A Cost-Effect Na and K Ion-Conducting Amorphous Covalent Organic Framework with High Ion Conductivity

Solid-state batteries utilizing sodium or potassium ions (Na- or K-ion) have emerged as promising alternatives to costly lithium-ion (Li-ion) batteries, offering a safer, more cost-effective, and sustainable solution for energy storage. Nonetheless, the practical application of these batteries is hindered by a significant challenge: the low ionic conductivity of most SEs designed for Na- or K-ion. Addressing this challenge, researchers have turned to covalent organic frameworks (COFs), a category of porous polymers characterized by their highly ordered network structures. These structures can lower the energy barrier for ion migration, thereby offering rapid pathways for ions, making COFs promising candidates as ion conductors. Sulfonated cyanurate-linked covalent organic frameworks (COFs), specifically designed for sodium or potassium ion conduction (named i-COF-1 (Na) and i-COF-1 (K)), were synthesized through a straightforward, one-step process using affordable starting materials. Remarkably, these COFs demonstrate high ionic conductivity at room temperature─1.62 × 10–4 S cm–1 for i-COF-1 (Na) and 1.15 × 10–4 S cm–1 for i-COF-1 (K)─without the need for additional salt or solvent. This enhanced performance, including low activation energies of 0.221 eV for i-COF-1 (Na) and 0.273 eV for i-COF-1 (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-1 provide a promising SE candidate for the exploration of next generation of non-Lithium-ion secondary batteries.