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

In this study, we want to highlight the assets and restrictions of X-ray absorption spectroscopy (XAS) and Mossbauer 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 Mossbauer 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 Na3Sb. This step is accompanied by the formation of amorphous Sb as an intermediate. Upon inversion of the current Na3Sb is desodiated and an amorphous Sb phase, distinct from the pristine bulk Sb state, is gradually formed. However, both XAS and Mossbauer spectroscopy were unable to spot the formation of intermediate NaxSb 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.