Electrodeposition of lithium is of both fundamental significance and practical importance. However, Li deposition has to proceed beneath a solid-electrolyte interphase (SEI). Herein, we present a fundamental study on the formation of SEIs and their influences on Li initial-stage deposition on Cu in Py-14 cation-based ionic liquids (ILs) combined with different ratios of TFSI and FSI anions. Different SEIs are pre-formed on Cu by using cyclic voltammetry, followed by detailed characterizations by XPS and AFM, which elucidate that all the SEIs bear mosaic-like I-O structures but with different compositions and structures and varying thicknesses of approximately 60-150 nm. The Li initial-stage depositions are studied by using cyclic voltammetric and potential stepping techniques, and current-time transients are analyzed with the aid of the Scharifker-Hills model. Discrepancies in nucleation density and diffusion coefficients and their relationships with overpotentials are investigated in correlation with the SEI structures. The present work demonstrates that interfacial and bulk properties of SEI strongly affect the Li deposition processes, which provides opportunities for uniform Li deposition.