Next‐generation thermal management requires the development of low lattice thermal conductivity materials, as observed in ionic conductors. For example, thermoelectric efficiency is increased when thermal conductivity is decreased. Detrimentally, high ionic conductivity leads to thermoelectric device degradation. Battery safety and design also require an understanding of thermal transport in ionic conductors. Ion mobility, structural complexity, and anharmonicity have been used to explain the thermal transport properties of ionic conductors. However, thermal and ionic transport are rarely discussed in direct comparison. Herein, the ionic conductivity of Ag+ argyrodites is found to change by orders of magnitude without altering the thermal conductivity. Thermal conductivity measurements and two‐channel lattice dynamics modeling reveal that the majority of Ag+ vibrations have a non‐propagating diffuson‐like character, similar to amorphous materials. It is found that high ionic mobility is not a requirement for diffuson‐mediated transport. Instead, the same bonding and structural traits that can lead to fast ionic conduction also lead to diffuson‐mediated transport. Bridging the fields of solid‐state ionics and thermal transport, it is proposed that a vibrational perspective can lead to new design strategies for functional ionic conducting materials. As a first step, the authors relate the so‐called Meyer–Neldel behavior in ionic conductors to phonon occupations.

<p>In recent years, ternary halides Li₃<i>MX</i>₆ (<i>M</i> = Y, Er, In; <i>X</i> = Cl, Br, I) have garnered attention as solid electrolytes due to their wide electrochemical stability window and favorable room-temperature conductivities. In this material class, the influences of iso- or aliovalent substitutions are so far rarely studied in-depth, despite this being a common tool for correlating structure and transport properties. In this work, we investigate the impact of Zr substitution on the structure and ionic conductivity of Li₃InCl₆ (Li_3-<i>x</i>In_1-<i>x</i>Zr<i>_x</i>Cl₆ with 0 ≤ <i>x</i> ≤ 0.5) using a combination of neutron diffraction, nuclear magnetic resonance and impedance spectroscopy. Analysis of high-resolution diffraction data shows the presence of an additional tetrahedrally coordinated lithium position together with cation site-disorder, both of which have not been reported previously for Li₃InCl₆. This Li⁺ position and cation disorder lead to the formation of a three-dimensional lithium ion diffusion channel, instead of the expected two-dimensional diffusion. Upon Zr⁴⁺ substitution, the structure exhibits non-uniform volume changes along with an increasing number of vacancies, all of which lead to an increasing ionic conductivity in this series of solid solutions.</p>

Ultra-low lattice thermal conductivity as often found in superionic compounds is greatly beneficial for thermoelectric performance, however, a high ionic conductivity can lead to device degradation. Conversely, high ionic conductivities are searched for materials in solid-state battery applications. It is commonly thought that ionic transport induces low thermal conductivity and that ion and thermal transport are not completely independent properties of a material. However, no direct comparison or underlying physical relationship has been shown between the two. Here we establish that ionic transport can be varied independent of thermal transport in Ag+ superionic conductors, even though both phenomena arise from atomic vibrations. Thermal conductivity measurements, in conjunction with two-channel lattice dynamics modeling, reveals that the vast majority of Ag+ vibrations have non-propagating diffuson-like character, which provides a rational for how these two transport properties can be independent. Our results provide conceptually novel lattice dynamical insights to ionic transport and confirm that ion transport is not a requirement for ultra-low thermal conductivity. Consequently, this work bridges the fields of solid state ionics and thermal transport, thus providing design strategies for functional ionic conducting materials from a vibrational perspective.

Ultra-low lattice thermal conductivity as often found in superionic compounds is greatly beneficial for thermoelectric performance, however, a high ionic conductivity can lead to device degradation. Conversely, high ionic conductivities are searched for materials in solid-state battery applications. It is commonly thought that ionic transport induces low thermal conductivity and that ion and thermal transport are not completely independent properties of a material. However, no direct comparison or underlying physical relationship has been shown between the two. Here we establish that ionic transport can be varied independent of thermal transport in Ag+ superionic conductors, even though both phenomena arise from atomic vibrations. Thermal conductivity measurements, in conjunction with two-channel lattice dynamics modeling, reveals that the vast majority of Ag+ vibrations have non-propagating diffuson-like character, which provides a rational for how these two transport properties can be independent. Our results provide conceptually novel lattice dynamical insights to ionic transport and confirm that ion transport is not a requirement for ultra-low thermal conductivity. Consequently, this work bridges the fields of solid state ionics and thermal transport, thus providing design strategies for functional ionic conducting materials from a vibrational perspective.

<p>In recent years, ternary halides Li₃<i>MX</i>₆ (<i>M</i> = Y, Er, In; <i>X</i> = Cl, Br, I) have garnered attention as solid electrolytes due to their wide electrochemical stability window and favorable room-temperature conductivities. In this material class, the influences of iso- or aliovalent substitutions are so far rarely studied in-depth, despite this being a common tool for correlating structure and transport properties. In this work, we investigate the impact of Zr substitution on the structure and ionic conductivity of Li₃InCl₆ (Li_3-<i>x</i>In_1-<i>x</i>Zr<i>_x</i>Cl₆ with 0 ≤ <i>x</i> ≤ 0.5) using a combination of neutron diffraction, nuclear magnetic resonance and impedance spectroscopy. Analysis of high-resolution diffraction data shows the presence of an additional tetrahedrally coordinated lithium position together with cation site-disorder, both of which have not been reported previously for Li₃InCl₆. This Li⁺ position and cation disorder lead to the formation of a three-dimensional lithium ion diffusion channel, instead of the expected two-dimensional diffusion. Upon Zr⁴⁺ substitution, the structure exhibits non-uniform volume changes along with an increasing number of vacancies, all of which lead to an increasing ionic conductivity in this series of solid solutions.</p>