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Vol. 18 (2026)

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

https://doi.org/10.1007/s40820-026-02160-5
Muhammad Ali
School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
Moazzam Ali
School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
Hamid Hussain
School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
Samia Aman
School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
Ufra Naseer
College of Optical Science and Engineering, Zhejiang University (ZJU), Hangzhou 310058, People’s Republic of China
Asif Mahmood
Faculty of Science, Centre for Clean Energy Technology, School of Mathematical and Physical Sciences, University of Technology Sydney, City Campus, Broadway, NSW 2007, Australia
Muhammad Tayyab Ahsan
Institute for Sustainable Transformation, School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, People’s Republic of China
Yinzhu Jiang
School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
Muhammad Yousaf
School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China

Corresponding Author: Muhammad Yousaf

muhammadyousaf@zju.edu.cn

Nano-Micro Letters, Vol. 18 (2026), Article Number: 325

Abstract

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.


Highlights:
1 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.
2 Polar N–H/C=O motifs preferentially interact with PF6−, suppressing anion migration and boosting Na+ transference number, enabling uniform Na+ flux and lower polarization.
3 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.

Keywords

Sodium metal batteries Hydrogen-bonded organic framework Ion-selective transport Solid electrolyte interphase Flame-retardant separator
Ali, Muhammad, Moazzam Ali, Hamid Hussain, Samia Aman, Ufra Naseer, Asif Mahmood, Muhammad Tayyab Ahsan, Yinzhu Jiang, and Muhammad Yousaf. 2026. “A Flame-Retardant Hydrogen-Bonded Organic Framework Separator for Selective Sodium-Ion Transport Toward a NaF-Rich Interphase in Sodium Metal Batteries”. Nano-Micro Letters 18 (April):325. https://doi.org/10.1007/s40820-026-02160-5.
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References
  1. H. Pan, Y.-S. Hu, L. Chen, Room-temperature stationary sodium-ion batteries for large-scale electric energy storage. Energy Environ. Sci. 6(8), 2338 (2013). https://doi.org/10.1039/c3ee40847g
  2. A. Yao, S.M. Benson, W.C. Chueh, Critically assessing sodium-ion technology roadmaps and scenarios for techno-economic competitiveness against lithium-ion batteries. Nat. Energy 10(3), 404–416 (2025). https://doi.org/10.1038/s41560-024-01701-9
  3. M. Ali, A.N. Chishti, M. Ali, S. Iqbal, S. Aman et al., Recent development in sodium metal batteries: challenges, progress, and perspective. Mater. Today 88, 730–751 (2025). https://doi.org/10.1016/j.mattod.2025.06.014
  4. X. Wang, J. Lu, Y. Wu, W. Zheng, H. Zhang et al., Building stable anodes for high-rate Na-metal batteries. Adv. Mater. 36(16), e2311256 (2024). https://doi.org/10.1002/adma.202311256
  5. J. Huang, K. Wu, G. Xu, M. Wu, S. Dou et al., Recent progress and strategic perspectives of inorganic solid electrolytes: fundamentals, modifications, and applications in sodium metal batteries. Chem. Soc. Rev. 52(15), 4933–4995 (2023). https://doi.org/10.1039/d2cs01029a
  6. M. Ali, M.T. Ahsan, A. Mehmood, A. Ishfaq, G. Ali et al., MOF-derived AlCuSe2 embedded in a carbon matrix for an economical anode of lithium-ion battery. ACS Omega 7(34), 30440–30446 (2022). https://doi.org/10.1021/acsomega.2c03819
  7. W. Gao, Y. Lu, X. Tan, T. Wang, Y. Yu et al., Sodiophilic design for sodium-metal batteries: progress and prospects. Energy Environ. Sci. 18(4), 1630–1657 (2025). https://doi.org/10.1039/d4ee05871b
