Formation Mechanism and Stability of the Solid Electrolyte Interphase in Propylammonium- and Pyrrolidinium-Based Ionic Liquid Electrolytes

A new in-situ method capable of probing the solid electrolyte interphase (SEI) formation mechanism and stability is reported. In this method, the electrochemical window of the electrolyte is first obtained and is followed by measurement of the so-called differential capacitance. The different regions within the differential capacitance plot are indicative of several processes (electrolyte degradation, electrolyte reorientation), and they are then compared against the findings of a third technique, the electrochemical quartz crystal microbalance. Overall, the use of 3 distinct techniques allows us to demonstrate that the SEI is formed as a result of both electrolyte reorientation/nanostructured interfacial layer (NIL) composition changes as well as electrolyte degradation and adsorption. Our study of 2 different electrolyte systems (propylammonium- and pyrrolidinium-based) was able to discern significant differences, with the propylammonium-based electrolyte having a less rapid desorption/dissolution when the electrode returned to the OCP condition, indicating that the SEI shows a higher degree of stability. Given the importance played by both the NIL structure and the SEI layer in the cycling performance of a battery, the proposed methodology will help provide a new electrolyte screening method. The SEI stability is used as a selection criterion for the electrolyte candidates likely to show the best cycling performances.