Infrared Nanospectroscopy Characterization of Initial Stages of the SEI Layer Formation at The Amorphous Si/Electrolyte Interface

The solid electrolyte interphase (SEI) layer is a crucial component in the function and lifetime of Li-ion batteries; however, the nanoscale mechanism of its growth and evolution are often elusive due to the difficultly in characterizing its structure under in situ conditions. Recently, our group has been utilizing nano-FTIR spectroscopy to investigate the structure and composition of battery interfaces with nanometer scale chemical resolution in a nondestructive manner. For instance, this technique was used to probe the graphene/polymer electrolyte interface <i>in situ</i>¹ as well as the liquid/graphene interface thus allowing opportunities to characterize the electrical double layer.²<br/>The nano-FTIR technique is implemented within an atomic force microscope using a metallically coated tip. By illuminating the tip/sample interface with a broadband infrared laser, plasmonic enhancement occurs at the apex of metallic tip inducing a nanoscopic near-field interaction with the sample. This near-field interaction couples with the infrared active vibrational modes of the sample and causes a change in the amplitude and phase of the backscattered infrared light which can be detected and then separated from the far-field signal with lock-in amplification at the tapping amplitude of the probe. The result is nanoscale, broadband infrared reflection and absorption of the sample with a probing area on the order of the tip radius (~20 nm), far beyond the diffraction limit of conventional infrared microscopy.<br/>In this presentation, we will discuss how nano-FTIR spectroscopy is used to characterize and observe the initial formation of the SEI layer at the interface between an amorphous Si surface and a carbonate electrolyte. By fabricating a 25 nm thin, free-standing amorphous Si film and encapsulating electrolyte with this thin layer, the Si can act as a window for the infrared light as well as an electrode to apply potential to the surface and induce electrolyte decomposition. Because of the nanometer scale spatial resolution and sensitivity to the electrolyte components, we observe subtle nanoscale heterogeneities in the electrolyte composition at the interface prior to the polarization of the Si electrode. After polarization of the cell to 0.5 V vs Li/Li⁺, we observe that LiPF₆ reacts to form LiF at the surface of the Si and find further inhomogeneity in the formed layer with spatial dependent measurements. Based on the technique’s high chemical sensitivity, spatial resolution, and nondestructive nature, <i>in situ</i> nano-FTIR spectroscopy is anticipated to be an important tool to unravel the elusive nanoscale formation and growth mechanisms of the SEI layer.<br/>This research was supported by the US Department of Energy (DOE)’s Vehicle Technologies Office under the Silicon Consortium Project directed by Brian Cunningham and managed by Anthony Burrell. The work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This research used resources of the Advanced Light Source from beamline 2.4, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231<br/><b>References</b><br/>(1) He, X.; Larson, J. M.; Bechtel, H. A.; Kostecki, R. In Situ Infrared Nanospectroscopy of the Local Processes at the Li/Polymer Electrolyte Interface. <i>Nature Communications</i> <b>2022</b>, <i>13</i> (1), 1398.<br/>(2) Lu, Y. H.; Larson, J. M.; Baskin, A.; Zhao, X.; Ashby, P. D.; Prendergast, D.; Bechtel, H. A.; Kostecki, R.; Salmeron, M. Infrared Nanospectroscopy at the Graphene-Electrolyte Interface. <i>Nano Letters</i> <b>2019</b>, <i>19</i> (8), 5388–5393.