When a carbon electrode in a non-aqueous electrolyte is charged for the first time from the open circuit potential toward 0V vs. Li/Li+, Li ions are intercalated into the carbon structure. Along with Li intercalation, other (electro)chemical reactions that occur during the first few cycles are the decomposition of the electrolyte on the carbon and the formation of a surface layer, often referred to as the solid electrolyte interface (SEI) layer. The properties and chemical composition of the SEI layer have been a subject of intense scientific interest because the SEI layer plays a critical role in the safety and the cycle life of the Li-ion secondary batteries. Our work entails characterization of SEI layer by combining two techniques, that is, electochemical quartz crystal microbalance(EQCM) and ellipsometry. This work is a joint project with Dr. Frank McLarnon at LBNL and supported by US Department of Energy