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 Na3Zr2Si2PO12, featuring a cross‑linked siloxane network and terminal ─NH2 groups. Covalent anchorage transforms inert particle surfaces into dispersible units, while ─NH2 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.