Electrode/electrolyte interfaces largely determine the performance of batteries. A fundamental molecular understanding of the interfacial electrolyte structure and the evolution of interphases is lacking. Vibrational spectroscopy offers molecular insights into the electrolyte. Raman and infrared (IR) spectroscopy probe the bulk of the electrolyte. Vibrational sum frequency generation (SFG) spectroscopy, with its intrinsic surface specificity, enables the characterization of electrode/electrolyte interfaces and is, therefore, a unique tool to address the knowledge gaps at interfaces [1,2,3]. The complexity of SFG measurements makes reliable peak assignment challenging, especially in the presence of IR absorption, IR dispersion, interphases, and alloy formation [2,3].

In our current work, we compare the ion-solvent interaction at the interface to the bulk for a broad range of concentrations. Furthermore, SFG has been used to monitor the formation of the solid-electrolyte interphase (SEI) during initial potential cycling. For these applications, thin-layer electrolyte cells were applied. To suppress nonresonant signals, typically fixed delays of 400 fs have been applied between the IR and visible pulse, generating the SFG signal [4,5,6]. For our system, we observed strong dispersion of the IR pulse, prohibiting the use of the common nonresonant suppression. We measured SFG spectra for broad ranges of IR-VIS delays, concluding that a detailed understanding of these artifacts is necessary for each system to ensure a reliable interpretation of SFG spectra. Furthermore, these artifacts may depend on the applied potential and the electrolyte used, thus limiting comparability under varying conditions.

Overall SFG proves a promising technique to investigate electrode/electrolyte interfaces. In our current work, we provide methods to understand SFG artifacts. Future work aims to advance the understanding of the interfacial electrolyte structure and to observe ion (de-)solvation at battery interfaces.

[1] Ge et al. (2020) J. Chem. Phys., 153(17), 170902
[2] Weiling et al. (2022) Adv. Energy Mater., 12(46), 2202504
[3] De et al. (2022) Chem. Eur. J., 28(55), e202200407
[4] Lagutchev et al. (2010) Spectrochim. Acta A, 75(4), 1289-1296
[5] Mukherjee et al. (2013) J. Electrochem. Soc., 159(3), A244-A252
[6] Nicolau et al. (2015) J. Phys.Chem. C, 115(19), 10227-10233