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SnF2-Modified Thin Composite Electrolyte With Ultra-Stable Interface for Solid-State Sodium Batteries

Jinbo Zhang,Yanxia Su,Zeran Ding,Yuqian Qiu,Chong Li,Yue Ma,Ningyu Zhang,Fei Xu,Hongqiang Wang

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.