Geometry-Guided Screening of Metal/Oxide Heterointerfaces Enables Superior Sodium Metal Batteries at -40 °C

Despite their high theoretical energy density, sodium metal batteries (SMBs) remain limited by unstable interfacial chemistry and sluggish Na⁺ transport kinetics, particularly under ultralow-temperature conditions. Herein, guided by a geometry-driven design principle linking structural matching at metal/metal-oxide heterointerfaces with current density distribution, we identify a Y/Y₂O₃ heterostructure as an optimal surface modification layer and fabricate it on commercial aluminum foil via magnetron sputtering. The resulting Y/Y₂O₃ heterojunction, featuring a built-in electric field and moderate sodiophilicity, homogenizes Na⁺ flux, and directs reversible deposition. Meanwhile, the heterointerface facilitates PF₆ ^- adsorption, directing interfacial reactions toward the generation of inorganic solid electrolyte interphase components, which contributes to improved interfacial stability under ultralow-temperature conditions. Consequently, the Y/Y₂O₃-modified current collector promotes highly reversible Na plating/stripping, enabling Na@Y/Y₂O₃@Al||Na₃V₂(PO₄)₃ full cells to retain robust cycling stability at -40 °C. The Y/Y₂O₃ heterointerface design provides an effective platform for enabling durable SMB operation under harsh ultralow-temperature conditions.