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

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 anions and facilitate desolvation of NaPF, 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), and stable cycling for over 1000 h at 3 mA cm are achieved. In Na||NVP full cells, COF-F@PP separator delivers an initial discharge capacity of 83.51 mAh g at 50 C and retains 88.42% of its capacity after 10 000 cycles.