  8. J. Lin, P. Huang, T. Naren, C. Liang, L. Zhou et al., Salt-assisted recovery of sodium metal anodes for high-rate capability sodium batteries. Adv. Mater. 36(39), 2409976 (2024). https://doi.org/10.1002/adma.202409976
  9. M. Ali, S. Iqbal, M. Ali, S. Aman, A.N. Chishti et al., Tailoring the NaI-rich solid electrolyte interphase for enhanced stability in sodium metal batteries. J. Power. Sources 640, 236733 (2025). https://doi.org/10.1016/j.jpowsour.2025.236733
  10. Z. Xue, T. Zhang, X. Li, F. Wang, G. Xu et al., Simultaneous regulation of organic and inorganic components in interphase by fiber separator for high-stable sodium metal batteries. Angew. Chem. Int. Ed. Engl. 64(3), e202415283 (2025). https://doi.org/10.1002/anie.202415283
  11. Y. Liu, Z. Tai, I. Rozen, Z. Yu, Z. Lu et al., Ion flux regulation through PTFE nanospheres impregnated in glass fiber separators for long-lived lithium and sodium metal batteries. Adv. Energy Mater. 13(24), 2204420 (2023). https://doi.org/10.1002/aenm.202204420
  12. S. Yao, T. Zhang, C. Ma, C. Zhang, W. Zhang et al., Comparative study of the electrochemical performances of different polyolefin separators in lithium/sulfur batteries. Mater. Res. Bull. 171, 112604 (2024). https://doi.org/10.1016/j.materresbull.2023.112604
  13. G. Zhi, Z. Hu, G. Zhou, Z. Zhang, H. Wang et al., Sodiophilic Au-diamane polypropylene separator enabled dendrite-free sodium metal batteries. Nanoscale 17(18), 11752–11761 (2025). https://doi.org/10.1039/d5nr00743g
  14. J. Zhu, M. Yanilmaz, K. Fu, C. Chen, Y. Lu et al., Understanding glass fiber membrane used as a novel separator for lithium–sulfur batteries. J. Membr. Sci. 504, 89–96 (2016). https://doi.org/10.1016/j.memsci.2016.01.020
  15. Y. Ji, L. Dong, J. Liu, H. Xie, S. Zhong et al., A Li+-flux-homogenizing separator for long-term cycling of Li metal anodes. Energy Environ. Sci. 17(12), 4078–4089 (2024). https://doi.org/10.1039/d4ee00115j
  16. Q. He, Z. Li, M. Wu, M. Xie, F. Bu et al., Ultra-uniform and functionalized nano-ion divider for regulating ion distribution toward dendrite-free lithium-metal batteries. Adv. Mater. 35(39), e2302418 (2023). https://doi.org/10.1002/adma.202302418
  17. Y. Ansari, K. Virwani, S. Yahyazadeh, L.E. Thompson, E. Lofano et al., A highly stable sodium–oxygen battery using a mechanically reinforced membrane. Adv. Energy Mater. 8(36), 1802603 (2018). https://doi.org/10.1002/aenm.201802603
  18. X. Zhang, H. Cao, W. Shi, C. She, X. Shi et al., A nano-SiO2 embedded HPC/P(VDF-HFP) composite gel membrane for fast-charging sodium metal batteries with long span life. Appl. Mater. Today 35, 101927 (2023). https://doi.org/10.1016/j.apmt.2023.101927
  19. T. Zhu, X. Zuo, X. Lin, Z. Su, J. Li et al., High-wettability composite separator embedded with in situ grown TiO2 nanops for advanced sodium-ion batteries. Energy Technol. 10(10), 2200409 (2022). https://doi.org/10.1002/ente.202200409
  20. J. Li, G. Yin, Y. Wang, Y. Xiang, N. Yu et al., Enhancing the electrochemical performance and safety of lithium-ion battery via fluorinated MOF-based multifunctional separator. Chem. Eng. J. 507, 160148 (2025). https://doi.org/10.1016/j.cej.2025.160148
  21. M. Ali, H. Hussain, M. Ali, S. Aman, W. Yang et al., Regulating a NaF-rich SEI layer for dendrite-free sodium metal batteries using trifunctional halogenated covalent organic framework separators. Adv. Sci. 12(37), e03693 (2025). https://doi.org/10.1002/advs.202503693
  22. J. Seo, J. Im, M. Kim, D. Song, S. Yoon et al., Recent progress of advanced functional separators in lithium metal batteries. Small 20(33), e2312132 (2024). https://doi.org/10.1002/smll.202312132
  23. T. Zhao, P. Xiao, S. Nie, M. Luo, M. Zou et al., Recent progress of metal-organic frameworks based high performance batteries separators: a review. Coord. Chem. Rev. 502, 215592 (2024). https://doi.org/10.1016/j.ccr.2023.215592
