Structure and Ionic Conductivity of Lithio-Borate Thin Films
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STRUCTURE AND IONIC CONDUCTIVITY OF L1THIO-BORATE THIN FILMS
P. DZWONKOWSKI, M. EDDRIEF, C. JULIEN and M. BALKANSKI Laboratoire de Physique des Solies, associ6 au CNRS Universit6 P. et M.Curie, 4 place Jussieu, 75252 Paris Cedex 05, France ABSTRACT Thin films of vitreous solid electrolyte in the boron-oxide/lithium oxide system have been grown using a thermal evaporation technique. Solid electrolyte films of composition B2 0 3-xLi2O with 0.5•x< 5 are obtained from a mixture of lithium metaborate and lithium oxide in good proportion. Structure and ionic conduction are studied as a function of the glass modifier concentration and process conditions. The structure is investigated by the mid-infrared absorption spectroscopy of films deposited on a silicon wafer. Comparison with bulk materials whose structures are known shows that the films have a similar structure, and exhibit the transformation of boroxol rings into triborate or di-triborate units as the Li 2O concentration increases. The ionic conductivity has been studied over wide frequency and temperature ranges using the complex impedance spectroscopy in a sandwiched geometry. The ionic conductivity increases with increasing Li content and exhibits a maximum value for B2 0 3 -3Li2O. INTRODUCTION Fast ion conducting glasses process properties which make them attractive materials for technological applications in high energy density, high performance batteries [1]. In addition to their good ionic conductivities, isotropic, and grain boundary free structure and ease of fabrication, they display wide compositional flexibility. We have explored this last attribute and investigated thin films of vitreous solid electrolyte in the boron-lithium oxide system grown using a thermal evaporation technique. Boron oxide glasses and boron sulphide glasses have been widely studied [2, 3]. In the system B O3-L'.iO-Li.2 SO4 the maximum value for ionic conductivity is 10.6 S/cm at room temperature. This value is not very high but for a 1 p±m film thickness the ohmic drop should be only few mV. This material is slightly hygroscopic so it can be handled in air without trouble. Recent papers have reported the elaboration and characterization of lithium conducting thin films glasses prepared from the binary boron-lithium oxide glassy material [4-7]. Advances have been made particularly in the engineering of lithium-glass solid electrolyte -layered compound cells and the feasibility of secondary galvanic generators has been demonstrated [8-9]. In this paper, we present the structural analysis and electrical conductivity of alkali-oxide electrolyte glass thin films in a wide range of composition in lithium oxide component. Using both the structural and electrical measurements, the correlation between composition and ionic conductivity is discussed. EXPERIMENTAL Alkali-boron oxide thin films have been prepared using a thermal evaporation process in a vacuum chamber with 1-10 mPa gas pressure conditions. Thin films of 500-1800 nm thick have been formed on silica glass substrate. The starti
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