Simultaneous operando Nuclear Forward Scattering and transmission X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy measurements were carried out in order to investigate the electrochemical mechanism of NaFeO₂ vs. Na metal using a specifically designed in situ cell. The obtained data were analysed using an alternative and innovative data analysis approach based on chemometric tools such as Principal Component Analysis (PCA) and Multivariate Curve Resolution - Alternating Least Squares (MCR-ALS). This approach, which allows the unbiased extraction of all possible information from the operando data, enabled the stepwise reconstruction of the independent “real” components permitting the description of the desodiation mechanism of NaFeO₂. This wealth of information allows a clear description of the electrochemical reaction at the redox-active iron centres, and thus an improved comprehension of the cycling mechanisms of this material vs. sodium.

X-ray absorption spectroscopy (XAS) is a synchrotron-based, element-specific, short-range structural method largely used in the study of materials, already widely employed for the study of the electrochemical processes in battery systems. The high penetration of the X-rays makes XAS particularly suited for the study of battery materials under operando conditions using specifically developed in situ electrochemical cells. In this paper, the application of XAS to the study of battery materials carried out at Institut Charles Gerhardt Montpellier (France) will be outlined, illustrating the type of information provided by XAS. The challenging study of the sodiation of Bi_0.50Sb_0.50 followed individually and simultaneously at both metals will be also presented in order to highlight the typical advantages of this technique even when it is pushed at its technical limits.

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.