Ionic Covalent Organic Framework Membranes with Single Li-Ion Conduction

Two-dimensional covalent organic frameworks (2D-COFs) are chemically and thermally stable, and their ionic COFs have recently been reported as solid electrolytes that exhibit lithium-ion (Li-ion) conductivity due to their well-ordered one-dimensional ion channels. However, there are very few examples of high Li-ion conductivity (σr.t. > 10–5 S cm–1) achieved with pure 2D-COF, not combined with other materials (e.g., solvents, other electrolytes, and electrolyte salts). One concern is that composites with other materials may cause undesirable side reactions. In addition, Li-ions may conduct through other materials, leading to incorrect Li conductivity measurements of the 2D-COFs. This study developed a synthesis process for ionic COF (TpBD-(SO3Li)2) membranes, which are Li salts of a β-ketoenamine-type COF with a high density of sulfonic acid groups fixed in the framework. After the Li-ion exchange reaction, the system was rinsed with LiOH solution and washed with cold water to obtain TpBD-(SO3Li)2 in a stable fashion. The resulting TpBD-(SO3Li)2 membrane achieved a high Li-ion conductivity of 4.0 × 10–5 S cm–1 at 30 °C as well as a high (∼1) transference number. A Li-ion transference number near 1 signifies both the suitability of the membrane as a substitute for inorganic solid electrolytes and a clear distinction from polymer-based organic solid electrolytes. Therefore, this study provides guidelines for designing solvent-free and additive-free single Li-ion conductors based solely on COF materials.