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

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.

Molybdenum Polysulfide Chalcogels as High-Capacity, Anion-Redox-Driven Electrode Materials for Li-Ion Batteries

“…Collecting operando PDFs allows for tracking the evolution of the local structure for materials that lack long-range ordering, which is particularly useful for chalcogels. 48,[72][73][74][75][76][77] Here, PDFs of the MoS x electrode cycled at constant current of 0.61 (9) mA g −1 (≈ C/16) (Figure 8a) capture the Mo-S bonds centered at 2.45Å in Figure 8b, which is unchanged upon discharge to 600.5 mAh g Coulombic efficiency is maintained at 98.9% for the second and third cycles. One of the many strategies for increasing Li ion kinetics includes increasing the electrode surface area.…”

Alloy‐Based Anode Materials toward Advanced Sodium‐Ion Batteries

“…[104,105] However, Sb also meets the challenges of drastic volume change (volume expansion about 390%) and slow reaction kinetics during Na + insertion and extraction process, and thus capacity fading. The maximum stoichiometry of Na-Sb alloy is Na 3 Sb, corresponding to a theoretical capacity of 660 mA h g −1 .…”

“…The maximum stoichiometry of Na-Sb alloy is Na 3 Sb, corresponding to a theoretical capacity of 660 mA h g −1 . [104,105,108] Darwiche et al put forward a simple sodiation mechanism consisting of two steps: c-Sb → a-Na x Sb and a-Na x Sb → Na 3 Sb hex /c-Na 3 Sb cub → c-Na 3 Sb hex , where all the intermediate phases detected by X-ray diffraction (XRD) were amorphous. [106,107] In the case of Sb, the (de)sodiation process remains inconclusive.…”

Elucidating the origin of superior electrochemical cycling performance: new insights on sodiation–desodiation mechanism of SnSb from operando spectroscopy

“…It is noteworthy that unlike our previous study in which we were able to identify both cubic and hexagonal Na 3 Sb, 30 Resonance analysis in the case of pure Sb. 11 In that work, two different electrochemically formed intermediates were identified: (i) amorphous Na 3−x Sb (with x ≈ 0.4 to 0.5), having a local structure similar to crystalline Na 3 Sb but with a significant number of sodium vacancies and a limited correlation length, and (ii) amorphous Na 1.7 Sb, with a highly amorphous structure featuring some Sb-Sb bonding. Even though the first of these two phases seems to form during the first sodiation, the second one is expected to appear only during the following desodiation.…”

“…11,37 In order to increase the Lamb-Mössbauer factor of the obtained phases, an ex situ variable temperature Mössbauer spectroscopy study was carried out on two selected electrodes, which were put in test batteries and stopped after the reaction of 3 mol of Na per mol of Sn, and at the end of the first discharge, respectively.…”