Solid-state electrolytes overcome many challenges of present lithium-ion batteries such as safety hazards and dendrite formation. However, detailed understanding of the involved lithium dynamics is missing due to a lack of in operando measurements with chemical and interfacial specificities. Here, we investigate a prototypical solid-state electrolyte using linear and nonlinear absorption spectroscopies. Leveraging the surface sensitivity of extreme-ultraviolet second-harmonic generation spectroscopy (XUV-SHG), we obtained a direct spectral signature of surface lithium ions, showing a distinct blue-shift relative to the bulk absorption spectra. First-principles simulations attributed the shift to transitions from the lithium 1s state into hybridized Li-s/Ti-d orbitals at the surface. Our calculations further suggest a reduction in lithium interfacial mobility due to suppressed low-frequency rattling modes, which is the fundamental origin of the large interfacial resistance in this material. Our findings pave the way for new optimization strategies for these electrochemical devices via interfacial engineering of lithium ions.

See associated manuscript.