Solid state lithium batteries are considered an alternative to currently available lithium ion batteries. The development of a suitable electrolyte is essential for commercialisation of bulk-type solid state batteries. This thesis is concerned with the synthesis of a solid state electrolyte as a composite of ceramic Li7La3Zr2O12 (LLZO) and a polymer electrolyte based on poly(ethylene oxide) (PEO). First, the synthesis of Al substituted LLZO by means of co-precipitation is described. The obtained powder is characterised with regard to its structural, chemical and electrochemical properties. In the second part, free-standing and flexible composite membranes (containing up to 40 vol% LLZO in polymer electrolyte (PEO20LiClO4) matrix) are manufactured by tape casting. In the third part, the LLZO/PEO20LiClO4 interface is identified as obstructive to continuous Li ion conduction. A model system is developed and investigated by means of impedance spectroscopy
