Fast Ion Conductor - Classification

Classification

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 an arbitrary value but it should be greatly larger 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 superionic conductors.

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. The Ω_e – Ω_i 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 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.