Spuiter Deposition of Lithium Silicate-Lithium Phosphate Amorphous Electrolytes
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SPU'ITER DEPOSITION OF LITHIUM SILICATE - LITHIUM PHOSPHATE AMORPHOUS ELECTROLYTES N. J. DUDNEY, J. B. BATES, J. D. ROBERTSON* and C. F. LUCK Solid State Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6030 *Department of Chemistry, University of Kentucky, 800 Rose St. Lexington, KY 405060055 ABSTRACT Thin films of an amorphous lithium-conducting electrolyte were deposited by rf magnetron sputtering of ceramic targets containing Li4SiO 4 and Li 3PO 4 . The lithium content of the films was found to depend more strongly on the nature and composition of the targets than on many other sputtering parameters. For targets containing Li4 SiO 4 , most of the lithium was found to segregate away from the sputtered area of the target. Codeposition using two sputter sources achieves a high lithium content in a controlled and reproducible film growth. INTRODUCTION Lithium silicate - lithium phosphate glasses are promising materials for use as thin-film electrolytes in lithium cells. Amorphous films have been prepared using rf magnetron sputtering of ceramic targets containing Li3PO4 and Li4 SiO 4 in this and other laboratories [1-3]. Studies of the conductivity of these films have shown that a high lithium concentration is needed to minimize the film resistivity [4]. In this paper we demonstrate that the lithium content of the films is strongly dependent on the composition and homogeneity of the target used for sputtering. For targets containing Li4 SiO 4 , there is a tendency for much of the lithium to segregate radially on the surface of the target, rather than sputter from the target. This segregation results in lithium deficient films when compared to the target composition. To our knowledge, such pronounced segregation has not been reported in the literature for this or any other ceramic material. Codeposition with multiple sputter sources is a reliable way to compensate for the lithium deficiency. TARGET PREPARATION, SPUTTERING AND FILM ANALYSIS Most films were prepared using a planar magnetron source designed to accommodate cylindrical targets approximately 1 inch in diameter by 1/8th inch thick. Targets were prepared from either the pure compounds, Li 4 SiO 4 , Li3 PO 4 , and Li 2 0, or from mixtures of these compounds. Most mixtures were prepared by grinding the components together, other mixtures were ground and calcined at 800'C several times in order to prereact the silicate and phosphate compounds. The powders were then either cold pressed into pellets and sintered in air at 9001 100*C or hot-pressed at 5000 psi and 1000'C. Li20 targets were sintered at 7000 C in dry nitrogen. Typical target densities relative to the theoretical density were 75% when sintered and 96% when hot pressed. As expected, the conductivity of targets composed of mixed lithium orthosilicate and lithium orthophosphate is much higher than that of the pure compounds [5,6]. The conductivity of the target is taken as a good indication of the degree of solid solution formed by the silicate and phosphate phases and increa
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