3D Silsesquioxane Cage-Based Covalent Organic Frameworks Enabling Efficient Ion Transport in Quasi-Solid-State Lithium Metal Batteries

The resurgence of lithium metal batteries (LMBs) necessitates advancements in electrolyte engineering to regulate ion transport and manipulate interfacial characteristics. Noteworthy strategies encompass the development of high-efficiency lithium-ion conductors for quasi-solid-state composite electrolytes. In this context, two crystalline 3D COFs are presented that are thoughtfully designed by selecting decasilsesquioxane (T₁₀) cage building blocks and linear linkers to open up efficient ion-conducting pathways. The cage silsesquioxane-knotted COFs (CSQ-COFs) feature densely interconnected pore channels and a multimodal pore size distribution, which gives them the potential to function as ionic conductors. In addition, the dissociation of electrolyte salts by the silsesquioxane framework, along with the strong adsorption of anions, synergistically enhances ion transport. The coin cell assembled with CSQ-COF displays an ionic conductivity of 0.727 mS cm⁻¹ at 80 °C, an E_a of 0.12 eV, and t_Li+ of 0.83. Therefore, Li symmetrical cell demonstrates excellent Li plating/stripping behaviors for 600 h under 0.5 mA cm⁻². The Li/LiFePO₄ cell containing the CSQ-COF solid-state electrolyte delivers an initial discharge capacity of ≈159.6 mAh g⁻¹ at a rate of 0.5 C at room temperature with excellent capacity retention after 150 cycles. This work provides a novel insight on the development of 3D COF ionic conductors.