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

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.