In solid-state ionics, fast ion conductors, also known as solid electrolytes and superionic conductors, are materials that act as solid state ion conductors and are used primarily in solid oxide fuel cells. As solid electrolytes they conduct due to the movement of ions through voids, or empty crystallographic positions, in their crystal lattice structure. The most commonly used solid electrolyte is yttria-stabilized zirconia, YSZ. One component of the structure, the cation or anion, is essentially free to move throughout the structure, acting as charge carrier.
The important case of fast ionic conduction is one in a surface space-charge layer of ionic crystals. Such conduction was first predicted by Kurt Lehovec.[1] As a space-charge layer has nanometer thickness, the effect is directly related to nanoionics (nanoionics-I). Lehovec’s effect is used as a basis for developing nanomaterials for portable lithium batteries and fuel cells.
Fast ion conductors are intermediate in nature between crystalline solids which possess a regular structure with immobile ions, and liquid electrolytes which have no regular structure and fully mobile ions. Solid electrolytes find use in all solid state supercapacitors, batteries and fuel cells, and in various kinds of chemical sensors.
Proton conductors are a special class of solid electrolytes, where hydrogen ions act as charge carriers.
There is difference between solid electrolytes and superionic conductors. In solid electrolytes (glasses or crystals), the ionic conductivity Ωi is an arbitrary value but it should be greatly larger than electronic one. Usually, the solids where electronic conductivity Ωe is arbitrary value but Ωi is an order of 0.0001-0.1 Ohm−1 cm−1 (300 K) are called superionic conductors.
Superionic conductors, where Ωi is more than 0.1 Ohm−1 cm−1 (300 K) and activation energy for ion transport Ei is small (about 0.1 eV), are called by advanced superionic conductors. The famous example of advanced superionic conductor-solid electrolyte is RbAg4I5 where Ωi > 0.25 Ohm−1 cm−1 and Ωe ~10−9 Ohm−1 cm−1 at 300 K. The Hall (drift) ionic mobility in RbAg4I5 is about 2x10−4 cm2/(V•s) at room temperatures. [2] The Ωe – Ωi systematic diagram distinguishing the different types of solid state ionic conductors is given on the figure[3]
Fig. Classification of solid state ionic conductors by the lg Ωe - lg Ωi diagram (Ohm−1 cm−1).
2, 4 and 6 – known solid electrolytes (SEs), materials with Ωi >> Ωe;
1, 3, and 5 – known mixed ion-electron conductors;
3 and 4 – superionic conductors (SICs), i.e. materials with Ωi > 0.001 Ohm−1cm−1, Ωe – arbitrary value;
4 – SIC and simultaneously SE, Ωi > 0.001 Ohm−1cm−1, Ωi >>Ωe;
5 and 6 – advanced superionic conductors (AdSICs), where Ωi > 10−1 Ohm−1cm−1 (300 K), energy activation Ei about 0.1 eV, Ωe – arbitrary value;
6 – AdSIC and simultaneously SE, Ωi > 10−1 Ohm−1cm−1, Ei about 0.1 eV, Ωi >>Ωe;
7 and 8 – hypothetical AdSIC with Ei ≈ kBT ≈0.03 eV (300 К);
8 – hypothetical AdSIC and simultaneously SE.
Examples of fast ion conductors include beta-alumina solid electrolyte, beta-lead fluoride, zirconium dioxide, silver iodide.