Regulating a NaF-Rich SEI Layer for Dendrite-Free Sodium Metal Batteries Using Trifunctional Halogenated Covalent...

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 PF₆ ^- anions and facilitate desolvation of NaPF₆, 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.