Ultrathin Composite Separator Based on Lithiated COF Nanosheet for High Stability Lithium Metal Batteries

Abstract Lithium metal is considered as the ultimate anode material for high‐energy‐density rechargeable batteries. However, lithium metal batteries (LMBs) with commercial separators still face some challenges such as low cycling efficiency and uncontrollable Li dendrite growth, which seriously hampers the commercialization of LMBs. In this study, a novel kind of ultrathin (6.2 µm) multifunctional composite separator (TpPa‐SO₃Li@PE) is designed and prepared via coating lithiated covalent organic framework nanosheet (TpPa‐SO₃Li) on the surface of commercial polyethylene separator (PE). TpPa‐SO₃Li@PE integrates features of the nanochannel arrays and abundant immobilized anionic sites, leading to efficient Li⁺ conduction and homogeneous Li⁺ flow. As a result, TpPa‐SO₃Li@PE exhibits excellent Li⁺ conductivity (0.96 mS cm⁻¹) and Li⁺ transference number (0.83) at room temperature, and Li/Li symmetric cell using TpPa‐SO₃Li@PE separator possesses highly stable Li plating/striping (over 2600 h) at high current density (5 mA cm⁻²). Moreover, Li/LiFePO₄ full cells with TpPa‐SO₃Li@PE separator show excellent cycling performance (high capacity retention of 94.9% after 300 cycles at 1 C) and superior rate performance (high specific capacity of 113.6 mAh g⁻¹ at 5 C).