Multivalent Ion Conduction in Inorganic Solids

Multivalent ion-based batteries are the subject of substantial research interest as next generation battery technologies due to the potential for superior performance at a lower cost. A key fundamental process to their operation is the conduction of the multivalent ion in the solid state. Solid state conduction impacts the charge storage processes at the electrode materials, conductivity and stability of metal–electrolyte interfaces, and conductivity of potential solid state electrolytes. However, multivalent ionic conduction in inorganic solids has struggled to keep pace with the monovalent analogues because of the challenges posed by the high charge density. In this perspective, we discuss why it is difficult to conduct multivalent ions by considering electronic and structural properties of materials. Using literature reports, we highlight the specific challenges that arise from the high charge density of multivalent ions and consider strategies to address them. We discuss charge screening by electrons and water, the geometry of conduction pathways, the polarizability of the anions, the coordination environment, and the structural flexibility. We also highlight the difficulty in characterizing the conductivity of these unconventional working ions and emphasize the need for new characterization techniques to advance the field. Ultimately, we provide suggestions for structural factors that are likely to facilitate MV ion diffusion.