Fast ionic conductors are a key component for solid-state batteries, offering the potential for improved safety and energy density. While significant progress has been made in discovering materials with high ionic conductivity, a comprehensive understanding of the underlying ion transport mechanisms remains not fully established.

Our work contributes by integrating materials synthesis, advanced characterization, and theoretical modeling. We investigate the relationship between lattice dynamics and ion transport by utilizing isotope substitution to selectively probe the role of specific phonon modes in garnet-type electrolytes. To refine the principles of materials design, we have explored the application of the Meyer–Neldel rule to provide a more comprehensive framework for optimizing ionic conductivity. These mechanistic studies guide our development of new materials, including UCl₃-type oxychloride solid electrolytes with nonclose-packed structures. In parallel, we contribute to the field’s methodological advancement by critically reviewing and assessing classical and emerging characterization techniques for studying ion transport.