Magnetron sputtered thin film solid state electrolytes in the materials system Li-V-Si-O with increased ionic conductivity

C. Ziebert, Karlsruhe Institute of Technology (KIT), Institute for Materials Research I (IMF I), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany; A. Knorr, KIT, IMF I, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany; S. Ulrich, KIT, IMF I, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany; D. Gerthsen, KIT, Laboratory for Electron Microscopy, Kaiserstrasse 12, 76131 Karlsruhe, Germany

The basic demand on an electrolyte for a Li-ion battery is the ability to conduct Li-ions. Because the ionic conductivity in solid state electrolytes is smaller than in fluid systems, there is a strong need for the development of thin film solid state electrolytes, which can compensate this handicap by reducing the diffusion path of lithium ions due to their small thickness. In this work amorphous thin films in the materials system Li-V-Si-O (LVSO) were realized by applying a combinatorial materials science approach. The films were deposited by non-reactive r.f. magnetron sputtering from a segmented target that consisted of two half-parts of circular LiVO₃ and SiO₂ ceramics. In each experiment, coatings of different composition and/or microstructure were obtained simultaneously by placing different substrates in individual positions relative to a segmented target. The composition, crystal structure and topography were examined using inductive coupled plasma optical emission spectroscopy, X-ray diffraction (XRD), Raman spectroscopy, scanning electron and atomic force microscopy. Moreover intrinsic stress, ionic and electrical conductivity and film density were determined. The parameters working gas pressure (0.075 – 25 Pa) and bias voltage (0 V – -60 V) were systematically varied to find parameters for thin films that remain amorphous and Raman-inactive even after a heat treatment for 3 h at 600 °C (Ar:O₂ = 4,5:5). XRD and Raman spectroscopy revealed that films deposited at a pressure of 0.15 Pa and a substrate bias of -40 V fulfilled these requirements. From electrical impedance measurements a ionic conductivity at room temperature of 2.8·10^-5 S cm^-1 was determined, that was significantly higher than all values for the Li-V-Si-O system or any other thin film electrolyte systems reported in literature up to now. This result clearly confirms the potential of the combinatorial materials science approach with a segmented target arrangement.