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

Allan, Phoebe K. and Griffin, John M. and Darwiche, Ali and Borkiewicz, Olaf J. and Wiaderek, Kamila M. and Chapman, Karena W. and Morris, Andrew J. and Chupas, Peter J. and Monconduit, Laure and Grey, Clare P. (2016) Tracking Sodium-Antimonide Phase Transformations in Sodium-Ion Anodes : Insights from Operando Pair Distribution Function Analysis and Solid-State NMR Spectroscopy. Journal of the American Chemical Society, 138 (7). pp. 2352-2365. ISSN 0002-7863

Abstract

Operando pair distribution function (PDF) analysis and ex situ Na-23 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 NaxSb phases from the total PDF, an approach constrained by chemical phase information gained from Na-23 ssNMR in reference to relevant model compounds, identifies two previously uncharacterized intermediate species formed electro-chemically; a-Na3-xSb (x approximate to 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-Na1.7Sb, a highly amorphous structure featuring some Sb-Sb bonding. The first sodiation breaks down the crystalline antimony to form first a-Na3-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 amorphofis network reacts at higher voltages reforming a-Na1.7Sb, then a-Na3-xSb, whereas lower potentials are required for the sodiation of crystalline antimony, which reacts to form a-Na3-xSb without the formation of a-Na3-xSb. a-Na3-xSb is converted to crystalline Na3Sb at the end of the second discharge. We find no evidence of formation of NaSb. Variable temperature Na-23 NMR experiments reveal significant sodium mobility within c-Na3Sb; this is a possible contributing factor to the excellent rate performance of Sb anodes.