Particle‐Size‐Dependent Lithium‐Ion Transport in PEO/LLZO Composite Electrolytes

Lithium‐metal batteries with solid electrolytes can deliver higher energy density and improved safety than conventional Li‐ion batteries. Among solid electrolyte candidates, polymer/ceramic composite electrolytes are attractive because they combine polymer flexibility with the high ionic conductivity of ceramics. However, whether ceramic fillers synergistically reduce polarization losses in the polymer matrix remains unclear. A central unknown is the critical polymer/ceramic interfacial resistance ( R int,crit ), below which adding ceramics lowers electrolyte overpotential. Here, we present the first macroscale model framework to quantify R int,crit for composite electrolytes based on polyethylene oxide (PEO) and Ta‐doped Li 7 La 3 Zr 2 O 12 (LLZO). A 1D model for DC‐polarization of tri‐layer cells (PEO‐LiTFSI/LLZO/PEO‐LiTFSI) shows that LLZO surface functionalization reduces the PEO/LLZO interfacial resistance, consistent with electrochemical impedance measurements. Extending to a 2D composite model, we show notably that R int,crit scales linearly with LLZO particle diameter and shifts toward experimentally accessible values (e.g., 28.8 Ωcm 2 ) as particle size increases. At fixed ceramic volume fraction, larger LLZO particles reduce the number of interfacial crossings, driving more current through the ceramic phase and lowering concentration polarization. In contrast, R int,crit is largely independent of ceramic volume fraction. These results demonstrate that ceramic filler‐size engineering can enable synergistic, energy‐efficient transport in polymer/ceramic composite electrolytes.