Research On Spherical Lithium Iron Phosphate Cathode Materials Of Lithium Ion Batteries

Olivine-type LiFePO₄ is considered as the most promising candidate for the next generation cathode materials of lithium ion batteries because it has high theoretical specific capacity, is cheap and environmentally friendly, also has good cycling characteristics and excellent safety. However, it's poor electronic conductivity, which leads to initial capacity loss and poor rate capability. And it's low pile density, which leads to low volumetric specific capacity. That prevent LiFePO₄ to be put into commercially used.In this work, we aim at preparing spherical LiFePO₄. Used liquid deposit-high temperature sinter method to synthesis spherical LiFePO₄. Researched the influence of synthesis condition to NH₄FePO₄·H₂O precursor and LiFePO₄ product in structure, pattern and electrochemical performance. In addition, X-Ray Diffraction (XRD) was used to analyze the crystal structure of precursor and product, Scanning Electron Microscope (SEM) was used to observe the superficial morphology, charge-discharge test was used to study the specific capacity and cyclic properties, Cyclic Voltammogram (CV) and Electrochemical Impedance Spectroscope (EIS) was used to discuss the electrochemical reactive mechanism.The results of the study show that use FeSO₄·7H₂O, H3PO4 and NH3·H₂O as materials to prepare NH₄FePO₄·H₂O precursor, the structure of the precursor and LiFePO₄ are similarity, it is an effect of"structure preset"before sinter. Carbon doping can influence the tap density greatly, with the increase of carbon content the tap density decrease. The tap density is 1.28 g·cm^-3, when doped with 5 mass% carbon content. Using LiF as lithium source, sucrose as carbon source, carbon content is 10 mass%, 550℃sinter for 12 h to prepare LiFePO₄, the sample has a discharge specific capacity of 120 mAh·g^-1 (0.1 C). The discharge specific capacity of nickel doping sample is about 110 mAh·g^-1 (0.1 C), the discharge specific capacity and the cycle performance aren't change efficiently by nickel doping. After carbon doping, Rct decrease, j°increase, it accelerates the extraction of Li⁺, so the electrode reaction speed is accelerated, it is advantage to the electrochemical reaction, also enhance the real capacity.