Scanning electrochemical microscopy for the in situ characterization of solid-electrolyte interphases: Highly oriented pyrolytic graphite versus graphite composite

The solid-electrolyte interphase (SEI) is of key importance for the performance and safety of lithium-ion batteries. Recently, scanning electrochemical microscopy (SECM) has been used to study in situ the SEI. Here, we compare the SEI of graphite composite electrodes to the SEI at macroscopic samples of binder-free, highly oriented pyrolytic graphite (HOPG) where effects of mechanical processing and particle-particle as well as electrolyte-binder interactions should be absent. Both, short-term changes of SEI passivity on the timescale of seconds and minutes and long-term changes over hours are found on HOPG as well as on graphite composite electrodes, but the SEI at HOPG is more stable compared to graphite composite electrodes. In addition, the microelectrode probe of the SECM is used to induce local SEI damage to monitor its reformation. Subsequently, secondary processes such as local crack formation, gas formation due to electrolyte reduction, and HOPG expansion are studied.