Li+ Conduction of Soft-Base Anion-Immobilized Covalent Organic Frameworks for All-Solid-State Lithium–Metal Batteries

Abstract

Organic solid-state electrolytes (SSEs) offer improved safety and flexibility, but they face challenges with low ionic conductivity at room temperature. Covalent organic frameworks (COFs) present a promising solution by preventing segmental motion and facilitating Li⁺ ion transfer through nanoporous channels with regularly aligned anionic groups. In particular, dissociating Li⁺ ions from these immobilized anionic groups is crucial for increasing Li⁺ ion conductivity. However, the design of COFs with electron-delocalized and soft bases, such as fluorinated sulfonimides anionic groups, for easier Li⁺ dissociation has been hindered by the challenging synthesis of these building blocks. Here, we successfully synthesized sulfonyl(trifluoromethanesulfonyl)imide (TFSI^–)-functionalized COFs and demonstrated a remarkable Li⁺ ion conductivity of 7.65 × 10^–5 S cm^–1 at 25 °C, which surpasses all known organic SSEs. This single Li⁺ ion conductor achieved over 200 times cyclability in Li and LiFePO₄ cells, representing a substantial step toward developing better organic SSEs.