In situ ¹¹⁹Sn Mössbauer Effect Study of Li−CoSn₂ Electrochemical System

Crystalline CoSn 2 powders prepared by a ceramic route under an Ar/H 2 controlled atmosphere are examined as a negative electrode for lithium-ion batteries. Ex situ X-ray diffraction measurements show that an electrochemically driven solid-state transformation of CoSn 2 occurs during the first reaction period. The complete reaction mechanism of CoSn 2 is investigated by in situ 119 Sn Mössbauer spectroscopy using a Li-ion plastic cell. The conversion reaction proceeds through a biphasic system, with CoSn 2 direct transformation into both a lithium-rich Li y Sn (where y ≈ 3.5) compound and metallic Co nanoparticles. On charge, the electrochemical process is more complex and it includes the conversion of the Li y Sn compound into a modified CoSn 2 nanocompound with a variable composition, i.e., a Li y Co z Sn 2 matrix, which is the active species for the following cycles.