Single Na and K ion-conducting sulfonated -NH- linked covalent organic frameworks

Highly ion-conductive solid electrolytes of non-lithium ions (sodium- or potassium- ions) are necessary for pursuing more cost-effective and sustainable energy storage. Here, new 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)), was 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 S cm-1 and 1.02 × 10-4 S cm-1 for i-COF-2 (Na) and i-COF-2 (K) and 2.75 × 10-4 S cm-1 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 eV 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 candidate for the exploration of sustainable energy storage.