Development of metallurgical silicon / CNT / graphene multifunctional anodes for lithium ion batteries

Using the green energy is a solution that will solve the global warming problem not only because of the dependence on limited fossil fuel but also the greenhouse gas emission. But, using renewable energy resources requires energy storage. Li-ion batteries are the main energy storage systems that are used in different applications which changes from portable electronic circuits to electricle vehicles. Nowadays, generally graphite is used as anode which has maximum 372 mAh/g spesific capacity and is not sufficient for the high energy density demand in Li-ion batteries. As an alternative to graphite, Si is capable of making alloys with Li in high stoichiometry, it can exhibit a specific capacity of 3590 mAh g which is almost 10 times the theoretical capacity of graphite at room temperature. However, Si experiences volumetric expansion by more than 300% during lithation, and this large volume expansion causes pulverization of the active material, loss of electrical contact and consequently rapid capacity loss. In this PhD thesis, in order to address these problems with a low cost solution, structural and electrochemical characterization of silicon particles with yolk-shell morphology and CCSi-based electrodes which have different ratios of RGO and / or MWCNT were studied using metallurgical silicon as starting material. The production process of anodes was started with grinding the metallurgical grade silicon with high purity grade into micrometer silicon particles via milling methods, continued with obtaining yolk-shell structured silicon particles by carbon coating and etching with HF. Active materials of CCSi/MWCNT/RGO anodes were prepared via vacuum-filtration method. Hummers method was used to prepare GO and the obtained GO was reduced to RGO. The structures, morphologies and electrochemical performances of composite anodes were investigated by different techniques. Compared to Si electrode, CCSi/RGO/MWCNT composite electrode displayed a superior electrochemical performance. The electrochemical performance of the electrodes has been improved by using carbon coating layer, MWCNT and RGO, which have high electrical conductivity and mechanical strength.