Dendrite‐Free Sodium Metal Anodes Via Solid Electrolyte Interphase Engineering With a Covalent Organic Framework Separator

Abstract Solid electrolyte interphases (SEIs) play a crucial role in keeping sodium metal anodes (SMAs) intact and improving battery life. However, the SEIs arising from irreversible reactions between metallic Na and electrolytes fail to suppress Na dendrite growth and have sluggish Na⁺ kinetics. Herein, a functionalized separator modified by a sp² carbon conjugated‐covalent organic framework (sp²c‐COF) is proposed to induce a robust SEI. X‐ray photoelectron spectroscopy (XPS) analyses and theoretical calculations demonstrate that the SEI is rich in NaF because the structure of NaPF₆ is unstable due to influences from the COF separator. In situ observations show that the Na dendrite is effectively suppressed even at a high current density of 20 mA cm⁻². Satisfactorily, the COF separator exhibits a high transference number of 0.78, achieving a fast Na plating/stripping process. Based on these superiorities, a symmetric cell Na|COF|Na shows stable cycling for over 1500 h at 20 mA cm⁻². In addition, full cells Na|COF|NaTi₂(PO₄)₃ (NTPO) present good rate performance (30 and 50 C) and excellent cycling stability over 5000 cycles at 5 and 10 C. The application of COFs to improve SMAs in this work demonstrates a new strategy for improving sodium metal batteries.