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

The first discharge of a nanostructured FeSn2 based negative electrode for Li-ion batteries has been studied by combining operando Sn-119 Mossbauer spectroscopy and ex situ magnetic measurements. A modified Swagelok-type cell has been designed to perform in situ Mossbauer measurements, which allowed us to quantitatively follow the first discharge. The electrochemical mechanism consists in a conversion reaction that transforms FeSn2 into Li7Sn2. The Mossbauer spectrum at the end of the first discharge has been analyzed from first principles calculations of the Mossbauer parameters. The observed differences with bulk Li7Sn2 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 FeSn2 magnetization caused by interactions with iron nanoparticles.