Characterization of Li 7 La 3 Zr 2 O 12 Thin Films Prepared by Pulsed Laser Deposition
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Characterization of Li7La3Zr2O12 Thin Films Prepared by Pulsed Laser Deposition Jiajia Tan1 and Ashutosh Tiwari1 1 Nanomaterials Research Laboratory, Department of Materials Science and Engineering, University of Utah, Salt Lake City, UT 84112, U.S.A. ABSTRACT A pulsed laser deposition system was employed to fabricate thin films of Li7La3Zr2O12 solid electrolyte. The deposition process was carried out at room-temperature, resulting in amorphous films. These as-deposited films had a large optical band gap of 5.13 eV, and exhibited a lithium-ion conductivity of 3.35×10-7 S/cm. The films were then annealed, and the effect of annealing on the optical and electrical properties of the films was examined. After annealing at 1000 °C, the films were found to be cubic with a narrower band gap of 3.64 eV. In addition, these annealed films showed an inferior ionic conductivity than the as-deposited ones. INTRODUCTION Solid state micro-batteries have been under investigation for decades for usage in micro robotics (micro-electromechanical systems) and wireless sensors, etc [1]. A lot of good work has been done to enhance the overall performance of the micro batteries [2-5]. The main strategies have been aimed at improving the quality of the electrolyte and the compatibility between the electrolyte and electrode. A good electrolyte should have high ionic conductivity, large band gap, and wide electrolyte window that matches well with electrode work functions [6]. On the other hand, the electrolyte should have good chemical stability against moisture and be suitable for fabrication as a dense membrane [6, 7]. A newly developed electrolyte material of bulk Li7La3Zr2O12 (LLZO) was found to have not only high ionic conductivity (10-6 S/cm for tetragonal phase and 10-3 S/cm for cubic phase) but also good electrochemical stability against lithium (electrolyte window wider than 0 - 7 V) and excellent resistance to moisture [8, 9]. Since this garnet-type LLZO has shown nice performance as a solid electrolyte in the bulk form, it is expected to show satisfying properties as a thin film electrolyte. To the best of our knowledge, the LLZO electrolyte has only been synthesized as a bulk phase, with no research having been reported on LLZO thin films. Given that LLZO is a complex oxide material, the preservation of stoichiometry from the target to the film would be very hard by conventional evaporation techniques. Therefore, a pulsed laser deposition system was employed. Utilizing highly energetic laser pulses, the target surface absorbs over 1 GW power and as a result all the elements are ablated at the same rate despite their usually different partial pressures in the vapor phase; thus the stoichiometry of the deposited film is more likely to be maintained. Considering that thin film batteries are fabricated at low temperatures, we explored the pulsed laser deposition of LLZO films at room temperature. The as-deposited films were annealed ex-situ for achieving better crystallinity. In this work, we are reporting the pulsed laser deposition
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