Fast-ion conductor

Proton conductors are a special class of solid electrolytes, where hydrogen ions act as charge carriers. One notable example is superionic water.

Superionic conductors where σ_i is more than 0.1 Ω⁻¹ cm⁻¹ (300 K) and the activation energy for ion transport E_i is small (about 0.1 eV), are called advanced superionic conductors. The most famous example of advanced superionic conductor-solid electrolyte is RbAg₄I₅ where σ_i > 0.25 Ω⁻¹ cm⁻¹ and σ_e ~10⁻⁹ Ω⁻¹ cm⁻¹ at 300 K.[1][2] The Hall (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 transport E_i < k_BT (300 К) had been discovered in 2006.[6]

A common solid electrolyte is yttria-stabilized zirconia, YSZ. This material is prepared by doping Y₂O₃ into ZrO₂. 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 with calcium oxide to give an oxide conductor that is used in oxygen sensors in automobile controls. Upon doping only a few percent, the diffusion constant of oxide increases by a factor of ~1000.[7]

Other conductive ceramics function as ion conductors. One example is NASICON, (Na₃Zr₂Si₂PO₁₂), a sodium super-ionic conductor

Another example of a popular fast ion conductor is beta-alumina solid electrolyte.[8] Unlike the usual forms 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 the sodium–sulfur battery.

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

A textbook example of a fast ion conductor is silver 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 for copper(I) iodide (CuI), rubidium silver iodide (RbAg₄I₅),[9] and Ag₂HgI₄.

• Silver sulfide, conductive for Ag⁺ ions, used in some ion selective electrodes
• Lead(II) chloride, conductive at higher temperatures
• Some perovskite ceramics – strontium titanate, strontium stannate – conductive for O²⁻ ions
• ${\displaystyle {\ce {Zr(HPO4)2.{\mathit {n}}H2O}}}$ – conductive for H⁺ ions
• ${\displaystyle {\ce {UO2HPO4.4H2O}}}$ (hydrogen uranyl phosphate tetrahydrate) – conductive for H⁺ ions
• Cerium(IV) oxide – conductive for O²⁻ ions

• Many gels, such polyacrylamides, agar, etc. are fast ion conductors[10][11]
• A salt dissolved in a polymer – e.g. lithium perchlorate in polyethylene oxide[12]
• Polyelectrolytes and Ionomers – e.g. Nafion, a H⁺ conductor