Synthesis And Electrochemical Performance Of Li₂MnSiO₄ As Cathode Material For Lithium Ion Batteries

Li2MnSiO4 was firstly reported by Dominko's group as a new cathode material for lithium ion batteries in 2006. Because of its high theoretical capacity (333 mAh/g), low cost and environmental compatible, Li2MnSiO4 was considered as a potential cathode material for lithium ion batteries. However, the electron conductivity of Li2MnSiO4 is low and the cycle ability is poor, so Li2MnSiO4 is in dire need of system and deep researching. This thesis focused on Li2MnSiO4 and studied its synthesis process, modified technic and electrochemical performance.Solid state method and polyol process were used to synthesize the Li2MnSiO4 cathode material; XRD, FESEM, HRTEM, ICP-AES, elemental analysis and granularity analysis were employed to character the physical and chemical properties; Battery testing system was used to test the electrochemical properties.The basic process parameter reaction temperature of 800℃was chose via TG-DSC, and then optimized by orthogonal experiments. Effect of sythesis temperature and holding time on microstructure and electrochemical performance of Li2MnSiO4 material were studied. The results showed that along with the raise of sythesis temperature and prolong of holding time, the electrochemical performance of Li2MnSiO4 was improved. The sample synthesized at 900℃for 20 h has the better initial discharge capacity of 117.9 mAh/g. The relationship of phase purity, microstructure and electrochemical properties was discussed.EIS was employed to study the extraction insertion kinetics of Li2MnSiO4 material. The EIS of Li2-xMnSi04 at different charge-discharge state was tested, and the corresponding lithium ion diffusion coefficient was calculated. In charge cycle, the lithium ion diffusion coefficient of Li2-xMnSi04 increased with the increase of x. In discharge cycle, the lithium ion diffusion coefficient of Li2-xMnSi04 decreased with the decrease of x. Li2MnSiO4/C composites were synthesized by solid state method with carbon black, graphite, sucrose and citric acid as carbon source. Effect of carbon source and carbon content on microstructure and electrochemical performance of Li2MnSiO4/C was studied. The Li2MnSiO4 sample corresponding to the best sucrose carbon source has the best initial reversible capacity of 88 mAh/g, and a discharge capacity of 62.1 mAh/g was obtained after 10 cycles. In the scope of this study, whichever carbon source was chose, along with the increase of carbon content, the grain size and particle size reduced and the amount of impurity increased. When carbon content was 9%-10%, the Li2MnSiO4/C samples can always achieve the best electrochemical performance. The effect mechanism of carbon source and carbon content on the electrochemical performance of Li2MnSiO4/C was analysed.Li2Mni-xMxSi04 (M=Fe, Al, Mg, Ti) cathode material was synthesize by solid state reaction; the problem that the structure of Li2MnSiO4 is collapsed during the charge-discharge cycle has been effectively solved. The electrochemical performance of the Fe2＋doping samples was increased with the raise of the Fe2＋doping amount. The best initial discharge capacity of 72.3 mAh/g was achived when x= 0.9. Al3＋, Mg2＋and Ti4＋doing can improve the electrochemical performance of Li2MnSiO4 effectively, since ion doping can stabilize the crystal structure of Li2MnSiO4. The mechanism of that ion doping stabilize the structure of Li2MnSiO4 was illustrated. The crystal volume of Li2MnSiO4 was shrinked by doped positive ion which has small ion radius, thus the structure was stabilized. The crystal field theory in coordination chemistry was used to reveal that Fe2＋doping can improve the crystal field stabilization energies and thus the structure was stabilized.Li2MnSiO4/C material was synthesized by polyol method with sucrose as carbon source, and the effect of synthesis temperature on microstructure and electrochemical performance of Li2MnSi04/C was explored. The results shown, the obtained samples have pure Li2MnSiO4 phase and nanoparticles, which coated by a very thin amorphous carbon film. The sample obtained at 600℃has a best initial reversible capacity of 132 mAh/g. Al3＋doping can remarkably improve the charge-discharge property of Li2MnSiO4. When cycled at 0.1C rate, the initial discharge capacity of the sample doped 10% Al3＋was 2.4 times as that of the pristine sample.