Lithium-driven conversion and alloying mechanisms in core-shell Sn/SnOx nanoparticles

Abstract

Sn-based electrode materials have been envisioned as an alternative to graphite owing to their high theoretical capacity. In this work, we embarked on investigating the lithium electrochemical mechanisms occurring in core-shell nanoparticles made of a tin metal core covered by an amorphous tin oxide shell. The electrochemical properties evaluated using galvanostatic experiments point to a dual contribution of core and shell components to the overall capacity. Operando Mössbauer spectroscopy allowed the identification of the different species involved during Li-driven electrochemistry, thus enabling the thorough monitoring of the gradual conversion process of tin oxide. Subsequently, Sn originating from both the conversion processes and the core reacted with lithium forming typical intermediate phases such as LiSn, Li5Sn2 and finally Li7Sn5. Upon charge, not only metallic Sn is recovered but surprisingly also tin oxide, confirming the reversibility of the conversion process. These results also underline an overlapping between conversion and alloying processes. Finally, after one cycle, the pristine core-shell morphology is not recovered, particles of 2–3 nm in size are instead observed.