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

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 Ag₃PO₄ and metallic sodium. The resulting Ag₂Na/Ag/Na₃PO₄ interphase synergistically combines metallic, alloy, and ionic phases to simultaneously regulate ion transport and suppress dendrite formation. Specifically, the Ag₂Na alloy and metallic Ag components ensure strong interfacial adhesion and enhanced electronic conductivity, while the Na₃PO₄ phase promotes homogeneous Na⁺ ion flux and accelerates surface diffusion via its desolvation capability. Benefiting from this engineered interface, the Na/Ag₃PO₄ 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/Ag₃PO₄ 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.