Ionic Conductivity in Solids

Ionic conductivity, in the context of solids, refers to the passage of ions across a solid under the influence of an externally applied electric field. High ionic conductivity is vital to the operation of batteries and related devices. As outlined earlier (Section 2.4), in essence a battery consists of an electrode (the anode), where electrons are moved out of the cell, and an electrode (the cathode), where electrons move into the cell. The electrons are thus generated in the region of the anode and then move around an external circuit, carrying out a useful function, before entering the cathode. The anode and cathode are separated inside the battery by an electrolyte. The circuit is completed inside the battery by ions moving across an electrolyte. A key component in battery construction is an electrolyte that can support ionic conduction but not electronic conduction. 

Defects in Solids, by Richard J. D. Tilley Copyright 2008 John Wiley Sons, Inc. 

The output of a battery depends upon the concentrations and Gibbs energy of the components at the anode and cathode. Because of this, battery technology also underpins the operation of many sensors. 

For a battery to give a reasonable power output, the ionic conductivity of the electrolyte must be substantial. Historically, this was achieved by the use of liquid electrolytes. However, over the last quarter of a century there has been increasing emphasis on the production of batteries and related devices employing solid electrolytes. These are sturdy and ideal for applications where liquid electrolytes pose problems. The primary technical problem to overcome is that of achieving high ionic conductivity across the solid. 

There are three broad categories of materials that have been utilized in this endeavor. In the first, even in fully stoichiometric compounds, the ionic conductivity is high enough to be useful in devices because the cation or anion substructure is mobile and behaves rather like a liquid phase trapped in the solid matrix. A second group have structural features such as open channels that allow easy ion transport. In the third group the ionic conductivity is low and must be increased by the addition of defects, typically impurities. These defects are responsible for the enhancement of ionic transport. 

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In an ideal ionic crystal, all ions are held rigidly in the lattice sites, where they perform only thermal vibratory motion. Transfer of an ion between sites under the effect of electrostatic fields (migration) or concentration gradients (diffusion) is not possible in such a crystal. Initially, therefore, the phenomenon of ionic conduction in solid ionic crystals was not understood. 

The theoretical treatment of ionic conductivity in solids is very similar to that of diffusion, the main difference is the superimposition of the potential field upon the potential barrier to migration (Fig. 3). 

J.B. Goodenough, Fast ionic conduction in solids. Proc. Roy. Soc. (London), A393 (1984) 214-234. 

The ionic transport in solids is attributed to the hopping of ionic carriers between the equivalent positions in the crystal lattice. This mechanism is known as lattice diffusion and depends on the jumping distance and frequency of moved ions. The understanding of the influence of these factors on the ionic conductivity is very important for the development of material with enhanced ionic transport. The question of what is the limit of ionic conductivity in solids will be addressed by analyzing the ionic transport in cubic stabilized zirconia systems with different acceptor dopants. 

History of Ionic Conductivity in Solid Polymer Electrolytes... 

Certain crystallographic and glass stractrrres allow rather easy motion of iotrs, in both MIECs and SEs. For ionic conduction in solids to occtn, iotrs have to move through a rather dense matrix (whether crystalline or amorphous) consisting of ionic species of comparable size. To enable this, three conditiotrs must be fulfilled (a) an empty site exists in the forward direction, into which a corrducting ion can move (b) the propagation of the ion... 

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