Effects Of Synthesis Factors Of LiFePO₄ On Its Low Temperature Performance

In this paper,FeSO₄·7H₂O was used as raw material to prepare LiFePO₄/C by liquid phase coprecipitation method and carbothermal reduction method.The low temperature electrochemical properties of the material were investigated and analyzed.The further verification experiments were carried out by means of XRD,SEM,TEM,particle size and EIS.The effects of synthesis factors such as purity of raw material,particle size of the material and modification of graphene on the low temperature performance of LiFePO₄/C were investigated.Specific work is:?1?The low temperature electrochemical performance of LiFePO₄/C samples prepared by high purity FeSO₄·7H₂O,industrial grade FeSO₄·7H₂O and the mixture of them show that with the decrease of purity of raw materials,the low temperature discharge capacity gradually becomes low.The discharge capacity retention of 0.2C and 0.5C of LiFePO₄/C with high purity FeSO₄·7H₂O was 65.9%and 44.9%at-20?.But the discharge capacity retention of LiFePO₄/C prepared by industrial grade FeSO₄·7H₂O were only 53.3%and31.9%.The results of EIS showed that the lithium ion diffusion coefficient of them was4.53×10^-1313 and 8.56×10^-15cm²/s,respectively,the lithium ion diffusion coefficient of LiFePO₄/C prepared by high purity FeSO₄·7H₂O is higher two orders of magnitude.Therefore,the lithium ion diffusion coefficient is an important factor affecting the low temperature performance of LiFePO₄/C materials.Elemental analysis of FePO₄·2H₂O prepared from industrial and high purity raw materials showed that the impurity element Ti contained in the industrial raw material was introduced into the LiFePO₄/C lattice,which caused the lattice distortion to block the diffusion of lithium ions aisle.Therefore,improving the raw material purity is an important method to improve the low temperature performance of LiFePO₄/C.?2?The different particle size of LiFePO₄/C was prepared by carbothermal reduction method.With the decrease of particle size,the particle size of the precursor and LiFePO₄/C decreases.The D₅₀ of LiFePO₄/C prepared by precursors polishing with 0.7mm zirconium balls is about 0.4 and 0.8?m but the LiFePO₄/C prepared by 0.4mm ball milling has obvious agglomeration.Especially the capacity retentions ratio of LiFePO₄/C material prepared by the precursor grinding with 0.7mm grinding balls at-20?0.2C and 0.5C rate can reach 77.2%and 66.9%,respectively.With the decrease of particle size,the lithium ion diffusion coefficient of LiFePO₄/C increases gradually,and the diffusion coefficient of Li⁺increased gradually.This is because that the reduction of LiFePO₄/C sample particles shortens the distance needed for Li⁺diffusion through LiFePO₄/C,increases the effective utilization rate of LiFePO₄/C material,and improves the low-temperature performance.However,too small particle size will lead to a larger material surface area.This agglomeration can adversely affect the infiltration of the electrolyte and the de-embedding of Li⁺,which in turn results in a small Li⁺diffusion coefficient of the material.Therefore,reducing the particle size of the precursor and LiFePO₄/C properly can effectively improve the low-temperature discharge capacity.?3?The effects of graphene addition on the low temperature performance of LiFePO₄/C were investigated.The addition of graphene causes the presence of graphene as a gauze on the outside of the carbon coating,but the graphene is not easily coated on the surface of the LiFePO₄ particles.It is difficult to form an effective continuous conductive network between the particles.Therefore,the effect of graphene coating on the conductivity and Li⁺diffusion coefficient of LiFePO₄ is limited.The electrochemical performance of LiFePO₄/C was investigated.It was found that graphene as a conductive agent could improve the high rate performance of LiFePO₄/C.But its low temperature performance is not significantly improved.