Hole-like Nano-scale Lithium-ion Battery Cathode Material Lithium Iron Phosphate Preparation And Research

Olivine LiFePO4, as a promising cathode material for Lithium-ion power batteries, has attracted much attention due to its high theoretical capacity, environmental benign, high thermal stability, low cost, and good cycle performance. However, large-scale application of LiFePO4 cathode material is restricted by two main obstacles:poor electronic conductivity and low diffusion coefficients. In order to resolve these problems, LiFePO4 material with nano-porous structure was developed to improve its electrochemical properties and prompt its commercialization. The roughened and porous walls with increased surface-to-bulk ration possess more sites for Li+ to insert and extract at higher current density. On the other hand, low angle grain boundaries found in the wall is favorable to accelerate electrical conductivity and thus inhibit electrode polarization.In this thesis, the nano-porous LiFePO4/C were prepared by two kinds of synthesis methods:sol-gel method and template-directed method. The porous architecture is achieved during the degradation of citrate by sol-gel method. As to template-directed method, under a certain condition, these materials are assembled into the form of aggregating entities such as micelle/vesicle aggregates or liquid crystal phase, which restrict and direct the growth of a guest structures. After separating the template, porous structure can be fabricated. Furthermore, the effects of synthesis conditions on the dimension and distribution of porous were investigated, and the relation between the dimension and distribution of porous and the electrical performance of LiFePO4/C was studied. The structure, morphology, electrochemical performance and distribution of the porous of materials were also analyzed by means of XRD, SEM, TEM, AC impedance, electrochemical techniques and BET.LiH2PO4, iron (Ⅲ) citrate as raw materials were introduced to synthesize LiFePO4/C by sol-gel method. Firstly, the characteristics of materials synthesized at different synthesis conditions including different synthesis times, different synthesis temperatures, different atmosphere, the pH of solution and adding surfactant were discussed. In addition, the best parameters of synthesizing LiFePO4/C using different sources of lithium were investigated. The performance of LiFePO4/C is better when Li2CO3 as raw material mixed with surfactant were sintered at 700℃for 10h, its initial discharge capacity was 140.1mAh/g at a discharge rate of 0.1 C.Otherwise, there were Li4P2O7 as impurity phase.About template-directed method, three self-assembly mechanisms were used to prepare of nao-porous LiFePO4/C, including S+I-mechanism, S0I0 mechanism and S-X-I+ mechanism, to optimize the parameters of preparation this material, and discuss the effect of the distribution of pore diameter and specific surface areas on the performance of the samples. The following conclusions were drew. Firstly, the LiFePO4/C samples indicated the best performance by S+I- mechanism when it was heated at 700℃for 24h, its specific surface areas were 53.91m2/g, its discharge capacity was 150.24mAh/g at a discharge rate of 0.1C. In addition, the LiFePO4/C samples indicated the best performance by S0I0 mechanism when it was heated at 700℃for 12h, its discharge capacity was 150.24mAh/g at a discharge rate of 0.1C. At last, the effect of pH on the performance of the LiFePO4/C was investigated, this samples indicated the best performance when it was heated at 700℃for18h.