Solid state Li ion batteries have garnered attention due to their high safety and increased energy density. A key challenge towards commercialization is reduction of the high interface resistance affecting the charge transfer and in turn performance. In this thesis the aim is to analyze lithium phosphorous oxynitride (LiPON) properties. By variation of layer thicknesses and electrode conguration, LiPON bulk and interfacial properties were determined using electrochemical impedance spectroscopy. The results show a high degree of consideration with respect to surface morphology and deposition conditions is required for the reduction of interface resistance in solid state batteries.