Abstract
Superionic solid electrolytes have widespread use in energy devices, but the fundamental motivations for fast ion conduction are often elusive. In this Perspective, we draw upon atomistic simulations of a wide range of superionic conductors to illustrate some ways frustration can lower diffusion cation barriers in solids. Based on our studies of halides, oxides, sulfides and hydroborates and a survey of published reports, we classify three types of frustration that create competition between different local atomic preferences, thereby flattening the diffusive energy landscape. These include chemical frustration, which derives from competing factors in the anion–cation interaction; structural frustration, which arises from lattice arrangements that induce site distortion or prevent cation ordering; and dynamical frustration, which is associated with temporary fluctuations in the energy landscape due to anion reorientation or cation reconfiguration. For each class of frustration, we provide detailed simulation analyses of various materials to show how ion mobility is facilitated, resulting in stabilizing factors that are both entropic and enthalpic in origin. We propose the use of these categories as a general construct for classifying frustration in superionic conductors and discuss implications for future development of suitable descriptors and improvement strategies.This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.
Highlights
Superionic conductors are materials—typically ionic solids—that feature rapid ion conductivity approaching or even exceeding that of liquids
We further suggest that the categories of frustration presented here may be used as a universal framework for classifying superionic solids and emerging design strategies
We have drawn upon examples from molecular dynamics simulations of a wide variety of superionic conductors to illustrate chemical, structural and dynamical sources of frustration and their connection to rapid ion mobility
Summary
Superionic conductors are materials—typically ionic solids—that feature rapid ion conductivity approaching or even exceeding that of liquids. Even at the shortest distances, where the influence of the anion geometry becomes detectable, its symmetry is broadly compatible with the arrangement of occupied trigonal interstitial sites, minimizing the frustration and explaining the lack of fast cation mobility in this phase Results like those in figure 3 suggest it is possible to quantify the symmetry mismatch between the polyatomic anion and the lattice packing in order to provide a new descriptor for superionic conductivity in hydroborates and other systems with complex anions. The competition between the local and global environment preferences of the mobile species can have profound effects on the interstitial site occupancy of the cation sublattice, as can other factors such as ion size, temperature, stoichiometry and crystal structure In superionic materials, these factors can result in having multiple types of available sites occupied with similar probability and no clear preference (as in the flattened energy landscape of figure 1c) [23,57,58]. This avenue is ripe for further study to determine the best compositions for maximizing structural frustration
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