Spectroscopic Compositional Analysis of Electrolyte during Initial SEI Layer Formation

The energy density of an electrochemical capacitor can be significantly improved by utilizing a lithiated negative electrode and a high surface area positive electrode. During lithiation of the negative carbon electrode, the electrolyte reacts with the electrode surface and undergoes decomposition to form a solid electrolyte interphase (SEI) layer that passivates the surface of the carbon electrode from further reactions between Li and the electrolyte. The reduction reactions that the solvent undergoes not only form insoluble and gaseous byproducts but also electrolyte-soluble products. In this work, those liquid-phase products generated by reductive decomposition of a carbonate-based electrolyte, 1.2 M LiPF6 in EC/PC/DEC (3:1:4), were analyzed at different stages during the lithiation process of an amorphous carbon electrode. An LCMS analysis of the electrolyte during the initial lithiation process was correlated to a DRIFTS analysis of the carbon electrode surface and the results from the EIS and gas-phase analysis previously reported. It was concluded that the formation reactions of the electrolyte-soluble reduction products are dependent on electrochemical processes but are independent of those reactions that directly lead to the generation of the decomposition gases. This is the first time that the composition of the electrolyte during the initial stages of formation of the SEI layer was systematically determined through the use of an in situ electrochemical-MS analysis. The results presented in this work are focused on the initial analysis, elucidation of the empirical formulas, and determining possible structures for these compounds through a detailed analysis of the spectra from each of these unknowns.