Solid electrolyte interfaces and interphases in lithium batteries: In Situ studies using nonlinear optical probes

Solid-electrolyte interphase (SEI) regions play a critical role in stabilizing lithium batteries, but little is known about the detailed mechanism of growth and formation. We have developed a novel method for in situ study of the interfacial regions of SEI layers, using an interface-selective nonlinear vibrational spectroscopy method termed femtosecond broadband multiplex vibrational sum-frequency generation spectroscopy (SFG) and a lithium battery electrochemical cell with optical access. SFG has high sensitivity and high selectivity needed to study vibrational transitions of molecular species during the SEI growth. SFG is most sensitive to interfacial regions, so with SFG we ignore the bulk electrolyte and focus on interface regions just a few molecules thick. During SEI growth there are two such interfaces, the electrode-SEI interface and the electrolyte SEI interface. We will present results obtained using a lithium battery and model materials relevant to Li batteries, where during successive cycles of charge and discharge we selectively probe the structural evolution of these two interfaces on Au, Cu and carbon.