Accessing the solid electrolyte interphase on silicon anodes for lithium-ion batteries in-situ through transmission soft X-ray absorption spectroscopy

Silicon offers nine times higher theoretical storage capacity than commercial graphite anodes for Li-ion batteries. For cycling stability, the electrolyte needs to be kinetically stabilized by the so-called Solid Electrolyte Interphase (SEI), a layer which ideally forms once from decomposition products of the electrolyte. While it works for graphite, the SEI on silicon fails to stabilize the electrolyte sufficiently, partly due to the large volume changes upon de-/lithiation. To investigate the SEI on silicon anodes, we developed a novel approach for X-ray Absorption Spectroscopy (XAS), that puts a twist to conventional SiNx window-based liquid cells by utilizing a deliberately induced gas-bubble to form a soft X-ray transparent electrolyte layer. We demonstrate our approach to allow transmission XAS in the soft X-ray regime on liquids and electrode thin-film materials under in-situ conditions. In our case XAS study of the SEI on silicon anodes, we reveal the main SEI constituents as Li acetate, Li ethylene di-carbonate or Li ethylene mono-carbonate, Li acetylacetonate, LiOH, and LiF. Additionally, we see evidence for aldehyde species which we attribute to possible liquid inclusions within a porous SEI morphology. We consider our method an appropriate tool for the successful engineering of a stable, efficient SEI in the future.