Study On Synthesis And Modification Of Lithium Ion Battery LiFePO₄Cathode

With the traditional petrochemical resources consumption and environmental pollution, the development of power battery has very important social and economic significance, the olivine-structured LiFePO4are seen as one of the optimal candidates for power battery due to the merits of abundant raw materials, environmental friendliness, excellent cyclic stability, and high safety. However, the intrinsic crystal structure of LiFePO4results in poor electronic conductivity, ionic conductivity, and tap density, which seriously hinders its application. Therefore, this article aims to improve the material properties through optimizing material synthesis process, two-stage carbon coating, and metal ion doping.LiFePO4/C composite was synthesized by two-stage carbon-coating. First, the precursor Fe2P2O7/C was obtained by solid state reaction with Fe2O3, NH4H2PO4and glucose. And then, LiFePO4/C was synthesized by mixing the Fe2P2O7/C precursor, Li2CO3and glucose through high temperature solid state method. The effect of sintering temperature and sintering holding time on structure, morphology as well as electrochemical performance of LiFePO4was studied. Under the optimum conditions, LiFePO4/C composite have excellent physics and electrochemical performance, the discharge specific capacity of LiFePO4/C was157.2mAh/g at0.1C,136mAh/g at1C and114.5mAh/g at2C rate, and tap density reached1.15g/cm3. The diffusion coefficient of the LiFePO4/C composite was6.3×10-12cm2/s, which was calculated by the linear relation between peak point current and scanning speed.LiFe1-xVxPO4/C and LiFe1-xNbxPO4/C was synthesized by two-step solid state reaction with FeC2O4and LiH2PO4and in the process of synthesis no ammonia was produced, so that the synthetic process was environmentally friendly. The well-crystallized composites with homogeneous fine particle size were obtained. Metal ion doping and carbon coating work together to make the crystal defect internally generated, and form conductive network on the surface of materials particles, which effectively improved electronic conductivity and ionic conductivity of the LiFePO4/C composite materials. Through comparative study, the optimal conditions for synthesizing LiFe1-xVxPO4/C were as follows:Vanadium was added5%relative iron molar weight, PVA was added7%of the total quality, synthesis temperature was650℃, and synthesis time was6hours. Under the optimum conditions the LiFe1-xVxPO4/C (x=0.05)have excellent electrochemical performance. The discharge capacity of the LiFe1-xVxPO4/C were161.9,156.6,150.3and141.3mAh/g at0.1,0.5,1, and2C rates. Even at8C and10C rate, the discharge capacity of the sample remains122mAh/g and84.6mAh/g. LiFe1-xNbxPO4/C was synthesized with the same raw materials and process, at the optimum conditions the LiFe1-xNbxPO4/C (x=0.008)have excellent electrochemical performance. The discharge capacity of the LiFe1-xVxPO4/C were162.7,161.1,160.4,154.8and143.9mAh/g at0.1,0.2,0.5,1, and2C rates. Through the electrochemical and dynamics analysis of LiFe1-xVxPO4/C and LiFe1-xNbxPO4/C, it is known that LiFe1-xNbxPO4/C has better dynamic characteristics, such as smaller impedance and larger lithium-ion diffusion coefficient and good high rate charge and discharge properties, which make the LiFe1-xNbxPO4/C electrochemical performance better.