Sol-gel synthesis of thin solid Li7La3Zr2O12 electrolyte films for Li-ion batteries

•We report on a sol-gel processing method for the synthesis of garnet-type Yttrium-doped cubic Li7La3Zr2O12 (LLZ) samples•Powder, pellets and thin films were successfully obtained•The highly conductive cubic LLZ phase was achieved after the introduction of suitable RTP heating programs in argon•Prototypes of thin film half-cells with LiCoO2 and LLZ single phase layers were developed

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 (σ > 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 d50 = 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 ~ 50 nm. The desired single phase LLZ material could be obtained after thermal treatment at 800 °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 °C. It is shown that these unwanted processes can be prevented and that a structure with a single phase LLZ and LiCoO2 layer can be obtained by modifying the heating program to a rapid thermal treatment (10 K/s, 800 °C, no holding time).