In Situ Characterization of the Oxidation Behavior of Carbonate-Based Electrolytes for Lithium-Ion Batteries by Scanning Electrochemical Microscopy

Lithium-ion batteries (LIBs) have been widely employed as energy storage devices in portable electronics and electric vehicles. Many processes occurring at the electrode/electrolyte interphases lead to performance degradation over time and yet remain poorly understood. We demonstrate new methods based on scanning electrochemical microscopy (SECM) to characterize LIB electrolyte oxidation, which is a key process occurring at the cathode/electrolyte interphase. Our technique leverages a combination of feedback mode and generation/collection mode SECM to provide an electrochemical reversal technique like commonly used cyclic voltammetry or rotating-ring disk electrode methods but one potentially more easily applicable at shorter time scales. We use our method to probe the oxidation of LIB electrolyte components at nonintercalating electrodes including Pt and glassy carbon. We study the oxidation of a common commercially used electrolyte, 1.2 M LiPF₆ 30% ethylene carbonate and 70% ethyl methyl carbonate electrolyte (LP58), as well as formulations using only the individual carbonates. Our results indicate that all electrolyte formulations characterized oxidize through multiple processes that are detected at distinct voltages. Some processes generate soluble and reducible products that are detected by the SECM tip electrode and may be consistent with deprotonation of carbonate solvents. However, several other oxidation processes do not appear to generate soluble and reducible species and may be connected to the formation of either nonelectroactive products or of processes only occurring on the substrate electrode surface. This work provides information about the electrochemical oxidation of commonly used carbonate electrolytes for comparison to studies involving potentially more complex processes in the presence of LIB cathode materials.