Study On Synthesis Process Optimization And Performance Of FePO₄

The processing techniques of FePO₄ material which is the main raw material to prepare LiFePO₄ cathode material are discussed in detail.It is discussed how the process parameters of co-precipitation method affect the properties of FePO₄·2H₂O material.Then,the influencing factors on the properties of LiFePO₄ cathode material prepared with FePO₄have been investigated by XRD,SEM,TEM,TG/DTA,CV,EIS etc.The results show that the lattice distortion and size effect are the main factors influencing the low temperature performance of LiFePO₄/C materials.On this basis,the processing methods and technological routes are proposed to improve the low temperature performance and discharge capacity of LiFePO₄/C material.Main works are as follows:?1?The effects of process parameters of co-precipitation method on the properties of FePO₄·2H₂O material are discussed systematically.The results show that microscale Fe?OH?₃ colloid coating on the surface of FePO₄·2H₂O material is the main factor influencing the development of electrochemical activity of cathode material.Adjusting the reaction solution pH values can availably reduce or avoid the production of Fe?OH?₃ colloid,and obtain the FePO₄·2H₂O material with high electrochemical activity.The discharge specific capacity of LiFePO₄/C materials can stabilize above 155 mAh/g at 0.2C.Under the optimum technology conditions,the discharge specific capacity of LiFePO₄/C materials synthesized by FePO₄·2H₂O that prepared with low purity raw materials still can achieve156.7 mAh/g.?2?For the problem of the poor low temperature performance of LiFePO₄/C material,it is found that the impurities in FePO₄·2H₂O material which could incorporate into LiFePO₄ lattice by high-temperature diffusion and cause the lattice distortion,blocking the Li⁺diffusion channel.It can be effective solved by removing the impurities in the raw materials and improving the purity of the iron source to synthesize FePO₄·2H₂O material.The capacity retentions of LiFePO₄/C material prepared with this technique can reach 70%,at-20°C.On this basis,a new type of LiFePO₄/C material with a pomegranate-like spherical structure composed of smaller primary spherical particles is reported,which greatly optimize process-ability and improve low-temperature performance.?3?To further improve the electrochemical performance of material,in this part,LiFePO₄/C is modified by graphene.It is found that the graphene can help to form a homogeneous conductive network between the surface and the interspaces of LiFePO₄/C material,which can effectively improve the lithium ion diffusion rate and electrical conductivity and enhance the electrochemical activity.The specific discharge capacity of modified LiFePO₄/C material is 168.9 mAh/g,approximating theoretical value.?4?The lithium-ion diffusion coefficient and exchange current density were calculated in this part.It indicates that the lithium ion diffusion coefficient of high purity LiFePO₄/C material(2.73×10^-13 cm²/s)is higher two orders than that of lower purity LiFePO₄/C material which is 8.28×10^-15 cm²/s.It is found that the exchange current density of LiFePO₄/C material modified by graphene has increased by 6.89×10^-4 mA/cm² than that of uncoated products.It can conclude that graphene coating on LiFePO₄/C material contributes to improving the electrochemical catalyst activity and makes the electrode reaction more easy.