The Electrochemical Sodiation of FeSb₂: New Insights from Operando ⁵⁷Fe Synchrotron Mössbauer and X-Ray Absorption Spectroscopy

Operando Sb K-edge X-ray absorption spectroscopy and ⁵⁷Fe Synchrotron Mössbauer Spectroscopy, used for the first time in the field of operando energy storage materials, assisted by operando magnetic measurements, were combined to clarify the role of iron and antimony in the electrochemical reaction mechanism of FeSb₂ as negative electrode material for sodium-ion batteries (SIB). Both datasets were analyzed using an innovative chemometric approach involving principal component analysis (PCA) and multivariate curve resolution – alternating least square analysis (MCR-ALS) yielding new insights on the sodiation reaction. Our findings show that the reaction of Na with FeSb₂ during discharge leads to the formation of Na₃Sb along with superparamagnetic Fe amorphous nanoparticles which contain small amounts of Sb dissolved in their lattice. During the following desodiation, the pristine material FeSb₂ is not recovered while iron nanoparticles grow in size, and continue growing also along the following discharge. Even though such iron nanoparticles remain electrochemically inactive, they play a key role in the reduction and stabilization of the polarization as well as in the reversibility of the electrochemical sodiation of antimony.