Direct integration of lithium-ion battery (LIB) with electronic devices on the same Si substrate can significantly miniaturize autonomous micro systems. For achieving direct integration, a barrier layer is essential to be inserted between LIB and the substrate for blocking Li diffusion. In this paper, the feasibility of thermal SiO film as the barrier layer is investigated by electrochemical characterization and X-ray photoelectron spectroscopy (XPS). Due to the negligible side reactions of thermal SiO with electrolyte, the solid electrolyte interphase (SEI) layer formed on the surface of the barrier layer is thin and the SEI content mainly consists of hydrocarbon together with slight polyethylene oxide (PEO), Li PO F , and Li CO . Although 8-nm thermal SiO effectively prevents the substrate from alloying with Li , the whole film changes to Li silicate after electrochemical cycling due to the irreversible chemical reactions of SiO with electrolyte. This degrades the performance of the barrier layer against the electrolyte penetration, thus leading to the existence of Li (in the form of F-Si-Li) and solvent decompositions (with the products of hydrocarbon and PEO) near the barrier layer/substrate interface. Moreover, it is found that the reaction kinetics of thermal SiO with electrolyte decrease significantly with increasing the SiO thickness and no reactions are found in the bulk of the 30-nm SiO film. Therefore, thermal SiO with an appropriate thickness is a promising barrier layer for direct integration.