Tracking Sodium-Antimonide Phase Transformations in Sodium-Ion Anodes: Insights from Operando Pair Distribution Function Analysis and Solid-State NMR Spectroscopy

Operando pair distribution function (PDF) analysis and ex situ ²³Na magic-angle spinning solid-state nuclear magnetic resonance (MAS ssNMR) spectroscopy are used to gain insight into the alloying mechanism of high-capacity antimony anodes for sodium-ion batteries. Subtraction of the PDF of crystalline Na_xSb phases from the total PDF, an approach constrained by chemical phase information gained from ²³Na ssNMR in reference to relevant model compounds, identifies two previously uncharacterized intermediate species formed electrochemically; a-Na_3-xSb (x ≈ 0.4-0.5), a structure locally similar to crystalline Na₃Sb (c-Na₃Sb) but with significant numbers of sodium vacancies and a limited correlation length, and a-Na_1.7Sb, a highly amorphous structure featuring some Sb-Sb bonding. The first sodiation breaks down the crystalline antimony to form first a-Na_3-xSb and, finally, crystalline Na₃Sb. Desodiation results in the formation of an electrode formed of a composite of crystalline and amorphous antimony networks. We link the different reactivity of these networks to a series of sequential sodiation reactions manifesting as a cascade of processes observed in the electrochemical profile of subsequent cycles. The amorphous network reacts at higher voltages reforming a-Na_1.7Sb, then a-Na_3-xSb, whereas lower potentials are required for the sodiation of crystalline antimony, which reacts to form a-Na_3-xSb without the formation of a-Na_1.7Sb. a-Na_3-xSb is converted to crystalline Na₃Sb at the end of the second discharge. We find no evidence of formation of NaSb. Variable temperature ²³Na NMR experiments reveal significant sodium mobility within c-Na₃Sb; this is a possible contributing factor to the excellent rate performance of Sb anodes.