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



Eric Daniel Rus, Joseph A. Dura


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 (EC) 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 labelling/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 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 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.
ACS Applied Materials and Interfaces


lithium ion battery, electrochemistry, neutron reflectometry, fitting, solid electrolyte interphase
Created December 17, 2019, Updated September 8, 2020