Built‐In Electric Field of In Situ Formed Artificial Interface Layer Induces Fast and Uniform Sodium‐Ions Transmission to Achieve a Long‐Term Stable Sodium Metal Battery Under Harsh Conditions

Abstract Sodium metal batteries have emerged as potential rivals to lithium‐ion batteries. Nevertheless, maintaining a stable sodium metal anode under harsh conditions (current density >10 mA cm⁻²) is extremely challenging. The primary issue is the highly reactive sodium metal continuously reacts with the electrolyte to form a thick and loose solid electrolyte film. The tip effect causes sodium ions (Na⁺) accumulated in the bulge resulting in uneven deposition. In this paper, a unique artificial interfacial layer (Ag nanoparticles distributed in NaClO₄‐based matrix) is prepared on the surface of a metallic sodium anode using in situ displacement reaction. Ag particles arranged at the bottom of the artificial layer act as effective nucleation sites helping Na⁺ deposit uniformly, while the built‐in electric field formed between Ag in the middle/top of the artificial layer and the sodium accelerates the migration of Na⁺ in the interfacial layer during the deposition and reduce the polarization. Such important properties further allow the steady operation of modified sodium metal batteries in very harsh conditions. The symmetric battery achieves reversible stripping/plating for over 1000 h under high current density (10 mA cm⁻²). Moreover, the full cell coupled with a high‐loading Na₃V₂(PO₄)₃ cathode (20 mg cm⁻²) presents a stable cycle performance.