Sn-Enriched Bilayer SEI via a SnF ₂ Electrolyte for Stable Solid-State Sodium-Ion Batteries

Solid-state sodium metal batteries (SSSBs) hold promise for achieving high energy density and safety in next-generation energy storage systems. However, practical implementation of the Na metal anode is severely hindered by poor interfacial contact and rampant dendrite growth. Here, we design a bilayer solid-electrolyte interphase (SEI), comprising an inner Sn-enriched NaF layer and an outer NaF-organic composite layer through an in situ reaction enabled by SnF2-asymmetric modification of NASICON/PVDF-HFP hybrid solid electrolytes (HSEs). The in situ-formed metallic Sn within the SEI not only enhances interfacial Na+ transport kinetics but also dynamically homogenizes the electric field distribution, enabling uniform Na+ flux and deposition. Consequently, the Na/Na symmetric cell achieves a critical current density (CCD) of 2 mA cm–2, which is a significant advantage over previously reported HSEs. When paired with the Na3V2(PO4)3 (NVP) cathode, the Na metal anode equipped with the Sn-rich SEI delivers outstanding rate capability, stable cycling behavior, and remarkable capacity retention. This study provides new insights into the scalable interfacial design and modification in solid-state sodium metal batteries.