Consequently, the vibrational character is likely important to understand transport in other ionic conductors, with or without ultralow thermal conductivity. Outlook on engineering transport in ionic conductors. As we have shown, the magnitude of ionic conductivity can be changed independently without altering the thermal transport properties that are governed by diffusons. Therefore, strategies to lower the ionic conductivity, and consequently improve the long-term stability in thermoelectric devices, are not expected to have detrimental effects on the desirable low lattice thermal conductivities. This profound insight opens the possibility to tailor superionic conductors and expand their usage in commercial thermoelectric devices by using the known design principles in superionic conductors. In addition, our results lead to an important conclusion for the research of solid ionic conductors that are becoming increasingly relevant for the performance optimization of all solid-state batteries. Here, the Meyer-Neldel behavior is a bottleneck in achieving higher ionic conductivities since a lowering of the activation barrier has detrimental effects on the conductivity pre-factor. With that, reducing the slope of the Meyer-Neldel plot becomes a main goal. By deriving the multi-excitation entropy using phonon-fluctuation considerations, we show that the Meyer-Neldel slope can be related to the prominent vibrational modes of ionic conduction and a characteristic number of phonons found to be in the range of equilibrium phonon occupations (Supplemental Note 6). This leads to a novel concept and design principle in solid-electrolyte research: determining and manipulating phonon occupations (e.g. targeted phonon excitation , through opto-electronic stimulation ) in conjunction with tailoring the site-specific vibrational frequencies and activation energies.