Synthesis And Modification Of Cathode Material Spherical LiFePO₄ For Li-ion Battery

Lithium iron phosphate(LiFePO₄) with an ordered olivine structure becomes a new pop research subject among the cathode materials of lithium ion batteries due to its advantages such as abundant resources,low cost,non-toxicity and environmental benignity.However,lithium phospho-olivines suffer from a poor electronic conductivity and a low tap-density.Therefore,researchers mainly tend to focus on how to improve the conductivity and enhance the tap-density of LiFePO₄ powders. The spherical LiFePO₄ particles were synthesized by the precipitation and calcination process from starting reagents of FeSO₄,H₃PO₄,LiOH and NH₃·H₂O.The structure, morphology of particles and electrochemical performance were investigated by x-ray diffraction(XRD),scanning electron microscopy(SEM) and fourier transform infrared(FTIR),and the formation process of spherical LiFePO₄ was analysed by TG-DSC.The electrochemical performance was tested by multi-potentiostat system.The preparative process of lithium iron phosphate was as follows:first synthesized the spherical ferrous phosphate precursor,then the precursor of ferrous phosphate was mixed with the lithium phosphate,at last the lithium iron phosphate by sintering at high temperature in a protected atmosphere.The preparation process of spherical ferrous phosphate,formation mechanism and electrochemical performances of samples were investigated systematically.The results showed that the spherical Fe₃(PO₄)₂ particles with a high tap density and good fluidity have been obtained by the aqueous reaction precipitation.The material sintered for 10h at 700℃under nitrogen atmosphere has a complete olivine structure,uniform surface morphology, high tapped density and excellent electrochemical performances.The initial discharge capacity of 109.4 mAh/g was obtained at 0.2C charge-dischange rate.However after 20 cycles,the capacity of the sample exhibited a rapid degration.In order to improve the electrochemical performance of products,LiFePO₄/C by addition of glucose as the carbon source was prepared.The effect of sintered temperature,sintered time and carbon content on LiFePO₄/C was examined.The experimental results showe that the addition of carbon reduced the LiFePO₄ grain size and enhanced performance in terms of improved discharge capacity and cyclability. The material coated 10%carbon sintered at 700℃for 10 hours showed the best electrochemical performances.The initial discharge capacity of 130.1mAh/g was obtained at 0.2C charge-dischange rate,and the capacity loss was only 7%after 20 charge-discharge cycles.In order to improve the conductivity of materials and increase the electrochemical performances,the effects of doping Mn²⁺ was studied,which showed that the addition of Mn²⁺ improved the electrochemical performances LiFe_0.85Mn_0.15PO₄ exhibites the best electrochemical performances,with a discharge capacity of 133.8 mAh/g at 0.2C charge-dischange rate and the capacity loss was only 7.9%after 20 times charge-discharge cycles.