Decoding the thermal conductivity of ionic covalent organic frameworks: Optical phonons as key determinants revealed by neuroevolution potential

Abstract

Ionic covalent organic frameworks (ICOFs) are a unique subclass of covalent organic frameworks (COFs) that combine the advantages of metal-organic frameworks (MOFs) and COFs through the integration of ionic and covalent bonds. Using ICOF-10n-Li/Na as examples, we trained a machine learning-based neuroevolution potential (NEP) function and conducted a comprehensive study of the thermal transport properties of ICOFs through large-scale molecular dynamics simulations. We found that the thermal conductivity perpendicular to the pore channels (x-direction) reaches a maximum of 4.04 ± 0.20 W m⁻¹ K⁻¹ at room temperature, primarily driven by high-frequency optical phonons (contributing ∼94 %). In contrast, the thermal conductivity along the pore channels (z-direction) is 0.74 ± 0.02 W m⁻¹ K⁻¹, dominated by low-frequency acoustic phonons (contributing ∼67 %). Further analysis reveals that linker types strongly influence phonon lifetimes of optical phonons in the x-direction, while interlayer ions significantly impact group velocities of acoustic phonons in the z-direction. This work highlights the critical role of optical phonons in determining the thermal behavior of ICOFs and provides deep insights into the influence of linkers and interlayer ions on thermal transport properties. The superior thermal conductivity (4.04 ± 0.20 W m⁻¹ K⁻¹) achieved in the x-direction underscores the unique synergistic effects of ionic and covalent bonding in ICOFs, making them highly promising for applications requiring efficient thermal management and molecular separation.