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. Such conduction was first predicted by Kurt Lehovec (www.kurtlehovec.com) in the paper “Space-charge layer and distribution of lattice defects at the surface of ionic crystals” ( J. Chem. Phys. 1953. V.21. P.1123 -1128). As a space-charge layer has nanometer thickness, the effect is directly related to nanoionics (nanoionics-I). The Lehovec’s effect had 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 Ω_i is arbitrary value but it should be greatly large than electronic one. Usually, the solids, where electronic conductivity Ω_e is arbitrary value but Ω_i is an order of 0.0001-0.1 Ohm^-1 cm^-1 (300 K), are called by superionic conductors.

Fig. Classification of solid state ionic conductors by the lg (electronic conductivity, Ω_e) - lg (ionic conductivity, Ω_i) diagram

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Superionic conductors, where Ω_i is more than 0.1 Ohm^-1 cm^-1 (300 K) and activation energy for ion transport E_i is small (about 0.1 eV), are called by advanced superionic conductors. The famous example of advanced superionic conductor-solid electrolyte is RbAg4I5 where Ω_i > 0.25 Ohm^-1 cm^-1 and Ω_e ~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 (Stuhrmann C.H.J., Kreiterling H., Funke K. Ionic Hall effect measured in rubidium silver iodide // Solid State Ionics 2002. V.154-155. P.109-112.).

The Ω_e – Ω_i systematic diagram distinguishing the different types of solid state ionic conductors is given on the figure^[1]

Fig. Classification of solid state ionic conductors by the lg Ω_e - lg Ω_i diagram (Ohm^-1 cm^-1).

2, 4 and 6 – known solid electrolytes (SEs), materials with Ω_i >> Ω_e;

1, 3, and 5 – known mixed ion-electron conductors;

3 and 4 – superionic conductors (SICs), i.e. materials with Ω_i > 0.001 Ohm^-1cm^-1, Ω_e – arbitrary value;

4 – SIC and simultaneously SE, Ω_i > 0.001 Ohm^-1cm^-1, Ω_i >>Ω_e;

5 and 6 – advanced superionic conductors (AdSICs), where Ω_i > 10^-1 Ohm^-1cm^-1 (300 K), energy activation E_i about 0.1 eV, Ω_e – arbitrary value;

6 – AdSIC and simultaneously SE, Ω_i > 10^-1 Ohm^-1cm^-1, Ei about 0.1 eV, Ω_i >>Ω_e;

7 and 8 – hypothetical AdSIC with Ei ≈ k_BT ≈0.03 eV (300 К);

8 – hypothetical AdSIC and simultaneously SE.

[edit] 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 O^2- 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 O^2- 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

• Organic materials:
  • Gels - polyacrylamides, agar - polymer holds a solution of ion electrolyte
  • A salt dissolved in a polymer - eg. lithium perchlorate in polyethylene oxide
  • Polyelectrolytes
  • Ionomers - eg. Nafion, a H⁺ conductor

[edit] References