We investigate the early stage of the solid electrolyte interphase (SEI) layer formation process in four electrolyte solutions, using electrochemical quartz crystal microbalance (EQCM) and X-ray photoelectron spectroscopy (XPS). The EQCM result proves that the LiBH4 in THF solution form a negligible amount of SEI layer. The SEI formation process of the LiPF6 solution occurs at 2.3 V vs. Li, and the SEI layer continuously grows even during the anodic scan. The electrolyte solutions containing LiTFSA form SEI layer around 1.5 V vs. Li. The XPS study shows that the SEI layer formed at high electrode potential, in the electrolyte containing LiPF6 forms a dense LiF layer. The LiTFSA solution forms a thick SEI layer containing organic components such as lithium alkyl carbonate or polyether. (C) The Author(s) 2019. Published by ECS.