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.