Probing Solid-Electrolyte Interphase (SEI) Growth and Ion Permeability at Undriven Electrolyte-Metal Interfaces Using ⁷Li NMR

We examine here the exchange of Li ions between electrolyte and metallic lithium with ⁷Li NMR spectroscopy. The measurements quantify the liquid-solid exchange processes, as well as the growth of a solid-electrolyte interphase (SEI) layer. A numerical model including diffusion in the solid phase through atom hopping, radiofrequency penetration considerations through the skin effect, as well as surface exchange explains the experimental trends. Incorporation of the growth of an SEI layer explains the "missing" Li quantities, and, as the SEI layer grows, a decreased ion permeability in dependence on the layer thickness is modeled to explain the long-term trends. These measurements provide indirect probes for SEI growth and permeabilities and also provide a means for quantifying Li diffusion in the metal.