Probing lithium mobility at a solid electrolyte surface

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 materi..., See associated manuscript.,