SnF ₂ ‐Modified Thin Composite Electrolyte With Ultra‐Stable Interface for Solid‐State Sodium Batteries

The unstable Na/electrolyte interface, plagued by Na dendrites and interface degradation, critically hinders solid‐state Na batteries. We propose the design of thin poly(ethylene oxide) (PEO)‐based composite polymer electrolytes with a stabilized solid electrolyte interphase (SEI). Incorporating SnF 2 into PEO and infusing the composite into an ultrathin polyethylene (PE) scaffold (PEO‐xSnF 2 @PE) promotes a stable Na 15 Sn 4 /NaF‐rich SEI that facilitates uniform Na⁺ deposition, while the PE layer ensures a thin yet mechanically robust structure. Consequently, the optimized composite polymer electrolyte, featuring an ultrathin 20 µm thickness, low areal density of 1.9 mg cm −2 , and ultrahigh tensile strength of 35 MPa, demonstrates exceptional dendrite‐suppressing capability. Such synergistic effects enable Na symmetric cells employing PEO‐4SnF 2 @PE to achieve ultralong cycling exceeding 10800 h (>1 year), alongside a critical current density of 1.0 mA cm −2 . Full cells paired with a Na 3 V 2 (PO 4 ) 3 cathode exhibit exceptional cycling stability, achieving 97.6% capacity retention over 500 cycles. This work demonstrates that combining SnF 2 functional additives with a flexible, high‐strength supporting layer effectively mitigates interfacial instability and dendrite propagation in solid polymer electrolytes, offering new design principles for long‐life solid‐state sodium metal batteries.