SnF₂-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 SnF₂ into PEO and infusing the composite into an ultrathin polyethylene (PE) scaffold (PEO-xSnF₂@PE) promotes a stable Na₁₅Sn₄/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-4SnF₂@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 Na₃V₂(PO₄)₃ cathode exhibit exceptional cycling stability, achieving 97.6% capacity retention over 500 cycles. This work demonstrates that combining SnF₂ 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.