Engineered Crystalline Heterostructure Interphase Enabling Dendrite‐Free Sodium Metal Anodes with Long‐Term Stability

Abstract The advancement of sodium‐ion batteries (SIBs) critically depends on the development of stable sodium metal anodes (SMAs). However, practical implementation remains hindered by uncontrollable dendritic growth and uneven Na stripping/plating behavior associated with pristine sodium metal. In this study, the design of a robust triphasic heterojunction artificial interphase is reported, formed via a spontaneous in situ reaction between Ag 3 PO 4 and metallic sodium. The resulting Ag 2 Na/Ag/Na 3 PO 4 interphase synergistically combines metallic, alloy, and ionic phases to simultaneously regulate ion transport and suppress dendrite formation. Specifically, the Ag 2 Na alloy and metallic Ag components ensure strong interfacial adhesion and enhanced electronic conductivity, while the Na 3 PO 4 phase promotes homogeneous Na⁺ ion flux and accelerates surface diffusion via its desolvation capability. Benefiting from this engineered interface, the Na/Ag 3 PO 4 anode exhibits a remarkably low nucleation overpotential of 27 mV and delivers stable cycling performance exceeding 1600 h at 0.5 mA cm −2 (1 mAh cm −2 ) in symmetric cells. Moreover, a full sodium metal pouch cell incorporating the Na/Ag 3 PO 4 anode achieves a high energy density of 425.5 Wh kg −1 , underscoring the practical viability of this interfacial design for next‐generation high‐energy SIBs.