Thin film batteries (TFB) are discussed to be one of the future power sources for integrated circuits and miniaturized autonomous electrical devices. The open circuit voltage of a TFB is determined by the difference of the Fermi level positions of the two electrodes, which are electronically separated by an ionically conductive solid electrolyte. The stepwise deposition of one material on top of another combined with in-situ X-ray photoemission spectroscopy (XPS) offers the possibility to measure energy band alignments and band bending due to space charge layer formation at the electrode/electrolyte interfaces, allowing insights into the formation and the properties of the (electrochemical) interface. In this contribution we show results on in-situ XPS measurements of the interface formation between the cathodic interface between LiCoO2/LiPON and on the anodic interface between LiPON/Li. From these measurements an electronic energy band diagram of the full battery cell is determined. The contributions of dipole- and space charge layers to the formation of the electrified interfaces as well as the ionic contribution to the cell voltage is discussed. (C) 2016 Elsevier B.V.All rights reserved.