Engineering Stable Interphases for Na Metal Anodes via MgF ₂ ‐Based Spatial Confinement for In Situ Polymerization

The practical deployment of sodium metal batteries (SMBs) is severely plagued by an unstable solid electrolyte interphase and dendritic sodium growth. This work proposes a spatial confinement polymerization strategy utilizing an MgF 2 ‐functionalized glass fiber (MgF 2 /GF) skeleton to construct high‐performance gel polymer SMBs. Unlike corrosive soluble initiators, the immobilized MgF 2 Lewis acid triggers a heterogeneous “grafting‐from” polymerization of 1,3‐dioxolane precisely within separator pores and at interfaces. This spatial regulation yields a heterogeneous electrolyte and a conformal electrode‐electrolyte interface, extending the oxidative stability window to 4.44 V vs. Na/Na + . Beyond catalysis, the multifunctional MgF 2 /GF framework tailors both ion kinetics and interfacial chemistry by preferentially anchoring TFSI − to boost the Na + transference number to 0.75 and constructing a fluorine‐rich, sodiophilic interphase reinforced by in situ formed metallic Mg. This synergistic design effectively suppresses dendritic growth and alleviates concentration polarization. Na || Na cells demonstrate stable cycling for over 1000 h at 0.1 mA cm −2 , while Na || Na 3 V 2 (PO 4 ) 3 full cells retain 89.0% capacity after 1000 cycles at 2 C. Remarkably, practical pouch cells operate stably for over 3500 cycles. This work presents a scalable, separator‐centric engineering approach for safe and high‐energy‐density SMBs.