June 12-17, 2011 - Arcachon
The development of alternative solid state electrolytes is a critical issue for advanced lithium ion batteries; because present batteries based on liquid organic electrolytes have several disadvantages, such as the formation of a solid electrolyte interface (SEI) at the anode leading to a large irreversible capacity loss during cycling, safety and self-discharge issues. However, the limited ionic conductivity of solid state electrolytes, which is 3-5 orders smaller than in typical liquid electrolyte systems, has to be compensated. Thin films are especially suited for this purpose, because they reduce the diffusion path of lithium ions due to their small thickness and provide a good thermal and chemical stability.
In this work a combinatorial materials science approach based on non-reactive magnetron sputtering from segmented targets in a Leybold Z550 coating device was applied in order to develop thin film solid state electrolytes in the material system Li-V-Si-O (LVSO). In each experiment, several thin films of different composition and/or microstructure were simultaneously deposited by placing different substrates in five individual positions relative to the segmented target. For the deposition onto Si and stainless steel substrates two different segmented target setups have been used: setup 1 consisting of two half parts of circular LiVO3 and SiO2 ceramics and setup 2 composed of Li4SiO4 and V2O5. The influence of the variation of the sputter pressure (0.075 –25 Pa), the bias voltage (up to -60 V) and the sample position on composition, crystal structure, morphology, topography and properties (residual stress, film density, ionic conductivity) will be shown.
Films deposited at
a pressure of 0.15 Pa and a substrate bias of -40 V using setup 1 remained X-ray-amorphous and Raman-inactive even after furnace
annealing (3 h at 600 °C in Ar:O2 = 4.5:5 atmosphere of 10 Pa).
The ionic conductivity of these Li1.33V0.77Si0.35O4 films at room temperature was determined to be 2.8´10-5 S/cm.
Using setup 2 amorphous Li1.2V1.3Si0.7O4 thin films could be deposited at a pressure of 0.5 Pa without bias voltage that transformed into a crystalline orthorhombic phase after annealing and showed an even higher ionic conductivity of up to 6.5´10-5 S/cm, which is significantly higher than all values for the Li-V-Si-O system or any other thin film electrolyte system reported in literature up to now. First results for thin film Li-ion batteries realized by combining these optimized LVSO thin films with standard materials for cathode and anode thin films will be presented.