Double layer capacitance as a sensitive metric to monitor the formation of solid electrolyte interphases in Li–ion batteries†

In Li–ion batteries with conventional liquid electrolytes, the formation of solid electrolyte interphases (SEIs) at carbonaceous anodes prevents continuous electrochemical decomposition of the electrolyte. Typically, SEI formation and electrolyte decomposition are examined with linear potential scans, where time and potential dependencies are intertwined. Herein, a stepwise potential variation in combination with amperometry and electrochemical impedance spectroscopy (EIS) is used to characterize the impacts of time and potential as individual degrees of freedom on the SEI formation. Based on EIS data, the double layer capacitance (DLC) is introduced as a sensitive in situ metric to monitor the SEI formation. This technique is used to show the similarities and differences in the SEI formation processes with typical Li–ion battery electrolytes consisting of hexafluorophosphate and carbonate solvents. A polished glassy carbon electrode is employed to provide model-like EIS data with reliable interpretation. Changes in the electrochemical interface within only few atomic layers are tracked with DLC, indicating that SEIs are formed below 1.9 V vs. Li/Li⁺ with the employed electrolytes. Amperometry measurements show that the decomposition of the employed electrolytes starts at approximately 2.7 V vs. Li/Li⁺, displaying smaller electrochemical windows than those previously reported.