Fast ion conductor

In materials science, fast ion conductors are solids with highly mobile ions. These materials are important in the area of solid-state ionics, and are also known as solid electrolytes and superionic conductors. These materials are useful in batteries and various sensors. Fast ion conductors are used primarily in solid 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 rigid crystal structure.

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.

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

Proton conductors are a special class of solid electrolytes, where hydrogen ions act as charge carriers.

Unlike conventional solid electrolytes and superionic conductors.

Superionic conductors where Ω_i is more than 0.1 Ohm⁻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 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 2×10⁻⁴ cm²/(V•s) at room temperatures. The Ω_e – Ω_i systematic diagram distinguishing the different types of solid-state ionic conductors is given in the figure.

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