Permselective Covalent Organic Framework Membrane as Self‐Extinguishing Separator for High‐Safety Lithium‐Ion Battery

Abstract As electric vehicles continue to gain popularity, the demand for high‐energy‐density battery technologies is growing rapidly. The separator, the most critical component in a battery, plays a key role in ensuring battery safety. However, uncontrolled ion transport and persistent safety concerns in promising lithium‐ion batteries (LIBs) still present significant challenges. In this study, we introduce a novel fluorine‐functionalized COF (TF‐COF) membrane as a battery separator to enhance the overall safety and cycling stability of LIBs. Its permselective nature effectively suppresses the growth of lithium dendrites and minimizes the risk of short circuit. When exposed to open flames, its excellent flame‐retardant properties allow it to self‐extinguish the fire up to three times, significantly prolonging the safe evacuation time when an accident occurs. Additionally, the intrinsic micropores of strongly lithophilic TF‐COF separator facilitate uniform lithium‐ion flux, enabling high values of ionic conductivity (8.79 × 10 −4 S cm −1 ) and Li⁺ transference number ( t Li ₊ , 0.86). The NCM811||Li cell with TF‐COF separators demonstrated high‐capacity retention of 87.6% after 200 cycles at 4.5 V and 0.5 C. Our approach, leveraging the fire‐retardant capabilities of permselective COF separators, offers a promising new pathway toward developing high‐safety, high‐energy‐density battery technologies with long‐life cyclability.