Evidence for stepwise formation of solid electrolyte interphase in a Li-ion battery

Replacement of graphite with alloying and conversion materials, having high specific capacity, has emerged as versatile route to increasing the energy density of Li-ion batteries. A key challenge is the large volume change in these materials, which leads to an unstable solid electrolyte interphase (SEI). The use sacrificial electrolyte additives, such as fluoroethylene-carbonate (FEC), has been established as an effective strategy for considerably improving cycling stability, but a mechanistic understanding of the underlying processes has been lacking so far. Here, we present an in-depth chemical and morphological study of the FEC-based interphase on graphite and SnO2–graphite model electrodes. We found that the FEC decomposition products aggregate first into spherical particles, whose growth depends on the cell medium and follows the laws of crystal-growth theory, before forming a continuous carbonate-rich film. The discrimination of the chemical composition of the FEC-derived particles from the rest of the electrode was obtained by X-ray photoemission electron microscopy (XPEEM) due to the high lateral resolution of this technique. The obtained understanding of SEI formation in fluorine-rich electrolytes should help to guide future designs of sacrificial fluorine-based additives.