Synthesis And Properties Of Lithium Iron Phosphates As Cathode Materials For Lithium-ion Batteries

With the gradual decrease of non-renewable petroleum and natural gas resourcesand the increasingly serious environment problems, development and utilization forthe renewable resources becomes more and more meaningful. A lithium-ion battery isa new generation of green and high-energy battery, which is used in portableelectronic products and new resources transportation vehicles widely. Cathodematerial is the key component of lithium-ion batteries. Recently, the olivine-typeLiFePO₄cathode material, which offers advantages such as low-cost, environment-friendly, high-capacity and steady cycling performance, is gradually replacingconventional LiCoO₂material and becoming cathode material for high-rate dischargedynamic Lithium-ion batteries. LiFePO₄is being considered as one of the mostexpected cathode material for lithium-ion batteries.In the paper, LiFePO₄material was synthesized by gel-network method andsol-gel method, respectively. LiOH·H₂O, FeC₂O₄·2H₂O and NH₄H₂PO₄were used asraw materials. Particle size distribution, structure and morphology of the preparedLiFePO₄samples were characterized by XRD, SEM, laser particle size analyzer andelectrochemical testing system.At first, this paper discussed the characteristics of materials synthesized bygel-network method at different synthesis conditions including gelatin contents, rawmaterials concentration, and LiOH concentration. The optimum conditions wereobtained as follow: gelatin content was25%, FeC₂O₄and NH₄H₂PO₄concentrationwas0.2mol·L^-1respectively, LiOH concentration was0.2mol·L^-1. The influence ofconcentration was explored from the view of process of particles formation and iondiffusion. The influencing mechanism of gelatin in gel-network process was analyzed.The effects of process parameters, such as pH of gelation preparation, feedingmode, temperature of gelation preparation, calcination temperature, calcinationheating rate, and the ratio of [CA]/[Fe²⁺], for the characteristics of LiFePO₄in thesol-gel method were discussed. The optimum conditions were obtained as follow: pHof gelation preparation was3.57, feeding mode was LFP, temperature of gelationpreparation was120℃, calcination temperature was800℃, calcination heating rate was7℃·min^-1, the ratio of [CA]/[Fe²⁺] was3.5:1. The gelation process was analyzed.Calcination process mechanism and the key step in calcination process were proposed.The effect of citric acid for particle size distribution of LiFePO₄and the mechanismwere discussed.