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/Y 2 O 3 heterostructure as an optimal surface modification layer and fabricate it on commercial aluminum foil via magnetron sputtering. The resulting Y/Y 2 O 3 heterojunction, featuring a built‐in electric field and moderate sodiophilicity, homogenizes Na + flux, and directs reversible deposition. Meanwhile, the heterointerface facilitates PF 6 − 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/Y 2 O 3 ‐modified current collector promotes highly reversible Na plating/stripping, enabling Na@Y/Y 2 O 3 @Al||Na 3 V 2 (PO 4 ) 3 full cells to retain robust cycling stability at −40 °C. The Y/Y 2 O 3 heterointerface design provides an effective platform for enabling durable SMB operation under harsh ultralow‐temperature conditions.