Grain-boundary engineering of Na3Bi/NaF dual-functional heterogeneous protective layer for highly stable sodium metal anodes

Sodium-metal batteries have been extensively recognized as a potential alternative to lithium-metal batteries. However, the huge volume expansion, inhomogeneous distribution of the electrical field, and sluggish Na⁺ diffusion at the electrolyte/electrode interface are insurmountable challenges to achieving high cycling performance and a long lifespan. In this work, a dual-functional heterogeneous protective layer consisting of Na₃Bi/NaF was constructed on the surface of metallic sodium (abbr. BiF₃/Na) by the spontaneous reduction reactions between metallic sodium and BiF₃ powder at room temperature. Attributing to the in-situ formation of rich grain boundaries and the built-in electric field between the components, the charge transfer, Na⁺ diffusion rate as well as mechanical strength of the BiF₃/Na anode were extensively improved. Consequently, the sodium metal anode with the Na₃Bi/NaF dual-functional heterogeneous layer achieves an excellent cycling capability and long cycling lifespan of more than 2000 h at a large current density of 2 mA cm^-2 and an area capacity of 1 mAh cm^-2. In addition, by further matching with the commercialized NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) cathode, the prepared BiF₃/Na||NFM full cell exhibits high durability (68.8 mAh g^-1 after 2000 cycles at 2 C). This work utilizing grain boundary engineering has provided a promising strategy for achieving dendrite-free sodium metal anodes and high-energy density sodium metal batteries.