Reflecting on interphases

Abstract

The use of neutron reflectometry is crucial since the light elements lithium and hydrogen are practically invisible to more commonly used X-ray techniques. A number of detailed insights about the exchange of species between the SEI and electrolyte are revealed in a series of experiments. First, the identity of the Li salt has a profound impact of the SEI formation with a more well-defined layer observed in LiBF₄ than in LiClO₄. Specifically, the structure consists of a thick, diffuse outer layer with a thin compact inner layer at low current density and low cycling that merge into a single layer after increased current cycling. Then, the use of isotopically (deuterium) labelled solvent (THF) and proton donor (ethanol) reveals that the proton donor impacts the inner layer while the solvent controls the outer. In the absence of any proton donor, dendritic growth of Li occurred at the expense of Li₃N and LiH. Finally, neutron absorption also allowed the observation of boron incorporation into the SEI in the case of LiBF₄.

These insights contribute towards explaining the previous observations that the selectivity and stability of Li-mediated N₂RR are improved by the use of fluorinated electrolytes. More broadly, it exemplifies the need for deepened understanding of SEI evolution and the power of SEI engineering through electrolyte additives.