The requirement for energy density of Lithium ion batteries has been dramatically increased in the past decades with the development in consumer electronics and electrical vehicles (EV). For example, the USABC target for next generation EV batteries is 350 Wh/kg and 600 Wh/l. To achieve these targets, high capacity positive electrode (cathode) and negative electrode (anode) materials need to be developed. Among all the potential materials, Si is the most attractive material due to its high theoretical capacity (~4200mAh/g), ten times higher than the capacity of graphite. However, it suffers from large volume expansion and contraction (~400%) during lithiation and de-lithiation, which leads to many degradation mechanisms. Firstly, because of the large volume change during cycling, cracking and pulverization of active particles and the surrounding matrix lead to the disconnection of active materials from conductive carbon or current collector, and the loss of the active material will contribute to capacity degradation of the cell. Secondly, solid state interphase (SEI) acts like a protection layer on the surface of anode to prevent electrolyte decomposition. In carbon-based anode, a stable SEI film normally develops in the first cycle at 0.5–1.0 V versus lithium, and contributes mostly to the first-cycle irreversible capacity. In contrast, the SEI formation on Si anodes appears to be a dynamic process of breaking off and reforming due to the constant volume changes of the Si particles during cycling. This process involves continuous electrolyte decomposition and lithium consumption, which also leads to capacity degradation. In this study, we will report SEI study on several Si based anode materials from the end of life (EOL) cells.