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

Operando Sb K-edge X-ray absorption spectroscopy and Fe-57 Synchrotron Mossbauer 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 sodiumion 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.