The solid electrolyte interphase (SEI) layer is a critical component to the performance, stability and safety of Li-ion/solid-state batteries. Despite the importance of this passivation layer, its initial formation mechanisms and relationship to electrode/electrolyte heterogeneity are still not clear. In this presentation, I will highlight our recently published works that advance techniques for probing this nanoscale interface with vibrational spectroscopy along with new understandings of their formation mechanisms.

In the first part of the talk, I will discuss how we use the unique capabilities of the broadband IR light source at the ALS to probe the SEI formation on Si negative electrodes in carbonate-based electrolytes. By encapsulating the liquid electrolyte with a 25 nm IR transparent Si layer, we probe the electrode/electrolyte interface with the IR near-fields of metallically coated AFM probe to obtain nano-FTIR spectra of the interface with high surface sensitivity and nanoscale lateral resolution. This enabled us to observe the potential dependent reaction of PF₆^- ions at the interface to form a layer of LiF during the initial SEI formation and its resulting heterogeneity. Reasons for how heterogeneity arises at the Si/electrolyte interface and its impacts on cell performance are discussed.

In the second part, I’ll discuss our work using operando Raman spectroscopy to probe the SEI layer formation at the Cu/Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) solid electrolyte interface in the context of solid-state batteries. By using a thin sputtered Cu layer (25 nm) as the current collector, the Cu also acts as a window for the 532 nm Raman laser allowing the detection of the LAGP vibrational modes. We observed that the LAGP irreversibly decomposes at the interface at 1.6 V by reducing Ge⁺⁴ to amorphous Ge⁰. Raman mapping then shows considerable heterogeneity at the reaction front which likely arises from the inhomogeneities of the LAGP surface based on AFM imaging. These results highlight that operando Raman spectroscopy can be an effective tool to evaluate solid electrolyte stability and reaction homogeneity at the interface.