Carboxyl‐Functionalized Covalent Organic Framework Engineered Separator Enables Stable Sodium Deposition for High‐Performance Sodium Metal Batteries

Abstract Sodium metal batteries (SMBs) have attracted extensive interest owing to the high natural abundance, high theoretical specific capacity, and low redox potential of sodium metal. However, uncontrolled sodium dendrite growth and unstable solid electrolyte interphase (SEI) formation severely hinder their practical application. Herein, a carboxyl functionalized covalent organic framework (TP‐COF‐COOH)‐modified commercial polypropylene (PP) separator is designed to address these challenges. Benefitting from the abundant sodiophilic functional groups, ordered skeleton, and regular porous structure, the TP‐COF‐COOH@PP separator not only demonstrates excellent electrolyte wettability and achieves uniform Na + ion flux to inhibit Na dendrite nucleation, but also induces the formation of the NaF‐rich SEI. Consequently, the TP‐COF‐COOH@PP separator achieves an outstanding Na⁺ transference number of 0.8 and a remarkable ionic conductivity of 1.78 mS cm −1 . The Na//TP‐COF‐COOH@PP//Na cells demonstrate ultralong cycling stability exceeding 2500 h at 1 mA cm −2 and over 1500 h at 5 mA cm −2 . The enhanced effects of TP‐COF‐COOH@PP separator are validated by experiments, density functional theory calculations, and finite element method simulations. Furthermore, the Na//TP‐COF‐COOH@PP//Na 0.67 Ni 0.33 Mn 0.67 O 2 full cells demonstrate a high specific capacity of 143 mAh g −1 and stable cycling performance over 200 cycles with 80% capacity retention. The study proposes a promising strategy for developing dendrite‐free and long‐life SMBs.