Synergistic Engineering of Bulk Integrity and Interfacial Reversibility via a Hierarchical Triphase Matrix for Stable Na Metal Batteries

Considerable volume fluctuation and sluggish Na⁺ diffusion capability have been identified as the key factors triggering uncontrolled growth of dendrite, hindering the practical exploration of sodium metal batteries (SMBs). Herein, a triphase Na₃Bi/Na₂S/Na₂O composite anode (abbr. BOS-Na), with both enhanced bulk mechanical strength and interfacial structural stability, is fabricated through a two-birds-with-one-stone strategy by impregnating 2D Bi₂O₂S nanosheets into molten metallic Na. The formed hierarchical high sodiophilic Na₃Bi skeleton not only withstands dramatic volume expansion, improving the structural stability upon long-term cycling, but also facilitates charge transfer throughout the bulk electrode, lowering the reaction energy barrier. Concurrently, the uniformly distributed Na₂S accelerates Na⁺ diffusion, regulating ion flux and electrical field on the Na surface, contributing to compact Na⁺ deposition. Additionally, the electronically insulating Na₂O component on the interface suppresses electron transport between the active material and organic electrolytes, avoiding the occurrence of side reactions and "dead Na". Crucially, the Na⁺ deposition behavior and mechanism are comprehensively revealed through ex situ characterizations, in-depth XPS analysis, and density functional theory (DFT) calculations. Consequently, the BOS-Na symmetric cell operates smoothly for 1300 h at 0.5 mA cm⁻²/1 mAh cm⁻², while the BOS-Na//NVP full cell also achieves a lifespan of 2000 cycles at 5 A g⁻¹.