In Situ Neutron Reflectometry Study of Solid Electrolyte Interface (SEI) Formation on Tungsten Thin-Film Electrodes

• Published in: ACS applied materials & interfaces Vol. 11; no. 50; pp. 47553 - 47563
• Main Authors: Rus, Eric D, Dura, Joseph A
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 18-12-2019
• Subjects: electrochemical neutron reflectometry; scattering length density; solid electrolyte interphase; diethyl carbonate; differential evolution adaptive metropolis; lithium-ion battery; ethylene carbonate; X-ray reflectometry
• ISSN: 1944-8244; 1944-8252
• DOI: 10.1021/acsami.9b16592

Tungsten, a non-Li-intercalating material, was used as a platform to study solid–electrolyte interface/interphase (SEI) formation in lithium hexafluorphosphate in mixed diethyl carbonate (DEC)/ethylene carbonate electrolyte solutions using in situ neutron reflectometry (NR). A NR measurement determines the neutron scattering length density (SLD)-depth profile, from which a composition-depth profile can be inferred. Isotopic labeling/contrast variation measurements were conducted using a series of three electrolyte solutions: one with both solvents deuterated, one with neither deuterated, and another with only DEC deuterated. A two-layer SEI formed upon polarization to +0.25 V vs Li/Li+. Insensitivity of the inner SEI layer to solvent deuteration suggested limited incorporation of hydrogen atoms from the solvent molecules. Its low SLD indicates that Li2O could be a major constituent. The outer SEI layer SLD scaled with that of the solution, indicating that it either had solution-filled porosity, incorporated hydrogen atoms from the solvent, or both. Returning the electrode to +2.65 V removed lithium from both surface layers, though the effect was more pronounced for the inner layer. Potential cycling had the effect of increasing the solution-derived species content in the inner SEI and decreased the contrast between the inner and outer layers, possibly indicating intermixing of the layers.