The application of a solid state electrolyte layer could greatly improve current Li-ion batteries in terms of safety and reliability. Garnet-type Li7La3Zr2O12 (LLZ) appears as a candidate material, since it shows the highest reported Li-ion conductivity of all oxide ceramics at room temperature (sigma > 10(-4) S cm(-1)) and at the same time chemical stability against lithium. In this paper, a sol-gel process is presented for fabricating homogeneous thin film LLZ layers. These layers were deposited using dip-coating and spin-coating methods. A stable Yttrium-doped Li-La-Zr-based sol with a particle size of d(50) = 10 nm was used as coating liquid. Successful deposition of such layers was accomplished using a sol concentration of 0.04 mol/l, which yielded for each coating step a layer thickness of similar to 50 nm. The desired single phase LLZ material could be obtained after thermal treatment at 800 degrees C for 10 min in Argon. Ionic conductivity of the layers was demonstrated with impedance spectroscopy. Continuing work on the development of half-cells is also presented. Half-cells which contain the novel LLZ electrolyte layer, a LiCoO2 cathode and a steel support were synthesized and investigated. Of considerable importance was the prevention of Lanthanum diffusion and the formation of non-conductive phases (e.g. La2Li0.5Co0.5O4) at the required heating temperature of 800 degrees C. It is shown that these unwanted processes can be prevented and that a structurewith a single phase LLZ and LiCoO2 layer can be obtained by modifying the heating program to a rapid thermal treatment (10 K/s, 800 degrees C, no holding time). (C) 2016 Elsevier B.V. All rights reserved.