Single Na- and K-Ion-Conducting Sulfonated -NH-Linked Covalent Organic Frameworks

Highly ion-conductive solid electrolytes of nonlithium ions (sodium or potassium ions) are necessary for pursuing a more cost-effective and sustainable energy storage. Here, two classes of sulfonated -NH-linked covalent organic frameworks (COFs), specifically designed for sodium or potassium ion conduction (named i-COF-2 (Na or K) and i-COF-3 (Na or K)), were synthesized through a straightforward, one-step process using affordable starting materials. Remarkably, these COFs demonstrate high ionic conductivity at room temperature─3.17 × 10^-4 and 1.02 × 10^-4 S cm^-1 for i-COF-2 (Na) and i-COF-2 (K) and 2.75 × 10^-4 and 1.42 × 10^-4 S cm^-1 for i-COF-3 (Na) and i-COF-3 (K)─without the need for additional salt or solvent. This enhanced performance, including low activation energies of 0.21 eV for both i-COF-2 (Na) and i-COF-2 (K) and of 0.24 and 0.25 eV for i-COF-3 (Na) and i-COF-3 (K), is attributed to the strategic incorporation of sulfonate groups and the directional channels within the COF structure. The Na⁺ and K⁺ ion high conductivities, low cost, and intrinsic framework stability of i-COF-2 (Na or K) and i-COF-3 (Na or K) provide promising solid electrolyte candidates for the exploration of sustainable energy storage.