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

Phoebe K Allan,Andrew J Morris,Kamila M Wiaderek,Peter J Chupas,Ali Darwiche,Olaf J Borkiewicz,Clare P Grey,John M Griffin,Laure Monconduit,Karena W Chapman

doi:10.1021/jacs.5b13273

Abstract

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 NaxSb 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-Na3–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-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 amorphous 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-Na1.7Sb. 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 23Na NMR experiments reveal significant sodium mobility within c-Na3Sb; this is a possible contributing factor to the excellent rate performance of Sb anodes.

Highlights

Sodium-ion batteries (NIBs) are of great interest as a complementary technology to lithium-ion batteries in applications such as grid-storage, where cost and sustainability are of greater importance than high energy density
Alloying anodes consisting of metals or metalloids that form alloys with sodium are of great interest due to their low operating voltage and high gravimetric capacities, reversible capacities of 480, 350, 500, and 580 mAhg−1 having been reported for Pb, Ge, Sn and Sb, respectively.[2−4] The performance of antimony at high rates sets it apart from other alloying materials; micrometric antimony can retain a capacity of 580 mAhg−1 at a rate of C/6 and 520 mAhg−1 at a rate of 3/4 C, something that has not been reported for lead, germanium or tin elemental anodes in NIBs.[3,5,6]
We focus on using in-depth analysis of pair distribution function (PDF) data, constrained by chemical information from 23Na ssNMR experiments referenced to synthesized model compounds of the two known sodiumantimonide compounds NaSb and Na3Sb, in order to isolate amorphous and crystalline phases present in the electrode, and understand their transformations during thesodiation process

Summary

Introduction

Sodium-ion batteries (NIBs) are of great interest as a complementary technology to lithium-ion batteries in applications such as grid-storage, where cost and sustainability are of greater importance than high energy density. The large volume expansions that take place on sodiation can lead to harmful mechanical processes and side-reactions that cause capacity fade with cycling. This can be somewhat mediated by careful electrode formulation or nanocompositing the material

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Conclusion