A Flame-Retardant Hydrogen-Bonded Organic Framework Separator for Selective Sodium-Ion Transport toward a NaF-Rich Interphase in Sodium Metal Batteries

A multifunctional melamine cyanurate hydrogen-bonded framework separator provides intrinsic flame retardancy, high-temperature dimensional stability, and dendrite suppression, offering a practical alternative to commercial polyolefin separators for sodium metal batteries. Polar N–H/C=O motifs preferentially interact with PF 6 − , suppressing anion migration and boosting Na + transference number, enabling uniform Na + flux and lower polarization. Separator-driven interphase regulation forms an inorganic, NaF-rich solid electrolyte interphase confirmed by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry 3D mapping, improving Na plating/stripping reversibility and long-term full-cell durability at elevated temperature.