Research On Preparation And Modification Of LiFePO₄as Cathode Material In Lithium Ion Battery

Olivine lithium iron phosphate（LiFePO₄） has been widely used as cathode material for lithiumion batteries, based on its abundant resources, lowcost, high safety performance, fine structural andelectrichemical stability. However,the application of LiFePO₄is limited to a large extent to be largescale applied for its light tap density, bad electrical conductivity and low ionic conduction coefficient.The main methods of LiFePO₄modification was kept on coating conductcive agent on the surface ofLiFePO₄and metal ion doping. In this article, research was focused on carbon coating process andstudy of ion doping mechanism.（1） The sample of carbon coated on LiFePO₄were prepared using critic acid with differentconcentration as carbon source. The result showed that when the doping content up to30%the samplepossessed better charge platform and particle morphology. But the apacity of the sample was just149.2mA/g, which could be correlated with secondary particle morphology. Research was carriedthrough in carbon coated on LiFePO₄by using critic acid as carbon source. The sample has goodcharge platform and best particle morphology while the content of critic acid reach30%by comparing.The capacity of the sample was just149.2mA/g, affected by secondary particle morphology.（2） Modification research of doping ammonium meta-vanadate with0、3%、5%、7%、10%content was carried on. The structural analysis results show that all the samples were olivine phaseand a small mount of vanadium ion enter into crystal lattice, bringing ahout the increase of latticeconstant. With increase of vanadium content, secondary particle ratio reduce and primary particle ratioincrease. And the morphology of the particle became better. The discharge capacity of samples at0.1Crate was149.2mAh/g、166.8mAh/g、146.5mAh/g、159.4mAh/g、155mAh/g respectively. The sampleLFP-V5had good high-rate performance, cryogenic property and cycle performance. The chargecapacity of it has66.7mAh/g at30C rate. XPS measurement demonstrated that the valence of Li, Pdidn’t change and Fe, V, O was+2,+3,-2respectly. Analysis showed that vanadium was doped intoFe position, leading to deformation of O6octahydron.（3） The mechanism of doping in lithium iron vanadium phosphate was studied with content15%,20%,25%,30%of vanadium. With the increase of doping concentration, many impurity phasesoccurred like Li₃V₂（PO₄）₃, VOPO₄, Fe2P, Li₃VO₄and V2O. SEM and TEM pictures stated that theexistence of impurities can reduce the pertilce. Three platforms was detected at4.07V,3.68V,3.61Vin discharge curve of LFP-V30, showing the discharging curve feature of Li₃V₂（PO₄）₃.