Synchrotron Near-Field Infrared Nanospectroscopy and Nanoimaging of Lithium Fluoride in Solid Electrolyte Interphases in Li-Ion Battery Anodes

Lithium fluoride (LiF) is a ubiquitous component in the solid electrolyte interphase (SEI) layer in Li-ion batteries. However, its nanoscale structure, morphology, and topology, important factors for understanding LiF and SEI film functionality, including electrode passivity, are often unknown due to limitations in spatial resolution of common characterization techniques. Ultrabroadband near-field synchrotron infrared nanospectroscopy (SINS) enables such detection and mapping of LiF in SEI layers in the far-infrared region down to ca. 322 cm–1 with a nanoscale spatial resolution of ca. 20 nm. The surface sensitivity of SINS and the large infrared absorption cross section of LiF, which can support local surface phonons under certain circumstances, enabled characterization of model LiF samples of varying structure, thickness, surface roughness, and degree of crystallinity, as confirmed by atomic force microscopy, attenuated total reflectance FTIR, SINS, X-ray photoelectron spectroscopy, high-angle annular dark-field, and scanning transmission electron microscopy. Enabled by this approach, LiF within SEI films formed on Cu, Si, and metallic glass Si40Al50Fe10 electrodes was detected and characterized. The nanoscale morphologies and topologies of LiF in these SEI layers were evaluated to gain insights into LiF nucleation, growth, and the resulting nuances in the electrode surface passivity.

Table S1.Longitudinal Optical (LO) mode and transverse optical (TO) mode frequencies from the literature for LiH, LiF, and Li2O.

Figure S1
. SEM of a FIB cross section of the LiF thin films of different thicknesses that were analyzed in Figure 2.

Figure S2 .
Figure S2.HAADF image and STEM EDS mapping of the cross section of the 228 nm LiF thin film.

Figure S3 .
Figure S3.XPS spectra of the evaporated LiF thin film with thickness of 228 nm.Survey spectra is at the top with the Li 1s and F1s at the bottom.

Figure S4 .
Figure S4.XPS spectra of ca. 10 nm evaporated LiF thin-film on a Si wafer.The thickness estimate is based on the fact that Si underlayer is present in the XPS, so the LiF thickness must be less than the photoelectron escape depth which is around 10 nm.

Figure S5 .
Figure S5.AFM topography image of the 228 nm thick, thin film of evaporated LiF.

Figure S8 .
Figure S8.A comparison of SiO2 and LiF thin films and their thickness dependent spectral shape.The 20 nm SiO2 spectra was taken from the TGQ1 calibration sample, and the 300 nm was collected from a

Figure S9 .
Figure S9.SINS amplitudes are referenced to a Si wafer of the 137 nm LiF reference to the spectra of the Cu electrode in Figure3.The 137 nm LiF shows a much higher amplitude response compared to the Cu electrode at 400 cm -1 suggesting that there is less LiF present on the Cu electrode or less near-field coupling.

Figure S10 .
Figure S10.Height profile of the line scan corresponding to Figure 4 showing that the thickness of the LiF is around 24 nm.

Figure S11 .
Figure S11.SINS amplitudes referenced to a Si wafer of the line scan shown in Figure 4.The amplitude values above 1.0 when referenced to a Si wafer are indicative of a phase that is more electronically conductive than Si which strongly supports the presence of Cu metal being present in this region.When the probe is over LiF/Cu the amplitude attenuates significantly due to the electronic insulating nature of LiF.

Figure S12 .
Figure S12.LSV of the thin film a-Si electrode to 0.05 V at 0.01 mV/s.

Figure S13 .
Figure S13.Voltage profile of the formation cycle (from 1.5 to 0.05 V) of the Si40Al50Fe10 splat quenched metallic glass electrode at 47 mA/g of Si + Al (0.05 C).

Figure S14 .
Figure S14.(a) AFM topography of metallic glass after formation cycle with line profiles.(b) Height profile over the areas corresponding to Spot 4 and Spot 6 as seen in Figure 5.

Figure S15 .
Figure S15.SINS amplitudes referenced to a Si wafer of Spots 1-6 shown in Figure 5. Spots 1-3 show higher amplitude values compared to Spots 4-6 indicating a significantly different interaction with the broadband IR light.