Probing polymer effects on interfacial Li-ion transport in hybrid solid electrolytes: LLZO-PCL compared with LLZO-PEO

Hybrid polymer-ceramic electrolytes are promising candidates for solid-state lithium-ion batteries, making it essential to identify the key factors governing interfacial ion transport. Atomistic molecular dynamics simulations at elevated temperatures are used to investigate differences in Li⁺ transport between PCL- and PEO-based polymer-ceramic interfaces. The investigated interface is built of Li₇La₃Zr₂O₁₂ (LLZO) ceramic and poly(ε-caprolactone) (PCL) polymer doped with bis(trifluoromethanesulfonyl)imide (LiTFSI) (LLZO-PCL), and a direct comparison is established with an analogous interface composed of LiTFSI-doped poly(ethylene oxide) (PEO) and LLZO (LLZO-PEO). The LLZO-PCL interface exhibits pronounced interfacial structuring, including strong adsorption of hydroxyl oxygen atoms at the ceramic surface. Li⁺ transfer across the interface is quantified from transition statistics and analyzed using free-energy profiles. Remarkably, LLZO-PCL exhibits an approximately 35-fold larger phase-exchange barrier (PEB) rate than LLZO-PEO, primarily due to a significantly lower PEB height for polymer-ceramic transfer. Analysis within the framework of overdamped Kramers theory indicates strongly damped interfacial dynamics and suggests spatially heterogeneous friction along the reaction coordinate. Overall, the results demonstrate that the substantially different interfacial Li⁺ transfer in PCL and PEO is mainly associated with differences in the PEB height. Our previously developed multiscale framework is used to evaluate which of the two polymer systems is more likely to enhance the overall conductivity in hybrid solid electrolytes by facilitating Li⁺ transport through the fast ceramic phase. Finally, effective Li-polymer coordination strength and polymer adhesion at the ceramic surface emerge as key factors governing interfacial ion transport and the efficient utilization of ceramic transport pathways in hybrid solid electrolytes.