The
first discharge of a nanostructured FeSn2 based
negative electrode for Li-ion batteries has been studied by combining
operando 119Sn 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 FeSn2 into Li7Sn2. 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 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.
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