite cathode materialhas much better electrochemical performance than LiFePO4/C and Li3V2(PO4)3/C. At0.1C rate, the first discharge capacity of7LiFePO4·Li3V2(PO4)3/C composite cathodematerial is up to145.6mAh/g with the charge and discharge efficiency of95.3%. At1C,2C,3C and5C rate, the first discharge capacities of7LiFePO4·Li3V2(PO4)3/Ccomposite cathode material are140.1mAh/g,120.6mAh/g,110.8mAh/g and100.8mAh/g, respectively. After50cycles, the capacity retentions are99.5%,98.7%,98.9%and98.4%, respectively, showing a good cyclic performance and rateperformance. At the same time, the composite cathode material exhibits the lowerpotential difference and good reversibility, the conductivity is1.0110-2S cm-1. The microscopic analysis show that the composite cathode material has fineparticle sizes between50-150nm, and with homogenous sizes distribution, thepraticles are uniformly wraped by an amorphous carbon layer, which connect all theparticles in an orderly fashion and forming nano network structure. The coalescencemachanism for the7LiFePO4·Li3V2(PO4)3/C composite cathode material was studiedand showed that some LiFePO4and Li3V2(PO4)3in the composite cathode materialwere doped by Fe and V, respectively. The valence of Fe and V after doping inLi3V2(PO4)3and LiFePO4was+2and+3, respectively.2. xLiFePO4·yLi3V2(PO4)3/C composites are synthesized by sol-gel method andthe optimized molar ratio of LiFePO4and Li3V2(PO4)3is7:1. At0.1C rate, the firstdischarge capacity of7LiFePO4·Li3V2(PO4)3/C composite cathode material is up to129.7mAh/g with the charge and discharge efficiency of96.0%. At1C,2C and5C rate,the first discharge capacities of7LiFePO4·Li3V2(PO4)3/C composite cathode materialare104.6mAh/g,89.3mAh/g and71.6mAh/g, respectively. After30cycles, thecapacity retentions are99.9%,95.1%and98.6%, respectively, showing a goodelectrochemical stability.3. The electrochemical performance of composite cathode mate rial with theMg2+-doped amount of0.01,0.02,0.03and0.04has carried on the contrast analysis,the results show that when Mg2+-doped amount is0.03, the composite cathodematerial has best electrochemical performance. At0.1C rate, the first dischargecapacity of composite cathode material is up to130.4mAh/g with the charge anddischarge efficiency of95.7%. At1C and3C rate, the first discharge capacities ofcomposite cathode material are112.1mAh/g and101.1mAh/g, respectively. |
PDF Full Text Request