Which Interfaces Matter Most? A Non-monotonic Relationship between Grain Boundary Defect Chemistry and Ionic Conductivity in a Concentrated Solid Electrolyte

High-conductivity solid electrolytes are critical components of solid oxide fuel/electrolysis cells, solid-state batteries, information storage devices, and other electrochemical systems. Like many electrolytes, polycrystalline GdxCe1-xO2-δ (GCO) can suffer from low grain boundary (GB) conductivity relative to grain interiors, attributed to local nanoscale oxygen vacancy ("V" _"O" ^"••" ) depletion which diminishes cross-GB ionic conductivity. To improve conductivity, microscopic analyses of GB structure and chemistry along with multiscale computational models are needed to accurately relate nanoscopic point defect concentrations to macroscopic ionic conductivity. Here, we present an experimental-computational framework that predicts which GBs in a polycrystalline electrolyte likely facilitate ionic conductivity. We developed a thermodynamic phase-field modeling framework and applied it to a model high-solute-content oxygen electrolyte Gd0.25Ce0.75O2-δ, wherein the microscopically measured GB-to-GB variability in defect concentrations was used to predict the GB-to-GB variability in cross-GB ionic conduction. Uniquely, our model prioritizes reproducing microscopically observed GB defect distributions and considers defect-defect interactions in highly concentrated solid solutions, making the framework applicable to most technologically relevant solid electrolytes. Across the GBs studied, we revealed a non-monotonic relationship between "V" _"O" ^"••" depletion and GB conductivity, with the highest conductivity predicted for intermediate "V" _"O" ^"••" depletion amount.