Solvation-derived amorphous motifs in lithium-metal SEI

Abstract

The dominant fraction of the solid electrolyte interphase (SEI) on Li metal is often amorphous, yet most structural assignments continue to rely on techniques that preferentially identify crystalline phases. Here we use total scattering and pair distribution function (PDF) analysis to interrogate the amorphous SEI formed from a localized high-concentration ether electrolyte, M47 (LiFSI: DME: TTE = 1:1.2:3), and to compare it with SEIs formed in related single-solvent electrolytes. The SEI total scattering patterns exhibit two diffuse maxima at high d-spacing, corresponding to the low-Q region of the diffraction experiment, together with sharp reflections from crystalline Li, LiH and LiF. The diffuse features are already present after 30 cycles and persist after 100 cycles, indicating a recurring amorphous structural motif. Strikingly, the two broad maxima resemble the patterns of 5 M and 6 M liquid electrolyte references, suggesting that the amorphous SEI inherits intermediate-range motifs from concentrated-liquid solvation structures. We assign the first, larger-d-spacing feature to a more spatially extended anion-solvent/diluent-cation correlated motif, whereas the second feature arises from a more compact local packing motif. The relative weight of the first feature decreases in the order DME > DEE > F4DEE > F5DEE, consistent with decreasing solvent solvating power. PDF analysis further shows that the short-range order of the amorphous SEI is strongly anion-like and that the enhanced 3.1 Å correlation is best explained by a Li–O-involving second correlation, implying that Li–O environments become more ordered in the amorphous SEI, particularly for the LHCE-derived interphase. These results support a solvation-derived picture of amorphous SEI formation and suggest that liquid-state organization leaves a measurable structural memory in the passivation layer.