Exploring Pathways toward High Conductivity in Solid Composite Electrolytes: A Multiscale Perspective

Composite solid electrolytes (CSEs) aim to combine the high ionic conductivity of ceramics with the mechanical compliance of polymers. Yet, it remains unclear under which conditions the intrinsic conductivity of a ceramic phase can effectively enhance macroscopic ion transport. Here, we establish a general multiscale framework that provides a quantitative criterion for when ceramic-supported transport becomes operative. Molecular dynamics (MD) simulations characterize the structural and energetic features of the phase-exchange barrier (PEB) governing lithium transfer across polymer–ceramic interfaces. A mesoscale hopping model captures the competition between fast intraphase transport and rare interfacial exchange events and yields an analytical expression for a critical PEB rate. Only if the actual exchange rate exceeds this value can the ceramic phase increase the overall ionic conductivity. The critical rate depends on ceramic size, aspect ratio, orientation relative to the electric field (for nanowires), the intrinsic conductivity contrast, the effective PEB width, and ion mobility in the polymer phase. These dependencies identify aligned high-aspect-ratio fillers as a particularly effective strategy to lower the threshold. Applying the framework to the prototypical LLZO/PEO system indicates that the estimated PEB rate is of similar magnitude as the predicted threshold required for conductivity enhancement via the LLZO bulk phase. Moreover, the interphase-mediated transport remains negligible under the present conditions. By linking atomistic interface properties to mesoscale transport through an analytically tractable criterion, this work establishes a unified theoretical framework and quantitative design guidelines for composite solid electrolytes.