Self‐Formed Fluorinated Interphase with Fe Valence Gradient for Dendrite‐Free Solid‐State Sodium‐Metal Batteries

Abstract The poor anode/electrolyte compatibility and dendrite growth have discouraged the development of solid‐state sodium‐metal batteries (SSSMBs). Here, a self‐formed interphase with a unique Fe valence gradient is designed to stabilize the anode interface of Na 3 Zr 2 Si 2 PO 12 solid electrolyte. This interphase consists of a Fe‐ and NaF‐containing outer layer near the Na anode and a Fe 2+ /Fe 3+ ‐rich inner layer, which are in situ formed upon contacting Na with a sodiophilic α‐Fe 2 O 3‐x F x interlayer specially coated on Na 3 Zr 2 Si 2 PO 12 . The NaF and Fe prevents continuous reduction reactions and homogenize the electric filed, while the iron oxide gradient layer offers buffering Na storage during plating, further suppressing dendrite growth. Benefiting from the dynamically stable, low‐impedance and dendrite‐free interface, the symmetric cells achieve a 20‐fold reduction in interfacial resistance and a fourfold enhancement in critical current density (CCD) at 25 °C, and powered a high CCD of 1.9 mA cm ‐2 and a 1000‐h cycle life at 1 mA cm ‐2 at 80 °C. The corresponding SSSMBs based on Na 3 V 2 (PO 4 ) 3 cathodes deliver a capacity retention rate of 96% over 120 cycles at 1 C. This robust interface design supports one of the best electrochemical performances ever reported and offers a comprehensive solution to stabilize the Na 3 Zr 2 Si 2 PO 12 /Na interface toward practical SSSMBs.