COF-10 as a Mesoporous Host for Solid-State Ionic Conduction of Lithium Iodide

Abstract

Covalent organic frameworks (COFs) provide structurally defined porous environments for solid-state ion transport, yet the extent to which a chemically neutral framework can modify the transport properties of an incorporated inorganic salt remains poorly understood. This study evaluates mesoporous boronate-ester COF-10 as a neutral host for lithium iodide (LiI) and examines the structural and electrochemical consequences of LiI incorporation. LiI incorporation preserves the covalent framework while strongly modifying its accessible porosity, decreasing the characteristic pore diameter from approximately 34.1 to 18.3 Å. The resulting COF-10@LiI composite exhibits an ionic conductivity of 1.44 × 10⁻⁶ S cm⁻¹ at 20 °C, which increases markedly with temperature and remains higher than that of LiI when measured under comparable conditions throughout the investigated temperature range. Potentiostatic polarisation at 100 °C yields an apparent lithium-ion transference number of approximately 0.63, indicating a substantial Li⁺ contribution while confirming mixed-ion transport. An oxidative current onset is observed near 2.9 V vs Li/Li⁺ under the measurement conditions, enabling evaluation with a low-voltage lithiated benzoquinone electrode, for which repeated lithium insertion and extraction are sustained over consecutive cycles despite significant polarisation. Together, these results show that incorporation of LiI into the mesoporous environment of a neutral COF modifies the transport behaviour of the salt and supports functional solid-state lithium transport. COF-10 therefore provides a structurally defined model platform for investigating how mesoporous confinement and host–guest interactions can regulate ionic transport in COF-based solid electrolytes.