Covalently Anchored Multifunctional Interlayer Enables Ultrastable and Fast ‐ Charging Composite Solid ‐ State Sodium Metal Batteries

ABSTRACT The practical application of composite solid‐state sodium metal batteries is critically limited by poor organic–inorganic compatibility, causing particle agglomeration, high interfacial resistance, and dendrite growth. Here, a covalent surface grafting strategy constructs a multifunctional interlayer covalently anchored on Na 3 Zr 2 Si 2 PO 12 , featuring a cross‑linked siloxane network and terminal ─NH 2 groups. Covalent anchorage transforms inert particle surfaces into dispersible units, while ─NH 2 groups anchor TFSI – and confine residual solvent via hydrogen bonding and Lewis acid‑base interactions. This dual regulation decouples ion transport from side reactions, yielding a high Na + transference number (0.58) and a stable, NaF‑rich, thin solid‑electrolyte interphase (SEI). The optimized electrolyte enables symmetric cells with exceptional cycling stability and high critical current density (CCD). Full cells achieve fast‑charging (92 mAh g −1 at 15 C) and ultralong cycle life (76.4% retention after 7000 cycles at 10 C). A flexible pouch cell retains 97% capacity after 150 cycles. This work establishes that precise molecular‑level interfacial design, rather than simple physical blending, is key to high‑performance, dendrite‑resistant solid‑state sodium batteries.