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

Abstract

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.

Keywords: LiF; SEI; SINS; anode; interface; interphase; nano-FTIR.

Figure 1

(a) Schematic representation of the operating principle of nano-FTIR spectroscopy. (b) Comparison of the nano-FTIR reflection of different broadband IR sources taken on a Si wafer.

Figure 2

(a) ATR-FTIR spectra of LiF powder, pellet, and single crystal along with the extinction coefficient as a function of the wavenumber from Palik. (b) ATR-FTIR spectra of evaporated thin films of LiF of different thicknesses. (c) SINS absorption and amplitude spectra of evaporated LiF thin films of different thicknesses. HRTEM images of the evaporated LiF thin film with a thickness of (d) 137 nm and (e) 228 nm.

Figure 3

(a) Four initial cyclic voltammograms of a Cu electrode at 0.1 mVs^–1 between 1.5 and 3.0 V. (b) AFM topography image (1 × 0.5 μm, 3.90 nm/pixel) and (c) IR white-light image of the Cu electrode after the CV cycles. (d) SINS absorption spectra at points shown in the topography and IRwhite-light images along with the average, SINS absorption spectra of the 137 nm LiF thin-film reference and the ATR-FTIR spectrum of the dried residual electrolyte (EC/LiPF₆).

Figure 4

(a) LSV of the Cu electrode to 1.5 V at 0.1 mV/s in the pouch exposed to air before assembly to introduce more water and promote more LiF formation. (b) AFM topography (1 × 1 μm, 3.90 nm/pixel) and (c) IR white-light image of the Cu electrode after the LSV to 1.5 V. (d) SINS absorption spectra at spots 1 and 2 and the reference spectrum of the 137 nm thick LiF on Cu. (e) SINS spectral intensity map along the 500 nm line scan shown in panels (b) and (c).

Figure 5

(a) AFM topography (2.5 × 2.5 μm, 12.5 nm/pixel) of the 50 nm thin-film amorphous Si electrode after LSV to 0.05 V at 0.1 mV/s. (b) SINS absorption spectra at spots 1–6 together with LiF thin-film reference spectra and lithium ethylene decarbonate (LiEDC) reference spectra.

Figure 6

(a) AFM topography (4 × 4 μm, 23.5 nm/pixel) and (b) IR white-light image of the Si₄₀Al₅₀Fe₁₀ electrode after the 1 formation cycle at C/25. (c) SINS absorption spectra recorded at different locations of the surface film (spots 1–6) together with LiF thin-film reference spectra.