Chemical additives, such as fluoroethylene carbonate (FEC), can be used to improve lithium-ion battery performance by facilitating the formation of a passivation layer on the negative electrode surface. We compare passivation layers formed by electrolytes with and without FEC, using the ferrocenium-ferrocene redox couple. To quantify experimental findings, we develop a mathematical planar-electrode impedance model to extract physical parameters related to the passivation layer.

Experimental cyclic voltammetry (CV) shows that the electronic passivity of the formed passivation layers is dependent upon the imposed electrochemical conditions. That is, potentiostatic holds cause surface passivation, leading to slower ferrocenium reduction kinetics. With increasing formation time, the high-frequency arc width of the impedance spectra increases for both electrolytes. However, the increase in high-frequency arc width is greater for electrolyte without FEC than that containing FEC. Formation potential influences the magnitude of electronic passivity of the solid electrolyte interphase (SEI), with more negative potentials increasing electronic passivity.

In Figure 1(a), the electrode surfaces have been passivated for 30 minutes under a 0.6 V potentiostatic hold. Two CVs are measured at ν = 10 mV s^-1, comparing electrolytes with and without FEC. The addition of FEC to the primary electrolyte enhances ferrocenium reduction kinetics, when comparing peak heights and position. Figure 1(b) shows experimental and simulated impedance responses, obtained after 30 minutes of passivation around the redox shuttle equilibrium potential (3.24 V). Experimental and simulated impedance responses show good agreement over the frequency range shown, suggesting that reliable conclusions can be drawn about the SEI. Particularly, impedance response simulations suggest that interfacial kinetics and porosity of the outer layer of the SEI greatly influences the high-frequency arc width.