Study Of Synthesis Of LiFePO₄ By Solid-state Method And Its Property Modification

Presently LiFePO₄ with a structure of olivine-type has become more concerned because of its high energy density, low raw materials cost, environmental friendliness and safety. So the material is particularly suitable for the production of electrochemical accumulators with high power. But the key barrier limiting its application is extremely low electronic conductivity; the electrochemical process of insertion/ extraction of lithium is controlled by Li⁺ diffusivity and the capacity of the LiFePO₄ electrode decreased dramatically when working at large current density. So the research must be carried out so as to improve the conductivity and lithium diffusivity.LiFePO₄ was prepared by high temperature solid-state method and the effects of calcining temperature on physical performance and electrochemical behavior of LiFePO₄ were studied in this paper.The principle of choosing carbon precursor was brought forward according to the carbon mechanical modeling and the suitable carbon precursor with more evident that had affluence than others on the electrochemical capacity of composite material was found on the basis of the principle.The influence of the main factors on the charge/discharge performance of product was studied through orthogonal experiment such as calcining temperature, carbon amount, calcining time and proportion of lithium and iron( II). The optimal synthesis condition was found out by analysis and discussion.The LiFePCVC composite was also synthesized by solid-state method. The carbon-coated samples with perfect olivine-type structure were characterized by XRD analysis. The initial discharge specific capacity reached the top value 162.0mAh/g when the carbon content in the composite was 5.29% and even could keep relative high specific capacity and excellent cycle performance even at 3C rate exceeding the pure LiFePO₄ prepared at same circumstance greatly.With the increase of carbon content, the volumetric energy density and the tap density decrease unfortunately, and it can also be an importantaspect to limit the usefulness of LiFePC>4 in some practical applications. Some synthesis affecting on the bulk density were also discussed in this paper and the bulk density of LiFePCVC composite with Fe3O4 as iron source was greatly improved in contrast to the samples prepared with FeC2O4 ? 2H2O as iron source.Cyclic voltammetry (CV) and Electrochemical impedance spectroscopy (EIS) measurements were used to study the kinetics behaviors of LiFePC>4. The physical processes of deintercalation-intercalation of Li+ were analyzed and a suited equivalent circuit diagram was found. The results showed R^ minished with the increasing potential of intercalation.