Mass and charge transport across the solid electrolyte interphase of lithium-ion batteries: experiments on a Model SEI

The solid electrolyte interphase (SEI) is known as a passivating layer, which is formed on the graphite anode during the first charging of a lithium-ion battery. In the ideal case, the SEI should be highly insulating for electrons, should inhibit the transport of solvent molecules to the surface of the graphite anode and should exhibit a high Li+ ion conductivity. The blocking of electrons and solvent molecule transport prevents further electrolyte decomposition and the co-intercalation of solvent molecules in the graphite anode. Thus, the SEI is essential for the cycling stability of the battery. Remarkably, the effective diffusion coefficient of the redox molecules in the SEI is virtually identical to the effective diffusion coefficient of the Li+ ions obtained from the SEI semicircle in the impedance spectra. Moreover, both diffusion coefficients show the same temporal evolution during the SEI formation process. This suggests that the Li+ ions are primarily transported in the liquid electrolyte phase inside the pores of the SEI and not across the solid phase of the SEI. These findings challenge the textbook knowledge about the transport properties of the SEI