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Fast ion conductor
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Fast ion conductors, also known as solid electrolytes and superionic conductors, are solid state electrical conductors which conduct due to the movement of ions through voids (or empty crystallographic positions)in their crystal lattice. One component of the structure, cationic or anionic, 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.
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Encyclopedia
Fast ion conductors, also known as solid electrolytes and superionic conductors, are solid state electrical conductors which conduct due to the movement of ions through voids (or empty crystallographic positions)in their crystal lattice. One component of the structure, cationic or anionic, 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.
As a space-charge layer has nanometer thickness, the effect is directly related to nanoionics (nanoionics-I). Lehovecs effect has given a basis for creation of multitude nanostructured fast ion conductors for portable lithium batteries and fuel cells.
Fast ion conductors are intermediate in nature between crystalline solids (see crystal) which possess a regular structure with immobile ions, and liquid electrolytes which have no regular structure and entirely 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 Oi is arbitrary value but it should be greatly large than electronic one. Usually, the solids, where electronic conductivity Oe is arbitrary value but Oi is an order of 0.0001-0.1 Ohm-1 cm-1 (300 K), are called superionic conductors.
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Superionic conductors, where Oi 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 Oi > 0.25 Ohm-1 cm-1 and Oe ~10-9 Ohm-1 cm-1 at 300 K. The Hall (drift) ionic mobility in RbAg4I5 is about 2x10-4 cm2/(Vs) at room temperatures.
The Oe Oi systematic diagram distinguishing the different types of solid state ionic conductors is given on the figure
Fig. Classification of solid state ionic conductors by the lg Oe - lg Oi diagram (Ohm-1 cm-1).
2, 4 and 6 known solid electrolytes (SEs), materials
with Oi >> Oe;
1, 3, and 5 known mixed ion-electron conductors;
3 and 4 superionic conductors (SICs), i.e. materials
with Oi > 0.001 Ohm-1cm-1, Oe arbitrary value;
4 SIC and simultaneously SE, Oi > 0.001 Ohm-1cm-1,
Oi >>Oe;
5 and 6 advanced superionic conductors (AdSICs),
where Oi > 10-1 Ohm-1cm-1 (300 K),
energy activation Ei about 0.1 eV, Oe arbitrary value;
6 AdSIC and simultaneously SE, Oi > 10-1 Ohm-1cm-1,
Ei about 0.1 eV, Oi >>Oe;
7 and 8 hypothetical AdSIC with Ei kBT 0.03 eV (300 ?);
8 hypothetical AdSIC and simultaneously SE.
Examples
Examples of fast ion conductors include sodium chloride, beta-alumina solid electrolyte, beta-lead fluoride, zirconium dioxide and silver iodide.
- Inorganic materials:
- Sodium chloride
- Zirconium dioxide doped with calcium oxide and yttrium oxide, which is conductive for O2- ions and is used in oxygen sensors
- Beta-alumina solid electrolyte used as a membrane in several types of molten salt electrochemical cells
- Lanthanum(III) fluoride, conductive for F- ions, used in some ion selective electrodes
- Silver sulfide, conductive for Ag+ ions, used in some ion selective electrodes
- Silver iodide, conductive at higher temperatures
- Beta-lead fluoride, exhibits a continuous growth of conductivity on heating. This property was first discovered by M. Faraday
- Lead(II) chloride, conductive at higher temperatures
- Rubidium silver iodide, conductive at room temperature
- Some perovskite ceramics - strontium titanate, strontium stannate - conductive for O2- ions
- Zr(HPO4)2.nH2O - conductive for H+ ions
- UO2HPO4.4H2O - conductive for H+ ions
- Conductive ceramics - eg. NASICON (Na3Zr2Si2PO12), a sodium super-ionic conductor
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