An in situ Study of Solid Electrolyte Interface (SEI) Formation on Tungsten Thin Film Electrodes by Neutron Reflectometry with Contrast Variation

Solid-electrolyte interphase (SEI) formation in LiPF₆ in mixed DEC/EC electrolytes was studied in situ by neutron reflectometry (NR) with scattering length density (SLD) contrast variation of the solvent. Tungsten, a non-Li-intercalating material, was used as the working electrode to greatly improve sensitivity to the SEI vs earlier studies.¹ A two-layer SEI was formed upon polarization to +0.25 V vs. Li/Li⁺. Insensitivity of the inner SEI layer to solvent deuteration at this potential suggested limited incorporation of hydrogen atoms from the solvent molecules. Its low SLD indicated Li₂O could be a major constituent. The outer SEI layer SLD scaled with that of the solution, indicating it either had solution filled-porosity or incorporated hydrogen from the solvent, or both. Returning the electrode to +2.65 V removed lithium from both SEI layers, though the effect was more pronounced for the inner SEI layer. Potential cycling increased the solution-derived species content in the inner SEI and decreased the contrast between the outer and inner layers, possibly indicating intermixing of the layers.

1 J E Owejan, J P Owejan, S C DeCaluwe, J A Dura, Chem. Mater.24 (11), 2133 (2012) [DOI: 10.1021/cm3006887]