Regulating a NaF‐Rich SEI Layer for Dendrite‐Free Sodium Metal Batteries Using Trifunctional Halogenated Covalent Organic Framework Separators

Abstract Uncontrolled sodium‐ion (Na + ) transport, fragile solid electrolyte interphase (SEI) layers, in and sluggish Na + desolvation using conventional separators drive dendrite growth, posing critical challenges to the development of sodium metal batteries (SMBs). Porous materials with tunable Na + transport pathways offer promise; however, simultaneously enhancing Na + kinetics, promoting NaF‐rich SEI formation, and lowering desolvation energy barriers remains a critical challenge. Herein, a trifunctional halogenated covalent organic framework (COF) integrated into a polypropylene (PP) separator (COF‐F@PP) is designed to address these issues. The COF‐F@PP separator features positively charged sites to anchor PF 6 − anions and facilitate desolvation of NaPF 6 , and in‐situ release of fluorine ions from halogenated COF promotes the formation of a robust NaF‐rich SEI layer. Additionally, its high‐porosity structure enables uniform Na + transport. Theoretical simulation demonstrates that the COF‐F@PP separator improves desolvation dynamics, ensures uniform Na + flux distribution, and mitigates local electric field concentration, resulting in smooth and dendrite‐free deposition. Consequently, a high Coulombic efficiency (99.2%), excellent ionic conductivity (1.13 mS cm −1 ), and stable cycling for over 1000 h at 3 mA cm −2 are achieved. In Na||NVP full cells, COF‐F@PP separator delivers an initial discharge capacity of 83.51 mAh g −1 at 50 C and retains 88.42% of its capacity after 10 000 cycles.