Grain-Boundary Engineering of Na3Bi/NaF Dual-Functional Heterogeneous Protective Layer for Highly Stable Sodium Metal Anodes

Abstract

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 hours 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 NaNi1/3Fe1/3Mn1/3O2 (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.