Identifying the Decomposition of Diethyl Carbonate in Binary Electrolyte Solutions in Contact with Silicon Anodes - A Sum Frequency Generation Vibrational Spectroscopy Study

The key factor in the use of lithium ion batteries is the formation of an electrically insulating solid layer that allows lithium ion transport but stops further electrolyte redox reactions on the electrode surface, therefore termed the solid electrolyte interphase (SEI). We have studied the preceding stages to the SEI formation, specifically the reduction of diethyl carbonate (DEC) mixed with ethylene carbonate (EC) with dissolved 1.0 M LiPF6, a common binary electrolyte. We used a p-doped crystalline silicon (100)-hydrogen terminated wafer as the anode in a lithium (Li) half-cell system. We employed in situ sum frequency generation (SFG) vibrational spectroscopy with interface sensitivity to probe the molecular composition of the SEI surface species under various applied potentials where only diethyl carbonate reduction to form a Si-ethoxy (Si-OCH2CH3) surface is expected. We found that even at open circuit potential (OCP) DEC decomposes on the silicon (100)-hydrogen surface to form Si-ethoxy bonds. These findings shed new light on the interfacial Si anode-binary electrolyte solutions chemistry at stages preceding the formation of the SEI on Si anodes.