Spatially and Chemically Resolved Degradation of Fluorine-Free Electrolyte on Silicon/Graphite Surfaces

Implementation of fluorine-free electrolytes that are safer and more sustainable than the state-of-the-art highly fluorinated electrolytes requires a thorough understanding of the interphase formation process. This work investigates the effects of LiPF6- and lithium bis(oxalato)borate (LiBOB)-based electrolytes on the electrochemical performance and surface chemistry of graphite, silicon, and silicon-graphite composite electrodes. The LiBOB-based electrolyte degrades more with the presence of silicon in the electrode, and tends to form a thicker solid electrolyte interphase (SEI) layer compared to the LiPF6-based electrolyte. Different degradation distributions were also found in the graphite-silicon composite electrode: The LiPF6 degradation products tend to form on silicon, while the LiBOB degradation products preferentially form on carbon species. These results provide insights into the relationship between electrolytes and electrodes in terms of electrochemical performance, as well as SEI composition and morphology.