Quantitative Characterization of SEIs and Lithiation/Delithiation Processes in Electrolytes with Additives Using In Situ Electrochemical Quartz Crystal Microbalance-Dissipation (EQCM-D)

In recent years, the crucial role of solid electrolyte interphases (SEIs) in Li-ion batteries (LIBs) has been well recognized by extensive studies. In spite of all available information, there are still issues relevant to the SEI formation, composition, and mechanical properties that have to be addressed. We have recently utilized Electrochemical Quartz Crystal Microbalance with Dissipation (EQCM-D) to study how SEI develops on tin thin film electrodes and probed the mass gain/loss in electrolytes with different amounts of fluoroethylene carbonate (FEC) additives upon electrochemical cycling. As shown in Figure 1, the detailed processes of the decomposition of the electrolyte on the electrode surface and Li alloying/dealloying with Sn were characterized quantitatively by measuring the decrease/increase of the resonance frequency in the cell. The effects of FEC on SEI formation and lithiation/delithiation cycles are also carefully studied and presented.  The results suggest that varying the concentration of FEC is important for adjusting the SEI mass and thickness on the anodes.

The key points of employing EQCM-D in this work are the extension of measurements to higher harmonics and incorporation of dissipation (D).  The capabilities enable the detection of the properties of interfacial films such as rigidity and viscoelasticity.  The final modeling results from EQCM-D demonstrate that the SEI layer is dynamically shifting in the mass, thickness and viscoelastic properties during every cycle. Therefore, given the unique advantage of EQCM-D, it is expected that the full picture of SEI formation, including mechanic properties and compositional change, can be reflected.