Preparation And Electrochemical Properties Of Cathode Material LiFeO₄Powder

With high voltage, high capacity, high energy and good cycle performance,lithium ion batteries developed rapidly since1990s．And high power lithium ionbattery becomes a new kind of power source with the emergency of energycrisis．Cathode material is very important part of the whole battery, and the researchof new cathode material with high electrochemical performance and low cost isnecessary．LiNiO₂and LiMn2O₄are studied widely now, but they all can not fit forrequirement of high power lithium ion batteries．In1997a new cathode materialLiFePO₄with olivine structure was reported．It has high theoretical capacity, goodcycle performance. low cost, safety and low harm．But its shortcomings are lowelectronic conductivity and low ion diffusion kinetics, which restrict LiFePO₄commercialized．In this thesis, Sol-Gel and solid phase method are used to prepareLiFePO₄．LiFePO₄powder performance is greatly improved through optimizing thepreparation technology. XRD, SEM, CV were used to study properties of products andreaction mechanism．The detailed results in this dissertation include:1. The stable sol is prepared with LiNO₃as the Li source, PAA as the chelatorand distilled water as the solvent via macromolecule complex at some temperature.The gel completely decomposes at430oC and gradually forms LiFePO₄and its sizebecomes larger with the increasing of sintering temperature. The stable sol is preparedwith LiNO3as the Li source, citric acid as the chelator. The gel completelydecomposes at400oC and gradually forms LiFePO₄and its size becomes larger withthe increasing of sintering temperature.2. The LiFePO₄power prepared by heating treatment gel prepared by PAA at650oC for12h exhibits uniform distribution, high crystallinity, low agglomeration andthe grain size is about350nm. The LiFePO₄power prepared by heating treatment gel prepared by citric acid at650oC for12h exhibits uniform distribution, high crystallinity,low agglomeration and the grain size is about400nm.Residual of carbon prepared byPAA is8.25%, while that prepared by citric acid is6.03%.3. With the increasing of calcination temperature, the LiFePO₄power prepared byPAA and citric acid discharge capacity has been improved. The LiFePO₄powerprepared with PAA at650oC for12h presented the discharge capacity of160mAh/g atdischarge rate of0.1C, it could reach140mAh/g at the100th cycle, which proves goodcycle performance.4. The initial discharge capacity of LiFePO₄power prepared by PAA is120mAh/g,100mAh/g and75mAh/g at discharge rate of1C,2C and2.5C. Even discharged at therate of2.5C, its capacity could keep as75mAh/g after20cycles, which proves goodelectrochemical performance at high discharge rate.5. The LiFePO₄power prepared by solid phase method exhibits seriousagglomeration and irregular, and its grain size is micron grade. The initial dischargecapacity is125mAh/g,100mAh/g and32mAh/g at discharge rate of0.05C,0.1C and1C.6. CV result showed that LiFePO₄powder prepared by PAA as the chelatorthrough Sol-Gel exhibits higher peak current and lower polarization voltage, whichproves good electrochemical performance.