X-ray photoelectron spectroscopy (XPS) has been used to study the depth profile of the solid-electrolyte interphase (SEI) formed on a graphite powder electrode in a Li-ion battery. The morphology of the SEI-layer, formed in a 1 M LiBF4 EC/DMC 2:1 solution, consists of a 900 Angstrom porous layer of polymers (polyethylene oxide) and a 15-20 Angstrom thin layer of Li2CO3 and LiBF4 reduction-decomposition products. Embedded LiF crystals as large as 0.2 mum were found in the polymer matrix. LiOH and Li2O are not major components on the surface but rather found as a consequence of sputter-related reactions. Monochromatised Al Kalpha XPS-analysis based on the calibration of Ar+ ion sputtering of model compounds combined with a depth profile analysis based on energy tuning of synchrotron XPS can describe the highly complex composition and morphology of the SEI-layer. (C) 2003 Elsevier Science B.V All rights reserved.