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

ABSTRACT 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 Na 3 Bi/Na 2 S/Na 2 O 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 Bi 2 O 2 S nanosheets into molten metallic Na. The formed hierarchical high sodiophilic Na 3 Bi 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 Na 2 S accelerates Na + diffusion, regulating ion flux and electrical field on the Na surface, contributing to compact Na + deposition. Additionally, the electronically insulating Na 2 O 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 .