Single-Crystalline Borate Covalent Organic Frameworks for Solid-State Lithium Metal Batteries

To advance lithium metal batteries, novel solid-state electrolytes are crucial. Covalent Organic Frameworks (COFs) are promising due to their crystalline, porous structure, light composition, strong bonds, high surface area, and stability. COFs can be engineered for enhanced ion conduction, with their porosity and ion-functional groups enabling fast ion movement and uniform lithium deposition. We synthesized a single-crystalline 3D borate COF (B-COF), achieving an ionic conductivity of 8.1 mS cm^-1 at room temperature and a lithium-ion transference number of 0.98 in a quasi-solid-state. In symmetric cells, B-COF supported stable lithium deposition/stripping for 2000 h, suppressing dendrite formation. Full cells with LiFePO₄ cathodes cycled stably at 0.5C, delivering 147 mAh g^-1 initial capacity, 91.8% retention, and 99.98% Coulombic efficiency over 600 cycles. These results highlight B-COF's potential as a high-performance solid electrolyte for lithium metal batteries.