The Electrochemical Sodiation of Sb Investigated by Operando X-ray Absorption and 121Sb Mössbauer Spectroscopy: What Does One Really Learn?

Abstract

In this study, we want to highlight the assets and restrictions of X-ray absorption spectroscopy (XAS) and Mössbauer spectroscopy for investigating the mechanism of the electrochemical reaction of antimony electrode materials vs. Na. For this, operando XAS was carried out during the first one and a half cycles, and the whole set of measured data was analysed using a statistical-chemometric approach, while low temperature Mössbauer spectroscopy measurements were carried out ex situ on selected samples stopped at different points of the electrochemical reaction. Complementary ab initio calculations were performed to support the experimental findings. Both techniques show that, upon the first sodiation, most Sb reacts with Na to form disordered Na 3 Sb. This step is accompanied by the formation of amorphous Sb as an intermediate. Upon inversion of the current Na 3 Sb is desodiated and an amorphous Sb phase, distinct from the pristine bulk Sb state, is gradually formed. However, both XAS and Mössbauer spectroscopy were unable to spot the formation of intermediate Na x Sb phases, which were evinced in previous works by operando Pair Distribution Function analyses. The results shown here clearly assign such failure to the intrinsic inability of both techniques to identify these intermediates.

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“…Figure 7B‐C shows the ex situ XANES analysis of the pristine and cycled Sb@void@C electrode at Sb K‐edge for SIBs, revealing that the Sb ions are primarily responsible for sodiation reaction during the electrochemical cycle. [ 5g , 29 ] Qualitatively, it is apparent that the Sb K‐edge spectra of cycled Sb@void@C electrode are irreversibly shifted towards lower absorption energy at discharge state relative to the non‐cycled electrode. [ 5g ] The Sb K‐edge (∼ 30490 eV) at a fully discharged state is different from that pristine Sb K‐edge (∼30493 eV) due to the formation of an amorphous Na x Sb/Na 3 Sb phase.…”

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