Electrochemical Analysis of the Cathode/Electrolyte Interface in Li-Ion Batteries by Scanning Electrochemical Microscopy (SECM)

Lithium ion battery (LIB) performance degradation is a complex process involving multiple overlapping, but still poorly understood mechanisms. Processes occurring at the anode-electrolyte interphase, commonly known as the “solid-electrolyte interphase” (SEI) in LIBs are known to stabilize cell performance and have been extensively studied. Similar processes occurring at the cathode-electrolyte interphase (CEI) are less well studied, however, CEI processes are known to form products which drive cell degradation. How these products form, and hence, how they can be controlled remains an active area of research. Multiple processes are known to occur at the CEI including oxidation of the electrolyte, dissolution of transition metals and gas evolution both from electrolyte decomposition as well as decomposition of the cathode material itself. Recent work has shown that electrolyte oxidation processes in the presence of metal oxide cathode materials may occur through multiple mechanisms including direct electron transfer from the electrolyte to the electrode as well as the potential chemical oxidation of the electrolyte by reactive species leaving the cathode at high voltage. Scanning electrochemical microscopy (SECM) is a powerful scanning probe technique that can be used to characterize near surface electrochemical processes under steady state conditions. The technique is particularly well suited to study short-lived reactive species as well as the presence of chemical reactions coupled to electron transfer processes. Our work uses these capabilities to help unravel some of the complex processes occurring as part of the electrolyte oxidation process at the CEI. We use SECM to explore electrolyte oxidation processes occurring at non-intercalating electrodes (Pt and glassy carbon) and compare to companion studies using model metal oxide cathode materials. This allows isolated study of direct electron transfer between the electrode and electrolyte and potential chemical oxidation processes that may occur at the metal oxide surface. Studies of these two processes appear to show both similarities and differences between apparent chemical and electrochemical oxidation routes for the LIB and results will be shared in this presentation.