Lithium Metal Diffusion in a Li-Mg Alloy by SIMS

In solid-state batteries with a lithium metal anode, fast diffusion of lithium atoms to and from the lithium/solid electrolyte interface is essential to limit the formation of voids during stripping and the preferential accumulation of lithium atoms during plating, i.e. to have a morphologically stable interface. Lithium metal has a self-diffusion coefficient around 10^-11–10^-10 cm²s^-1 at room temperature. This is not fast enough to sustain large current densities, so that the lithium metal electrode is often morphologically unstable. On the other hand, lithium alloys have shown enhanced morphological stability in contact with the solid-electrolyte, and they have been used as anodes or as interlayers between the lithium metal and the solid electrolyte. Such improved kinetics of lithium alloys have been attributed to a faster lithium diffusion in the lithium alloy than in pure lithium metal. However, only a few lithium diffusivity studies exist and most of them employ pulsed electrochemical techniques in liquid electrolytes, whose relevance to the solid-state case is unclear. Amongst various lithium alloys, the Li-Mg system has attracted particular interest because of the wide lithium solubility range in Mg. However, contrasting information on the lithium diffusivity in Li-Mg alloys can be found in the literature, with diffusion coefficient values ranging from 10^-7 to 10^-11 cm²s^-1 depending on the different methods and experimental conditions used. In this study we use an isotope tracer method to investigate the lithium diffusivity in pure lithium and, for the first time, in a Li-Mg alloy. We deposit by thermal evaporation a ⁷Li thin film on ⁶Li, and a ⁶Li thin film on ⁷Li_0.9Mg_0.1. We then employ secondary ion mass spectrometry (SIMS) to directly observe the isotope diffusion and provide a diffusion coefficient without relying on model assumptions. According to our SIMS results, the lithium diffusivity in Li_0.9Mg_0.1 is similar to the lithium self-diffusivity, both having a diffusion coefficient around 2x10^-11 cm²s^-1. This study challenges the widespread notion that lithium diffusivity in lithium alloys is faster than in pure lithium, hence suggesting that the bulk kinetics of a Li-Mg alloy electrode might be analogous to that of a pure lithium metal electrode in contact with a solid electrolyte.