Covalent organic framework-engineered separators enabling selective sodium ion transport for sodium metal anode storage

Abstract

Sodium metal energy storage devices with high power/energy densities offer scalability without requiring complex presodiation. However, the sluggish migration of Na⁺ and the uncontrollable growth of sodium dendrites have hindered their commercial adoption. Herein, we construct functionalized separators using quinoline carboxylic acid covalent organic frameworks (QL-COFs) to achieve selective Na⁺ transport and uniform deposition. Molecular dynamics simulations and theoretical calculations confirm that QL-COFs with uniform pore structures restrict PF₆^- ion migration while providing highly selective transport channels for Na⁺, doubling the Na⁺ transference number to 0.89 (vs. 0.43 for polyethylene separators), surpassing values reported for state-of-the-art functionalized separators and solid-state electrolytes. In-situ XRD directly visualizes the reversible Na⁺ deposition/stripping behavior on Cu foil, corroborated by the stable Coulombic efficiency of Na-Cu cells over 800 h, jointly demonstrating that the uniform pore architecture of QL-COFs guides homogeneous Na⁺ electrodeposition. The modified separator enables symmetric cells to achieve 1300-h cycling stability and empowers a sodium metal capacitor to deliver 203.33 Wh kg^-1 at an ultrahigh power density of 24,000 W kg^-1, surpassing the performance metrics of previously reported devices. This work first elucidates the mechanism of QL-COF-modified separators in accelerating Na⁺ migration, expands the application boundaries of COF materials, and proposes a new paradigm for constructing scalable sodium metal capacitors.