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Title: Lithium ion conducting solid electrolytes
Author: Bruce, Peter G.
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 1981
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A weak electrolyte theory of ionic conduction in solid electrolytes, applicable to a wide range of both crystalline and amorphous ionic conductors, is presented. At low temperatures this theory predicts linear log δT against 1/T plots, the associated prefactors and activation energies varying considerably with composition, at high temperatures, downward curvature of the Arrhenius plots is expected. Anomalously large, composition dependent prefactors and high temperature curvature of Arrhenius plots have been observed in several solid electrolyte systems, including a lithium ion conducting solid solution, LISICON, discussed in this thesis. Their interpretation is not possible with traditional strong electrolyte theories. For the first time it is shown that the AC conductivity of (a) a single crystal solid electrolyte and (b) the intra- and intercrystalline regions of polycrystalline electrolytes, can be accurately analysed in terms of equivalent circuits containing frequency dependent elements of the form Y* = Awn + jBwn. Such elements are related to the co-operative migration of ions which is known to exist in solid electrolytes. This is in contrast to previous approaches which are based on the physically unrealistic model of isolated ion migration. The analysis is applied to single crystal Na β alumina and polycrystalline LISICON. In particular this approach can explain distorted impedance semicircles and broadened, separated spectroscopic M" and Z" peaks. The phase diagram of the Li₄GeO₄-Zn₂GeO₄ system is determined. It contains a lithium ion conducting solid solution, LISICON. It is shown that interstitial Li+ ions, rather than cation vacancies, give rise to high conductivities. The solid electrolyte properties and possible applications of the solid solutions are evaluated. Irreversible decreases in conductivity (ageing effects) occur on annealing, even at room temperature. A variety of minor polymorphic transitions occur on annealing γII solid solutions below ~ 300°C, their relationship to the conductivity has been determined. Contrary to previous results the intra- and intercrystalline conductivities of LISICON solid solutions have the same activation energy. The grain boundary effect in LISICON arises from constriction of the conduction pathways as a result of air gaps at grain boundaries. A model of a polycrystalline electrolyte with such grain boundary behaviour is presented.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available