Fluorinated porous covalent polymer nanofilm assembled sandwich separator: Lithiophilic sites engender uniform Li+ flux towards lithium metal battery

Lithium metal is renowned for its high theoretical specific capacity and extremely low reduction potential. Nevertheless, the repeated breaking/forming of the solid electrolyte interface and uncontrolled growth of lithium dendrites in lithium metal batteries significantly impede their commercial application. In this study, we propose a novel sandwich separator (F-BtTp@PP) composed of fluorinated porous covalent polymer nanofilms (F-BtTp-Nfilm) assembled using roll-to-roll technology. The F-BtTp@PP improves the pore distribution of PP, enhances its resistance to dendrite penetration, and promotes a uniform distribution of lithium ions flux. Furthermore, Fourier-transform infrared spectroscopy and density functional theory calculations confirm the energy affinity. Abundant lithophilic sites and strong electronegative groups effectively control the deposition behavior of lithium ions, resulting in a transfer number of 0.87. This mitigates effectively the occurrence of significant concentration gradients and localized hot-spots during cycles with high current density. The symmetrical cells of F-BtTp@PP Li|Li can maintain a stable cycling voltage of 52.2 mV for 5600 hours under a current density of 20 mA cm−2 and capacity density of 10 mAh cm−2. This result robustly demonstrates the exceptional efficiency of F-BtTp@PP, which can be readily scaled up for protecting lithium metal anodes and provides valuable guidance for the design and industrial development of lithium metal batteries with modified separators.