Lithium-ion distribution and motion in two-dimensional covalent organic frameworks: the example of TAPB-PDA COF

Abstract

Two-dimensional covalent organic frameworks (2D COFs) are promising as solid-state electrolytes due to their high structural stability, lack of glass transition temperature, and molecular versatility. For more rational molecular and system design toward better ionic conduction in COFs, a detailed understanding of the ion distributions, ionic motions, and interactions among different species is needed. Achieving this goal requires insights at the molecular level, where computer simulations can be a powerful tool. However, efficient protocols that can model lithium-ion trajectories in COFs over long times are still to be explored, which are needed for the reliable consideration of Li⁺ motions as well as for the development of high-throughput molecular screening strategies for accelerated materials discovery. Here, we apply molecular dynamics simulations with electronic continuum correction to study lithium-ion distributions and motions in 2D COFs. The efficient force fields have allowed us to model trajectories over hundreds of nanoseconds and the consideration of experimentally relevant lithium loading. The imine-linked TAPB-PDA COF, which has become a model system in COF-related studies, is taken here as an example. LiClO₄ is used to provide Li⁺, and the role of propylene carbonate is studied. Our analysis provides a comprehensive understanding of the distributions and motions of lithium ions and anions. It was found that LiClO₄ tends to form clusters that attach to the walls of COF pores that limit ionic motions. The addition of propylene carbonate molecules helps the dispersion of Li⁺ in the COF pores for increased mobility. These results highlight the role of microscopic interactions between different components and point to potential optimization directions for higher ionic conductivity engineering of 2D COFs.