From the discovery of the first fast ionic conductor silver iodide in the early 20th century to the recent discovery of lithium fast ionic conductors with ionic conductivities surpassing those of liquid electrolytes, high ionic conductivity σ has often been associated with low activation energy Ea following the Arrhenius equation. However, the Meyer–Neldel rule (MNR) indicates that the Ea and prefactor σ0 are correlated, suggesting the relation between the Ea and σ is, in fact, complex. In this perspective, the use of the Meyer–Neldel–conductivity plot and a critical descriptor, Meyer–Neldel energy Δ0, to guide the search for fast ionic conductors is proposed. Reported lithium, sodium, and magnesium ionic conductors are categorized into three types, depending on the relative magnitude between the Δ0 and thermal energy (kBT) at the application temperature. The process by which σ can be optimized by tuning Ea for these types of ionic conductors is elaborated. This principle can be widely applied to all ionic conductors that obey the MNR at any application temperature. Furthermore, a pressure‐tuning approach to measure the Δ0 rapidly is developed. These findings establish a previously missing step for designing new ionic conductors with improved ionic conductivity.

Gao, Y., Li, N., Wu, Y., Yang, W., Bo, S.‐H., Rethinking the Design of Ionic Conductors Using Meyer–Neldel–Conductivity Plot. Adv. Energy Mater. 2021, 2100325. abstract

a) Prefactor versus activation energy plot. The dashed lines were obtained using Equation (4), corresponding to different ionic conductivities. These lines share the same slope, and the reciprocal of the slope equals the thermal energy at 300 K, that is, 26 meV. b–d) MNC plots of three types of materials with Δ0 > 26 meV for Li6MLa2TaO12 (b),[36] Δ0 < 26 meV for Li6+xP1−xSixS5Br (c),[37] and Δ0 ≈ 26 meV for Li6PS5Cl1−xBrx (d).[17] e) Conductivity versus activation energy plot of the selected materials in (b–d). The conductivities and prefactors were obtained from refs.