Study On The Synthesis And Modification Of LiFePO₄ Used As Cathode Materials For Lithium Ion Batteries

Lithium iron phosphate (LiFePO₄) is a promising candidate cathode material for lithium ion batteries, due to low cost, environment amity, excellent cycling stability and safety. The key for commercializing LiFePO₄ is to improve its electronic conductivity and lithium-ion diffusion coefficient. On the basis of optimizing the synthesis conditions and investigating the structural change and the kinetic process of the electrode reaction in the charge and discharge process, some improvements for the electrochemical performances of LiFePO₄ were made by selective doping of rare earth elements, preparing LiFePO₄ conductive polymer composites and LiFe_1-xY_xPO₄/C composites.The predecomposition and synthesis system were optimized by the orthogonal method to obtain the optimized technologcial conditions. The synthesis temperature is the key influence factor on the microstructures and performances of LiFePO₄. The crystallites in the sample synthesized at 650℃possesse perfect crystal structure, high crystalline degree and few defects. The granularity distribution is uniform. The sample has the best electrochemical performance, its initial discharge capcity (D1) is 127.44 mAh/g, its initial charge and discharge efficiency isη1=94.50 %. The plateau capacity and plateau ratio is 114.87 mAh/g and 90.11 %, respectively; it displays the best rate and cycling capability.The structural change of Li1-xFePO₄ was analyzed by XRD method in the charge and discharge process. In the charge process, the content of LiFePO₄ decreases, that of FePO₄ increases gradually with increasing the x value. When x is 0.88, the charge process is over. Meanwhile the content of LiFePO₄ is lowest, that of FePO₄ is highest. The content of LiFePO₄ and FePO₄ changes reversely in the discharge process. The electrode kinetic process in the charge and discharge process was investigated by EIS method. At the beginning of charge, the exchange current ( i 0) and lithium-ion diffusion coefficient (DLi) increases rapidly. When x is 0.29, i 0 and lithium-ion diffusion coefficient reach the maximum, the electrochemical delithium reaction occurs most. At the end of charge (x>0.71), DLi decreases slightly and i 0 decreases abruptly. When x is 0.88, the charge process is over. DLi and i 0 change reversely in the discharge process.LiFePO₄/PAn, LiFePO₄/PPy and LiFePO₄/PTh composites were prepared by in-situ polymerization method. The combining form between LiFePO₄ and the conductive polymer was investigated systematically by TEM and SEM method when 6.75 % PAn or 10.56 % PTh was applied, the thin and uniform polymer coating is covered on the surface of LiFePO₄ particles. And they display excellent electrochemical performace and strong adhesion. PPy grains are distributed on the surface of LiFePO₄ particles or among LiFePO₄ particles. The electrochemical performaces of LiFePO₄/PPy composites is worse than that of LiFePO₄/PAn and LiFePO₄/PTh composites, especially in the aspect of rate capability.LiFePO₄ was doped on Li site or Fe site by using various rare earth ions Re3+ (Re= La, Nd, Y, Er). The microstructures and performaces of Li_1-xRe_xFePO₄ and LiFe_1-xRe_xPO₄ were comparatively investigated. The results show that when the same Re3+ is doped on Li site, the crystal lattice parameters and cell volume increase with increasing the doping amount; when the same doping amount is applied, the crystal lattice parameters and cell volume increase with increasing the radius of doping ion. When Re3+ is doped on Fe site, La3+, Nd3+ make the crystal lattice parameters and cell volume increasing and the Y3+ makes crystal lattice parameters and cell volume nearly no changing, the crystal lattice parameters and cell volume decrease with applying Er3+. When the doping ion and doping amount are the same, the crystal lattice parameters and cell volume of Li_1-xRe_xFePO₄ are larger than those of LiFe_1-xRe_xPO₄. Compared with Li_1-xRe_xFePO₄,,LiFe_1-xRe_xPO₄ displays higher electronic conductivity, good lithium-ion diffusion coefficient and more excellent electrochemical performance. The rules of the microstructure changes for Li_1-xRe_xFePO₄ and LiFe_1-xRe_xPO₄ correspond with that of the macro-performance changes. Therefore it is feasible and reliable using the change of microstructures and macro-performance to determine the selective doping of Re3+ on Li and Fe site of LiFePO₄.Using phenolic resin and epoxy resin as carbon precursor, LiFe_1-xY_xPO₄/C composites with network structure were prepared by curing, predecomposition and synthesis. The network structure was analyzed systematically by TEM, XPS and EDS methods. It was discovered that carbon is used as framework and LiFe_1-xY_xPO₄ particles were adhered on the carbon framework. This structure is entirely different from carbon coating, the conduct mechanism of electron and the diffusion mechanism of lithium-ion in network structural composite are evidently different from carbon coating composite. The network structure provides mult-channels for lithium-ion diffusion, accelerates the diffussion of lithium-ion, improves the rate capability of LiFePO₄. The experimental results show that carbon content is the key of factor forming network structure, the doping amount has few influence on the formation of network structure. When 5% polymeric carbon was applied, FY2C5,EY1C5 samples posses the most perfect network structure. Comparing with LiFePO₄, the electronic conductivities and lithium-ion diffusion coefficient of FY2C5 and EY1C5 samples increase 8 and 3 orders, respectively. FY2C5 and EY1C5 samples exhibit the most excellent electrochemical performance, their initial discharge capacities are 160.71 mAh/g and 165.71mAh/g at C/12 rate, 131.43 mAh/g and 143.96 mAh/g at 1 C rate.