  24. Y. Yang, S. Yao, Y. Wu, J. Ding, Z. Liang et al., Hydrogen-bonded organic framework as superior separator with high lithium affinity C═N bond for low N/P ratio lithium metal batteries. Nano Lett. 23(11), 5061–5069 (2023). https://doi.org/10.1021/acs.nanolett.3c00801
  25. W. Wang, Y. Liu, Q. Wang, Synthesis of melamine cyanuric based flame retardant via hydrogen bond self-assembly and in-situ dispersion strategies for improving comprehensive performance of epoxy resin. Compos. Part A Appl. Sci. Manuf. 176, 107826 (2024). https://doi.org/10.1016/j.compositesa.2023.107826
  26. Y. Chu, Z. Chen, Y. Yu, T. Chen, Z. Chen et al., Synthesis of a novel phosphorus-containing melamine cyanurate derivative to enhance the fire resistance and mechanical properties of epoxy resin. Colloids Surf. A Physicochem. Eng. Aspects 652, 129789 (2022). https://doi.org/10.1016/j.colsurfa.2022.129789
  27. W. Li, H. Xu, H. Zhang, F. Wei, L. Huang et al., Tuning electron delocalization of hydrogen-bonded organic framework cathode for high-performance zinc-organic batteries. Nat. Commun. 14, 5235 (2023). https://doi.org/10.1038/s41467-023-40969-5
  28. N. Orekhov, N. Bukhtiiarova, Z.A. Brushevich, A.A. Muravev, E. Nadav et al., Altering the structures of 3D supramolecular assemblies from melamine and cyanuric acid derivatives in water. Chem. Commun. 60(77), 10680–10683 (2024). https://doi.org/10.1039/D4CC02817A
  29. Y.-L. Li, E.V. Alexandrov, Q. Yin, L. Li, Z.-B. Fang et al., Record complexity in the polycatenation of three porous hydrogen-bonded organic frameworks with stepwise adsorption behaviors. J. Am. Chem. Soc. 142(15), 7218–7224 (2020). https://doi.org/10.1021/jacs.0c02406
  30. H. Darjazi, M. Falco, F. Colò, L. Balducci, G. Piana et al., Electrolytes for sodium ion batteries: the current transition from liquid to solid and hybrid systems. Adv. Mater. 36(35), 2313572 (2024). https://doi.org/10.1002/adma.202313572
  31. R. Zhou, Y.-S. Xu, C. Han, Y.-N. Li, J. Wang et al., Constructing anion solvation microenvironment toward durable high-voltage sodium-based batteries. Adv. Mater. 37(10), e2416748 (2025). https://doi.org/10.1002/adma.202416748
  32. D. Guan, W. Wang, B. Chen, J. Wu, G. Hu et al., Does salt concentration matter? new insights on the intercalation behavior of PF6− into graphite cathode for the dual-ion battery. Adv. Funct. Mater. 33(30), 2215113 (2023). https://doi.org/10.1002/adfm.202215113
  33. Y.-X. Yao, N. Yao, X.-R. Zhou, Z.-H. Li, X.-Y. Yue et al., Ethylene-carbonate-free electrolytes for rechargeable Li-ion pouch cells at sub-freezing temperatures. Adv. Mater. 34(45), e2206448 (2022). https://doi.org/10.1002/adma.202206448
  34. Y.-H. Feng, M. Liu, J. Wu, C. Yang, Q. Liu et al., Monolithic interphase enables fast kinetics for high-performance sodium-ion batteries at subzero temperature. Angew. Chem. Int. Ed. 63(23), e202403585 (2024). https://doi.org/10.1002/anie.202403585
  35. J. Popovic, Insights into cationic transference number values and solid electrolyte interphase growth in liquid/solid electrolytes for potassium metal batteries. ACS Phys. Chem. Au 2(6), 490–495 (2022). https://doi.org/10.1021/acsphyschemau.2c00024
  36. Y. Yan, Z. Liu, T. Wan, W. Li, Z. Qiu et al., Bioinspired design of Na-ion conduction channels in covalent organic frameworks for quasi-solid-state sodium batteries. Nat. Commun. 14(1), 3066 (2023). https://doi.org/10.1038/s41467-023-38822-w
  37. D. Morales, L.G. Chagas, D. Paterno, S. Greenbaum, S. Passerini et al., Transport studies of NaPF6 carbonate solvents-based sodium ion electrolytes. Electrochim. Acta 377, 138062 (2021). https://doi.org/10.1016/j.electacta.2021.138062
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