Understanding the fast kinetics and mechanism of sodium storage in antimony using ab initio grand canonical Monte Carlo simulation and operando X-ray scattering

Sodium-ion battery (SIB) alloy anodes are attractive for their high gravimetric capacities, but they suffer from sluggish kinetics during charge storage caused by bulk diffusion during (de)sodiation-induced phase transformations. Among these SIB alloy anodes, antimony (Sb) exhibits one of the fastest (de)sodiation kinetics, with a rate capability comparable to that of intercalation electrodes. It is desirable to understand the origin of Sb fast kinetics, and herein, we use ab initio grand canonical Monte Carlo (ai-GCMC) to predict possible intermediate compounds that form during (de)sodiation of Sb and discover a family of layered glassy intermediates that are not only close to the convex hull of the Na-Sb phase diagram, but are also similar in composition, structure, and energy, suggesting that an amorphous phase may be observed during (de)sodiation. Further, we find that the diffusion barrier for Na in an amorphous/glassy phase can be as low as 6 kJ mol(-1). To experimentally validate our simulation results, we performed electrochemical studies including cyclic voltammetry (CV)-based kinetics analysis, which revealed a fast intermediate reaction; and operando wide-angle X-ray scattering (WAXS), which showed an alternating crystalline pattern indicative of an amorphous intermediate. Based on our results, we propose the following sodiation pathway: Sb (crystalline) -> NaySb (amorphous/glassy) -> NaxSb (amorphous/glassy) -> Na3Sb (crystalline), where y < 1.5, 1.5 <= x <= 2, and these amorphous/glassy intermediate phases are responsible for the fast kinetics.