Equilibration of ion distribution at polymer/ceramic interfaces

Composite solid electrolytes (CSEs), involving ceramic and polymer electrolytes, are a promising candidate for use in solid-state batteries. These non-homogeneous electrolyte systems have altered ion distributions compared to their respective bulk phases. Due to high energy barriers for ion exchange between phases, achieving interfacial ion equilibration can be an issue when classical molecular dynamics (MD) techniques are employed at temperatures relevant for battery applications (e.g. 300-360 K), as the accessible timescales are limited to <μs. In this work, we present MD simulations at elevated temperatures (400-700 K) to reach equilibrium ion distributions in CSEs and evaluate whether these high-temperature simulations can also provide insight into the structural and dynamic properties at lower temperatures. Specifically, we selected Li₇La₃Zr₂O₁₂ (LLZO) as the ceramic and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in polyethylene oxide (PEO) as the polymer electrolyte. We demonstrate that, at temperatures equal to or above 600 K, the interfacial structures reach a state of equilibrium that is essentially independent of temperature, involving a significant Li⁺ transfer to the ceramics relative to the bulk phase. Below 600 K, the Li⁺ distribution is not equilibrated. Furthermore, even below 600 K, the TFSI^- distribution within the polymer phase follows the Li⁺ distribution in the two phases. This results in apparent equilibrium ion distributions at temperatures below 600 K. From the equilibrium Li⁺ free energy curve at 700 K, together with the transition rates at 600 K and 700 K, we can estimate the free energy barrier for Li⁺ entering the LLZO phase at lower temperatures, and thus predict the expected hopping rates at experimentally relevant temperatures. The results suggest that simulations at 400 K or lower require initial high-temperature simulations to achieve an appropriate lithium partitioning. The resulting structure can then be used for simulations at lower temperatures.