Effect of halide ions on Li + ionic conductivity in oxyhalo-borate glasses
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I. INTRODUCTION Because of their fast ionic conductivity with potential for application to an energy storage battery system, the electrical properties have been studied for lithium oxyhalide glasses in the systems Li 2 O-B 2 O 3 -Li 2 X 2 (X = F, Cl, Br, I) '"5 as well as for thin films containing Lil or Li2SO4 prepared by vacuum evaporation and rf sputtering techniques.6 Although the halide ions in those glasses do not take part in the charge transport directly,7 they must affect the mobility of the L i + ion by changing its environment or interacting with it. It is intended in this study to clarify the effect of X ~ on the electrical conduction due to L i + for the lithium oxyhalide glasses in the systems B2O3-Li2O-Li2X, (X = F, Cl, Br) as a function of the identity and content of X ~. However, the electrical conductivity is governed by the concentration and mobility of the charge carrier. Therefore, in order to elucidate the effect of X ~ ions, the L i + ion content has been fixed by replacing Li 2 Z 2 for Li2O. Then the variation in the activation energy has been discussed in terms of the classical Anderson-Stuart theory in relation to the behavior of each halide ion.
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II. EXPERIMENTAL A series of glasses of the composition 70B2O3 • (30-x)Li2O-xLi2X2 (X = F, Cl, Br), later denoted as X glasses, was prepared from reagent grade B2O3, LiX, and Li 2 CO 3 . The batches giving about 20 g glass were fully mixed in an alumina mortar and melted in a covered platinum crucible at 1000 "C for 10-20 min in air. Fuming was observed during melting for higher \AX content. The melts were poured onto a stainless steel plate, pressed into the plate by an iron block, and immediately annealed at 300 °C for 30 min. The homogeneity of the glasses was confirmed by x-ray mapping. The ana606
J. Mater. Res. 2 (5), Sep/Oct 1987
lyzed content of B 2 O 3 and Li was in good agreement with the batch content in experimental error, whereas that of halogen experienced serious vaporization loss. The analyzed compositions are used in the following discussion. Both surfaces of the glass disks were parallel-polished with kerosene and fine emery paper. The polished disks that were thinner than 0.5 mm were used for the conductivity measurement. Both surfaces of the specimen were coated with silver paste as the electrodes. The resistance and reactance were measured by using a Hewlett-Packard 4192A LCR meter over the temperature range from 100 to 300 °C under vacuum in the frequency range from about 10 Hz to 10 MHz. A typical Cole-Cole plot is indicated in Fig. 1. The semicircle for the higher frequencies ( > 2 kHz) arises from the intrinsic property, while the semicircle for the lower frequencies ( < 2 kHz) is due to the polarization at the interface between the blocking electrodes and glass sample.8 The bulk resistivity has been obtained from the intersection
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