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

The incessant quest for high-energy-density batteries to meet the growing demand of electrification makes high-level safety operation a critical concern. Traditional polypropylene separators, susceptible to thermal instability and sodium dendrite growth, often lead to internal short circuits and catastrophic thermal runaway. Here, a flame-retardant, dendrite-suppressing, and ion-regulating hydrogen-bonded organic framework (HOF) separator is designed by a simple and scalable strategy. The HOF lattice, featuring abundant polar N-H and C=O sites, preferentially coordinates PF₆⁻. This selective interaction suppresses anion migration, yielding a high Na⁺ transference number (0.91). Concurrently, the liquid-filled pores and weakly coordinating channels of the HOF facilitate rapid Na⁺ transport, achieving an ionic conductivity of 1.57 mS cm⁻¹ at 60 °C. Interfacial analyses reveal that the HOF stabilizes Na⁺ deposition by fostering a NaF-rich solid electrolyte interphase with a high Young's modulus (~ 11 GPa), which suppresses dendrite penetration. Furthermore, thermogravimetric and combustion tests confirm exceptional resilience above 380 °C and the formation of carbon nitride layer that effectively suppresses heat release. Consequently, Na||Na symmetric cells cycle stably for over 2000 h at 2 mA cm⁻², while Na||Na₃V₂(PO₄)₃ full cells retain high capacity ~ 99% over 5000 cycles at 5 C. A pouch cell with a Prussian blue cathode further demonstrates practical applicability with consistent operation at 0.5 C. This multifunctional HOF separator establishes a new paradigm for stable, fast, selective, dendrite-free, and fire-safe sodium metal batteries.