<i>Operando</i> Infrared Nanospectroscopy\nof the Silicon/Electrolyte Interface during Initial Stages of Solid-Electrolyte-Interphase\nLayer Formation

The solid electrolyte interphase\n(SEI) is a critical\ncomponent\nin Li-ion batteries; however, its nanoscale structure and composition\nand unstable nature make it difficult to characterize and ascertain\nprimary functional mechanisms. We use <i>operando</i> nanoscale\nFourier transform infrared spectroscopy (nano-FTIR) with a broadband\nsynchrotron IR source to study the SEI formation on a thin-film Si\nelectrode at nanometer-scale spatial resolution as a function of time\nand voltage. By probing the Si/carbonate electrolyte interface through\na 25 nm-thick amorphous Si window/electrode, we detect molecular vibrational\nmodes within a 10s of nanometers region adjacent to the Si surface\nand observe that PF<sub>6</sub><sup>–</sup> anions react to\nform LiF at 0.5 V. Spatially resolved nano-FTIR spectra showcase subtle\nnanoscale heterogeneities in the initial solid/liquid interface and\nthe resulting deposited LiF. With its nanoscale resolution and high\nchemical specificity, <i>operando</i> nano-FTIR provides\nunique insights into the dynamics and heterogeneous formation of SEIs\nand opens opportunities for connecting nanoscale interfacial properties\nto bulk performance metrics.