High Frequency and Solid State NMR Techniques for the Study of Ionically Conductive Glasses
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HIGH FREQUENCY AND SOLID STATE NMR TECHNIQUES FOR THE STUDY OF IONICALLY CONDUCTIVE GLASSES
JOHN H. KENNEDY, ZHENGMING ZHANG and HELLMUT ECKERT Department of Chemistry, University of California, Santa Barbara,
CA 93106
ABSTRACT The synthesis, characterization, and electrochemical properties of sulfide-based lithium glasses are briefly reviewed. Of the many probes that have been used to study these materials two will be discussed in more detail, namel• solid state NMR and conductivity measurements at high frequency (10Z-109 Hz). Solid state NMR is particularly useful in understanding the network structure of these glasses and the role coformers play in modifying the glass network. Electrochemical measurements, including impedance at high frequency, give information concerning dynamic processes, and show that some of these glasses are potentially useful in miniature and thin-film solid state batteries.
INTRODUCTION One area of fast ion conductors (FIC) that has recently gained attention for batteries is lithium ion conducting glasses. For battery applications, lithium is an attractive anode material so that in the last 15 years considerable effort has been spent finding lithium-conducting FIC's. Li 3 N is probably the best crystalline Li+ conductor but is extremely anisotropic and also possesses a low decomposition potential. Anisotropic crystalline electrolytes are more difficult to interface to solid electrodes, leading to low exchange current densities. For this reason, much of recent solid electrolyte research has turned to non-crystalline materials, notably polymers and glasses. In searching for new solid electrolytes the basic conductivity equation must be kept in mind, namely
a -
nep
(1)
where n is the carrier concentration and A is the carrier mobility. The mobility, and sometimes the carrier concentration also (defect conductor), is thermally activated so that conductivity obeys an Arrhenius Law
a - aoe-Ea/kT
(2)
and is characterized by an activation energy, Ea. Defect conductors, in which a defect must be formed to create a carrier, typically exhibit large activation energies of 0.5 - 1.0 eV. On the other hand, compounds, such as RbAg 4 1 5 , that contain excess lattice sites have low activation energies of 0.1 - 0.2 eV. Glasses in which ions move through channels usually exhibit intermediate values for activation energy -- typically 0.3 - 0.5 eV. The goal, thus, is to find+glassy materials that have a high concentration of an ionic carrier, Li , and a channel for this carrier that has a low activation energy for mobility. Mat. Res. Soc. Symp. Proc. Vol. 210. 01991 Materials Research Society
612
Conductivity and activation energy may also be a function of frequency when ac signals are used for measuring impedance. Later, we will show how measurements at high frequencies give additional information concerning the conduction process and how activation energy at high frequency correlates better with activation energy determined from NMR than does activation energy measured at dc or relatively low
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