ionic transport in amorphous solid electrolytes

Rev. Mater. Sci. 1981. 1l:211-31 Copyright © 1981 by Annual Reviews Inc. All rights reserved

Ann.

IONIC TRANSPORT IN

+8663

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AMORPHOUS SOLID ELECTROLYTES J. L. Souquet

Laboratoire d'Energetique Electrochirnique, ENSEEG, Saint-Martin-d'Heres, France

BP 44, 38 401

INTRODUCTION

Until recently, electrochemistry has been mainly involved with liquid electrolytes, traditionally molten and dissolved electrolytes. Because their domain of electroactivity is often too narrow and their selectivity is poor with respect to the nature of the charge carrier, these electrolytes have proved unsuitable for energy storage in high density energy storage batteries. Consequently, in recent years a great deal of research has been devoted to another type of electrolyte: crystallized solid state electrolytes. Roughly, there are two types of solid electrolytes. First there are electrolytes conducting through point defects. These include alkali and alkaline earth halides and some oxides. They have a compact crystalline structure, and ion displacement is only possible because of point defects such as vacancies or interstitial ions. These conductors present an intrinsic conduction, associated with the con­ centration of defects of thermal origin, Frenkel or Schottky disorder, and an extrinsic conduction associated with the concentration of point defects in the ionic or cationic sublattice created by impurities. Intrinsic and extrinsic conduction are both characterized by an activation energy. The intrinsic conduction activation energy is always high since it is the sum of two terms-one representing the energy needed to form a point defect, and the second the migration energy of this defect. In the extrinsic domain, at lower temperature, the activation energy is lower since it reflects only the migration energy. The value I eV often constitutes a boundary between the two types of conduction. Secondly, there are solid state electrolytes that conduct through dis­ order of the sublattice of one of the ions. They include electrolytes such 0084-6600/81/0801-0211$01.00

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as £lAgI and /3AI203. One ionic species ensures the rigidity of the crystal lattice, whereas the sublattice of the mobile species presents many available energetically equivalent sites separated by weak potential bar­ riers. In the two kinds of electrolytes, defects in the structure or the lack of rigidity in one of the sublattices give rise to ionic conduction. So it is natural to expect that more disordered solids, noncrystalline solids, will present a significant and selective ionic conduction. Oxide glasses are by far the most studied among amorphous electrolyte materials. In this type of glass all the oxygen atoms are covalently bound to the cations of the "forming" oxides in elementary units (Si04, B04, P04 tetrahedra, B03 triangles, etc). The macromolecules are formed by an assembly of these units in which at least one atom of oxygen, called a bridging atom, is shared. Some oxygen atoms that are not bridging are negatively charged and maintain in their vicinity alkali cations or alkaline earth cations of the network-modifier oxides. In as much as the anions are fixed to the macromolecular chains by solid covalent bonds, only alkali cations move in an electric field. More complex glasses have been synthesized recently. They are ob­ tained by dissolving mineral salts, mainly halogenated alkali salts, in an oxide-base glass in order to increase the number of ions that can migrate in the macromolecular structure. More recently, the inorganic macro­ molecul�r structure described here has been replaced by organic polymer chains in which mineral or organic mineral salts are dissolved. None of these materials is in thermodynamic equilibrium: the rearrangement of the macromolecular chains into crystals is blocked at the habitual tem­ perature of use. GENERAL CHARACTERlSTICS OF AMORPHOUS

ELECTROLYTES Ionic Transport

As they move in the disordered lattice, the mobile ions interact with the macromolecular chains whose movements are temperature dependent. For instance, on Figure I an Arrhenius plot gives the variations of conductivity of sodium silicate over a temperature range of 50-1100°C. Over this temperature range, measurements of the transport number show that the current is carried solely by the alkali cations. At very high temperatures, the silicate is liquid; its conductivity decreases with a decrease in temperature. This process is accelerated for temperatures below r::, at which temperature the crystallized phase,

IONIC TRANSPORT

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400

, a=Q;exp-

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Annu. Rev. Mater. Sci. 1981.11:211-231. Downloaded from www.annualreviews.org Access provided by CAPES on 07/21/16. For personal use only.

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Figure 1

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Electrical conductivity of a 26.5 wt% Na20 , 73.5% Si02 silicate in the liquid state

(T> Tc)' supercooled liquid state (Tc > T> Tg), and glassy state. Adapted from Doremus

(I). Circles are from Seddon et at (3) and triangles are from Babcock (3).

which is thermodynamicaly stable, should be formed. For this composi­ tion, the macromolecular chains are long and the crystals cannot form if the cooling rate is sufficiently high. In the example given, this rate must be higher than 30°C/min (4). The rapid decrease in conductivity below � stabilizes at a temperature Tg, around 500°C and below this tempera­ ture the activation energy remains constant. For temperatures below Tg, conductivity follows a simple law of the type a = aD exp( - E/RT) with in this case a value of 0.7 eV for E. On the

214

SOUQUET

other hand, above 500°C, an equation of the type a = ao exp[ E' jR(T­ To)] correctly expresses experimental results. In this example, parameters E' and To are 0.23 eV and 162°C respectively. To, a temperature somewhat below �, is the temperature separating the two conduction domains. To is called the ideal vitreous transformation temperature and can be interpreted microscopically as follows: the macromolecular chains of silicate are made up of links, in our example Si04 tetrahedra, that can move somewhat around their bonding point, provided the energy kTo is at least equal to the potential barriers restricting this movement. Between To and Tg these movements are very unlikely; above Tg they are many and allow cooperative movements of chain segments. The system can then adopt a great number of structural configurations. Macroscopically this results in a significant increase in the configura­ tional entropy when the temperature exceeds �, and, at the same time, a notable increase in the heat capacity and expansion coefficient is ob­ served. Tg is then called the experimental vitreous transition temperature. The equation for the variation in conductivity above T has been g justified in two different ways (5, 6). Both justifications are based on a statistical theory of local movements, favoring ionic migration, that are only possible when the number of structural configurations is sufficiently high. Whatever the theory, the equation in exp[ EjR(T- To)] relies on limited expansions and its use is not justified other than for T