Inmaterials science,fast ion conductors aresolidconductors with highly mobileions. These materials are important in the area ofsolid state ionics, and are also known assolid electrolytes andsuperionic conductors. These materials are useful in batteries and various sensors. Fast ion conductors are used primarily insolid oxide fuel cells. As solid electrolytes they allow the movement of ions without the need for a liquid or soft membrane separating the electrodes. The phenomenon relies on the hopping of ions through an otherwise rigidcrystal structure.

Fast ion conductors are intermediate in nature betweencrystalline solids which possess a regular structure with immobile ions, and liquidelectrolytes which have no regular structure and fully mobile ions. Solid electrolytes find use in all solid-statesupercapacitors,batteries, andfuel cells, and in various kinds ofchemical sensors.

In solid electrolytes (glasses or crystals), the ionic conductivity σ_i can be any value, but it should be much larger than the electronic one. Usually, solids where σ_i is on the order of 0.0001 to 0.1 Ω⁻¹ cm⁻¹ (300 K) are called superionic conductors.

Proton conductors are a special class of solid electrolytes, wherehydrogen ions act as charge carriers. One notable example issuperionic water.

Superionic conductors where σ_i is more than 0.1 Ω⁻¹ cm⁻¹ (300 K) and the activation energy for ion transportE_i is small (about 0.1 eV), are calledadvanced superionic conductors. The most famous example of advanced superionic conductor-solid electrolyte isRbAg₄I₅ where σ_i > 0.25 Ω⁻¹ cm⁻¹ and σ_e ~10⁻⁹ Ω⁻¹ cm⁻¹ at 300 K.^[1][2] TheHall (drift) ionic mobility in RbAg₄I₅ is about 2×10⁻⁴ cm²/(V•s) at room temperatures.^[3] The σ_e – σ_i systematic diagram distinguishing the different types of solid-state ionic conductors is given in the figure.^[4][5]

No clear examples have been described as yet, of fast ion conductors in the hypothetical advanced superionic conductors class (areas 7 and 8 in the classification plot). However, in crystal structure of several superionic conductors, e.g. in the minerals of the pearceite-polybasite group, the large structural fragments with activation energy of ion transportE_i <k_BT (300 К) had been discovered in 2006.^[6]

A common solid electrolyte isyttria-stabilized zirconia, YSZ. This material is prepared bydoping Y₂O₃ intoZrO₂. Oxide ions typically migrate only slowly in solid Y₂O₃ and in ZrO₂, but in YSZ, the conductivity of oxide increases dramatically. These materials are used to allow oxygen to move through the solid in certain kinds of fuel cells. Zirconium dioxide can also be doped withcalcium oxide to give an oxide conductor that is used inoxygen sensors in automobile controls. Upon doping only a few percent, the diffusion constant of oxide increases by a factor of ~1000.^[7]

Other conductiveceramics function as ion conductors. One example isNASICON, (Na₃Zr₂Si₂PO₁₂), a sodium super-ionic conductor

Another example of a popular fast ion conductor isbeta-alumina solid electrolyte.^[8] Unlike the usualforms of alumina, this modification has a layered structure with open galleries separated by pillars. Sodium ions (Na⁺) migrate through this material readily since the oxide framework provides an ionophilic, non-reducible medium. This material is considered as the sodium ion conductor for thesodium–sulfur battery.

Lanthanum trifluoride (LaF₃) is conductive for F⁻ ions, used in someion selective electrodes.Beta-lead fluoride exhibits a continuous growth of conductivity on heating. This property was first discovered byMichael Faraday.

A textbook example of a fast ion conductor issilver iodide (AgI). Upon heating the solid to 146 °C, this material adopts the alpha-polymorph. In this form, the iodide ions form a rigid cubic framework, and the Ag+ centers are molten. The electrical conductivity of the solid increases by 4000x. Similar behavior is observed forcopper(I) iodide (CuI),rubidium silver iodide (RbAg₄I₅),^[9] and Ag₂HgI₄.

• Silver sulfide, conductive for Ag⁺ ions, used in someion selective electrodes
• Lead(II) chloride, conductive for Cl⁻ ions at higher temperatures^[10]
• Someperovskite ceramics –strontium titanate,strontium stannate – conductive for O²⁻ ions
• ${\displaystyle \text{Zr}{({\text{HPO}}_4^{})}_2^{}\cdot \mathit{\text{n}}{\text{H}}_2^{}\text{O}}$ – conductive for H⁺ ions
• ${\displaystyle {\text{UO}}_2^{}{\text{HPO}}_4^{}\cdot 4{\text{H}}_2^{}\text{O}}$ (hydrogen uranyl phosphate tetrahydrate) – conductive for H⁺ ions
• Cerium(IV) oxide – conductive for O²⁻ ions

• Manygels, suchpolyacrylamides,agar, etc. are fast ion conductors^[11][12]
• A salt dissolved in a polymer – e.g.lithium perchlorate inpolyethylene oxide^[13]
• Polyelectrolytes andIonomers – e.g.Nafion, aH⁺ conductor

The important case of fast ionic conduction is one in a surface space-charge layer of ionic crystals. Such conduction was first predicted byKurt Lehovec.^[14]As a space-charge layer has nanometer thickness, the effect is directly related tonanoionics (nanoionics-I). Lehovec's effect is used as a basis for developingnanomaterials for portable lithium batteries and fuel cells.

• Mixed conductor

• Electric and magnetic fields in matter
• Electrochemical concepts

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