Exploring Ionic Transport Mechanisms in Solid Conductors: A Dual Perspective on Static Structural Properties and Anion Dynamics

Solid Li-ion conductors require high ionic conductivity to ensure rapid Li⁺ transport within solid-state batteries, necessitating a thorough examination of the relationship between the structure and Li⁺ transport mechanisms. Factors such as crystal symmetries, anion electronegativity, and Li-anion bond lengths are critical in influencing the ionic conductivities of solid conductors. Furthermore, the relationship between Li⁺ transport and the dynamic behavior of anions, particularly through mechanisms such as the paddle-wheel effect, highlights the complexity of ionic transport in solid conductors. In this study, we focus on investigating the antiperovskite-type ionic conductor Li₂OHX (X = Cl or Br), which integrates various static structural features with dynamic anion behavior, to delve deeper into the structure–function relationship. Employing Rietveld refinement on neutron powder diffraction, maximum entropy method analysis, and ab initio molecular dynamics simulations, our findings reveal that Li⁺ transport is influenced not only by static structural properties like space groups, anion electronegativity, Li vacancies, and Li–O bond lengths but also, and more crucially, by the dynamics of OH^– anions. These insights highlight the pivotal role of anion dynamics and offer foundational guidelines for designing solid ionic conductors.