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

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.