Investigation of processes involved in the formation of the solid electrolyte interphase layer during the primary lithiation of the negative electrode for a lithium ion capacitor

Electrochemically-driven lithiation of an amorphous carbon electrode was evaluated under three configurations: unaided, lithium facing carbon (front-side), and lithium facing current collector (back-side). The results showed that electrochemically-driven pre-lithiation in the front-side configuration produced reversible capacities that were comparable to unaided lithiation, while the back-side configuration produced loading capacities that were approximately 95% of the loading capacities obtained in the front-side configuration. 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. The complex reduction reactions that the solvent undergoes will also generate gaseous and electrolyte-soluble products and they will also have a significant affect on the performance of the device.;During the primary lithiation process of an amorphous carbon electrode, the changes in the composition of the gas phase and the electrolyte was systematically determined at different cell potential stages through the use of in-situ electrochemical-MS analyses. These analyses were correlated with supporting analyses of the SEI layer itself using the DRIFTS and EIS techniques. The results from the gas phase analysis showed that the decomposition reactions that result in SEI layer formation and the generation of decomposition gasses occurs after two reduction steps at different cell potentials and that it is only after the second step are the decomposition gasses generated. LC-MS analyses were used to separate the electrolyte-soluble decomposition products and it was concluded that their formation are the result of other electrochemical processes occurring in the same cell potential range as the first reduction step. A detailed analysis of the mass spectra from each of these compounds first led an elucidation of possible structures for these compounds. Three time-study experiments were then performed to determine how the cell potential affects the formation rate of the electrolyte-soluble compounds. Through a statistical regression analysis of that data, the expected concentration of each electrolyte-soluble compound was extrapolated as a function of the cell potential, which led to a first approximation of the rate constants that govern those reactions.