Interfacial processes of a model lithium ion battery anode observed, in situ, with vibrational sum-frequency generation spectroscopy

Vibrational sum-frequency generation spectroscopy (SFG) was used for in situ studies of the electrified interface of the negative electrode (anode) of a lithium battery analog. In a lithium battery, lithium is deposited interstitially in a carbon anode. In the model system, which facilitates the study of the solid-electrolyte interphase (SEI) the anode was Au and lithium was deposited on the anode surface. The electrolyte was LiClO₄ plus ethylene carbonate (EC) and diluted with tetrahydrofuran (THF). The SEI created on the anode in this case consists primarily of lithium ethylene dicarbonate (LiEDC) and lithium salts. SFG experiments were run during multiple cycles of reduction and oxidation of a half-cell, corresponding to charging and discharging of a battery. The infrared (IR) pulses were tuned to EC carbonyl transitions or -CH₂ transitions of LiEDC and THF. A model is introduced to describe potential-dependent SFG intensities at electrified interfaces when the electrolyte has intense IR transitions. The electrified interface is defined as consisting of the electrode-electrolyte (or electrode-SEI) interface, plus bulk material located within a Debye length of the electrode. We show that SFG can detect and characterize the underpotential deposition and stripping of lithium on the Au anode, and we observe the growth and structural evolution of the SEI at the anode. During the first three charge/discharge cycles, SEI growth is hindered by the presence of THF, but after the third cycle the THF has been expelled and SEI growth accelerates and then levels off.