In situ observation of solid electrolyte interphase formation in ordered mesoporous hard carbon by small-angle neutron scattering

The aim of this work was to better understand the electrochemical processes occurring during the cycling of a lithium half-cell based on ordered mesoporous hard carbon with time-resolved in situ small-angle neutron scattering (SANS). Utilizing electrolytes containing mixtures of deuterated ( ²H) and nondeuterated ( ¹H) carbonates, we have addressed the challenging task of monitoring the formation and evolution of the solid-electrolyte interphase (SEI) layer. An evolution occurs in the SEI layer during discharge from a composition dominated by a higher scattering length density (SLD) lithium salt to a lower SLD lithium salt for the ethylene carbonate/dimethyl carbonate (EC/DMC) mixture employed. By comparing half-cells containing different solvent deuteration levels, we show that it is possible to observe both SEI formation and lithium intercalation occurring concurrently at the low voltage region in which lithium intercalates into the hard carbon. These results demonstrate that SANS can be employed to better understand complicated electrochemical processes occurring in rechargeable batteries, in a manner that simultaneously provides information on the composition and microstructure of the electrode.