Thermal Analysis Kinetics And Electrochemical Performance Of Li₂MnSiO₄ As Cathode Material For Lithium Ion Batteries

As a new cathode material for lithium ion batteries, Li₂MnSiO₄has the advantages of high theoretical capacity （333 mAh/g）,abundant natural resources, low cost, non-toxic, environmentalcompatible and safety performance. However, its own electronicconductivity is very low, this seriously limited the takeoff/embedded efficiency of lithium ion in the material; and theamorphization of crystal materials structure in the process ofcirculation leads Li₂MnSiO₄almost has no electrochemical activity.In view of the above questions, the main contents of this thesisresearch as following:Firstly, according to the characteristics of solid-state reaction,optimizing the synthesis of Li₂MnSiO₄ cathode materials.The Thermal Decomposition process of Li₂MnSiO₄Precursor wasinvestigated by thermogravimetric-differential scanningcalorimetry （TG-DSC） combined with Fourier transform infraredspectroscopy （FTIR）.The results showed that, There were two main weight loss stages due to the dehydration and thermaldecomposition of raw materials. There were five endothermic peaksappeared in DSC curves obtained at different heating rates.According to the thermal analysis, the apparent activate energies ofthree endothermic peaks （> 200℃） were calculated to be 384.12、120.63、263.43 kJ/mol and 350.78、117.16、227.59 kJ/mol using theDoyle-Ozawa method and the Kissinger method, respectively. In theKissinger method, the corresponding reaction order, frequencyfactor and reaction rate equation were further inferred.From the thermal dynamics results, proposed the optimizingtechnological of step-sintering method: step optimized thesyntheses process of direct rising temperature, the optimalsynthesis process of high temperature solid-state reaction is asfollows: 200℃（ 2h）�'grinding, compacting�'400℃（ 3h）�'500℃（2h）�'700℃（10h）. By comparing the chemical phase, morphology, sizeand electrochemical performance of the Li₂MnSiO₄cathodematerials before and after optimization showed that: the optimizingtechnological synthesized materials possess high purity, small sizeand particle uniformity, with favorable electrochemical properties.Secondly, the study of carbon compatiblizing modification ofLi₂MnSiO₄/C cathode materialsIn order to improve the low conductance of Li₂MnSiO₄materials, use glucose as carbon source, Li₂MnSiO₄/C cathode materials withdifferent amounts of carbon were synthesized by solid-state reactionunder the vacuum circumstance. The crystalline structure andmorphology of products were analyzed by X-ray diffraction （XRD）,scanning electron microscopy （SEM） and laser scattering technology（LS）, respectively. The results showed that carbon doping decreasethe crystallite sizes of products, but kept the aggregation of theparticles in the process of solid state sintering and made the Li₂SiO₃impurity increased instead. Constant current charge-discharge testsshowed that, the Li₂MnSiO₄cathode materials doping with carboncan improve the charge-discharge capacity and the coulombefficiency of the first cycle. While carbon content was 7.6%, the firstcharge capacity of the materials reached the maximum, is175.7mAh/g, but the capacity loss rates is still higher, and after 10cycles, it’s reduced to 119.1 mAh/g.