Synthesis and Electrical Properties of Garnet-type Solid Oxide Electrolyte Thin Films from Solution Route
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Synthesis and Electrical Properties of Garnet-type Solid Oxide Electrolyte Thin Films from Solution Route Hirofumi Matsuda, Eiji Hosono, and Haoshen Zhou Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba 305-8568, Japan ABSTRACT Development of Al-stabilized Li7La3Zr2O12 (LLZO) fast Li ion conducting thin films was attempted by chemical solution deposition (CSD) method with the nominal composition of Li5.95Al0.35La3Zr2O12. The films were crystallized at 600ºC on Pt-coated Si substrates in almost single phase without a preferred orientation to any crystallographic axis. Subsequent thermal annealing at 760ºC improved packing of LLZO grains with the film thickness of 1.1μm. AC impedance measurements were conducted both with Li reversible and Au blocking microelectrodes deposited on the films annealed at 760ºC. Total Li ion conductivity σtotal comprised of bulk and grain boundary contribution was studied. The temperature dependence of σtotal was described with single thermal activation process and σtotal∼ 2x10-5 S/cm and activation energy of Ea=0.58eV were estimated. These values may be attributed to tetragonal modification of LLZO crystal. INTRODUCTION Li ion conducting solid electrolyte materials attract attentions from the viewpoint of possible application for all solid-state Li ion batteries with improved long-life and safety by substituting volatile and combustive organic liquid electrolyte. Among various ionic conductors, a class of oxide materials with garnet structure [1] is one of the favorable candidates with respect to the reported fast ionic conductivity up to 0.3mS/cm at room temperature in the composition of Li7La3Zr2O12 (LLZO), stability in ambient atmosphere and highly inert character to elemental Li [2]. The fast Li ion conductivity of LLZO is attributed to the relatively high site occupancy at 96h position where a Li ion is octahedrally coordinated by surrounding O [3]. This crystal, however, exhibits transition to low-temperature ordered phase with tetragonal symmetry at 450K having poor total conductivity of ∼ 10-6 S/cm and high occupancy of 24d position where Li is tetrahedrally coordinated by O [4]. Geiger et al. [5] have reported that Al substitution for Li stabilizes cubic LLZO and disordered phase is quenched down to the room temperature. Needless to say that the attempt to develop faster Li ion conducting materials is important, decreasing the length of ion conduction pathway separating positive and negative electrodes is also crucial to ensure all-solid Li ion batteries with low internal resistance. Sintered ceramic body of oxide materials, however, finds a difficulty in decreasing the thickness below several tens microns because of the brittle nature. Various film deposition techniques, in contrast, can achieve the thickness far below one micron [6]. To obtain optimal composition and low growth temperature of various oxide materials, chemical solution deposition (CSD) techniques are advantageous in controlling composi
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