Detecting and observing changes in the surface electrolyte interphase (SEI) is difficult due to the small size and amorphous nature of this layer, while its reactivity calls for characterization to be performed in situ, further limiting the applicability of many techniques. Nuclear Magnetic Resonance (NMR) can be performed in situ, but not in conjunction with magic angle spinning (MAS), which is often needed to enhance sensitivity and resolve the broad signal associated with the crystalline environments making up the SEI. It has therefore been necessary to devise alternative NMR approaches to probe the SEI.

Even in an isolated lithium metal electrode in contact with a standard electrolyte, an SEI forms and acts as an intermediary in the exchange between lithium ions in the electrolyte and the metal. Self-diffusion in the metal further complicates this picture, although its impact is only apparent when changes in the local ⁶Li/⁷Li isotopic concentration can be considered. This is possible with NMR and, coupled with the skin effect that limits the detected NMR signal to the micron-thickness surface region of the lithium metal, provides a unique, time resolved “window” into the lithium ion dynamics at the metal-SEI-electrolyte interfaces. We will describe how this “window” can be used to extract information about the growth of the SEI layer and measure the diffusion of lithium ions through it using NMR experiments on well-designed, isotopically-enriched systems (see Figure).

Figure caption: Model of the ion dynamics at the lithium metal electrode – SEI – electrolyte interface. Information about the diffusion, exchange and transport properties of the ions through the SEI can be accessed via carefully designed ⁶Li and ⁷Li NMR experiments.