Quantitative Analysis of the Initial Restructuring Step of Nanostructured FeSn₂-Based Anodes for Li-Ion Batteries

The first discharge of a nanostructured FeSn 2 based negative electrode for Li-ion batteries has been studied by combining operando 119 Sn Mössbauer spectroscopy and ex situ magnetic measurements. A modified Swagelok-type cell has been designed to perform in situ Mössbauer measurements, which allowed us to quantitatively follow the first discharge. The electrochemical mechanism consists in a conversion reaction that transforms FeSn 2 into Li 7 Sn 2 . The Mössbauer spectrum at the end of the first discharge has been analyzed from first principles calculations of the Mössbauer parameters. The observed differences with bulk Li 7 Sn 2 have been explained by the small size of the electrochemically formed particles. The magnetic measurements of the electrode material at the end of the discharge show the existence of rather pure superparamagnetic iron nanoparticles with an average diameter in the range 2–3 nm as evaluated from three different methods. The electrode saturation magnetization increases during the discharge, due to the increasing number of formed iron nanoparticles, but unexpected two-step variations were observed. They are interpreted by changes of the FeSn 2 magnetization caused by interactions with iron nanoparticles.