In situ 119Sn Mossbauer effect study of Li-CoSn2 electrochemical system

Crystalline CoSn2 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 CoSn2 occurs during the first reaction period. The complete reaction mechanism of CoSn2 is investigated by in situ Sn-119 Mossbauer spectroscopy using a Li-ion plastic cell. The conversion reaction proceeds through a biphasic system, with CoSn2 direct transformation into both a lithium-rich LiySn (where y approximate to 3.5) compound and metallic Co nanoparticles. On charge, the electrochemical process is more complex and it includes the conversion of the LiySn compound into a modified CoSn2 nanocompound with a variable composition, i.e., a LiyCozSn2 matrix, which is the active species for the following cycles.