The Synthesis And Modification Of Olivine LiMPO₄(M=Fe,Mn) For Lithium Ion Batteries

The increasing concerns on energy and environment have more demands on batteries. Lithium ion batteries (LIB) are attractive for application with the favorable properties of high voltage, long cycling life, high energy density and pollution-free. It becomes the main direction for lithium ion batteries to improve the whole performance and decrease the cost of relative materials. On the base of reviewing the development of lithium ion battery and its cathode materials in detail, LiMPO₄ (M=Fe, Mn) were chosen and their synthesis and modification were studied by using various electrochemical methods in combination with modern analytic techniques, e.g., XRD, SEM and so on .The reaction mechanism to synthesize LiFePO₄ was put forward by using TG/DTA method. The effects of synthesis conditions on physical performance and electrochemical behavior of LiFePO₄ were studied and the synthesis conditions were optimized. With the rising of heat-treatment temperature, the growth of crystal is quickened and the crystal became perfect. Increasing heat treatment time was benefit to the growth of LiFePO₄ crystal. The physical properties of particle distribution and morphology, which play an important role on the electrochemical performance of LiFePO₄, were affected by synthesis conditions. According to the research, samples prepared at 650℃ for 20 hours has the perfect crystallization, small particle size, coarse surface and even particle size distribution.Elevated temperature performance together with room temperature performance was proposed to evaluate the LiFePO₄ materials. Capacity fading of LiFePO₄ cycled at elevated temperature (55℃) was more serious than that at room temperature. For the samples prepared by solid-state reaction, the capacity fading after 15 cycles was 9% at room temperature, but it was 21% at elevated temperature.The composites of LiFePO₄/C were synthesized by high temperature solid state reaction with the carbon gel used as addition materials. Theresults showed that the carbon was distributed between the crystal particles. There was no influence on the structure of the crystal, but the crystal sizes were reduced obviously. The samples with the carbon addition demonstrated the improved performance. The first specific discharge capacity of LiFePO4/C with 22%C were 143.4 mAh/g, and 142.7mAh/g after 6 times cycling.A new process to prepare LiFePO4 was explored by the combination of solution deposition to form FePO4 and carbon-thermal reduction of FePO4. Pure LiFePO4 was synthesized at temperature from 560 to 800°C. Paticles size increased and discharge capacity decreased with the rising of temperature. The discharge capacity at 560 °C reached to 150mAh/g, while decreased to lOOmAh/g at 800 "C. Electrochemical measurement at different charge/discharge rate showed that the sample prepared at 560 °C had good ratecapability with 82% capacity retention at 0.5C in comparison with that at 0.1C. Under this research, samples fired at 560°C for 20hours had the best performance.Olivine LiFexMn|.xPO4 was synthesized by the method of solid-state reaction combining with the addition of carbon black and ball-milling of reagents and precursors for 2 times. LiFeo.3Mno.7PO4 sample prepared at 600°C had the small and even particle size, and the favorable properties of high voltage and perfect cycle ability. Its discharge capacity was 130 mAh/g and two discharge platforms in theory were appeared. The capacity at 4.1 V platform was 80 mAh/g. There was no capacity fading after 25 cycles. Kinetic parameters on lithium ion diffusion were evaluated, which was useful for the extensive research of olivine materials.The intercalation/deintercalation mechanism of LIB cathode materials was studied. Under the hypothesis, the equivalent circuit was built, and electrochemical impedance spectroscopy of lithium ion intercalation/deintercalation process was deduced theoretically. The impedance spectra of cathode materials electrode consists of one separated arcs in the high and intermediate frequency and a line inclined at 45 degree to the real axis in the low frequency, which wereassociated with the charge transfer of reaction at the interface of the electrolyte/cathode electrode and the diffusion of Li+ through the electrode respectively. The intercalation/deintercalation process of several cathode materials, such as, LiFePO4, LiFePO4/C, and LiFeo.3Mno.7PO4 was studied using impedance technique, and the results were consistent with the theoretical deduction, which proved the correction of the proposed mechanism, the built model and theoretical deduction of intercalation/deintercalation process.