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

Lithium-ion batteries\n(LIBs) have been widely employed as energy\nstorage devices in portable electronics and electric vehicles. Many\nprocesses occurring at the electrode/electrolyte interphases lead\nto performance degradation over time and yet remain poorly understood.\nWe demonstrate new methods based on scanning electrochemical microscopy\n(SECM) to characterize LIB electrolyte oxidation, which is a key process\noccurring at the cathode/electrolyte interphase. Our technique leverages\na combination of feedback mode and generation/collection mode SECM\nto provide an electrochemical reversal technique like commonly used\ncyclic voltammetry or rotating-ring disk electrode methods but one\npotentially more easily applicable at shorter time scales. We use\nour method to probe the oxidation of LIB electrolyte components at\nnonintercalating electrodes including Pt and glassy carbon. We study\nthe oxidation of a common commercially used electrolyte, 1.2 M LiPF<sub>6</sub> 30% ethylene carbonate and 70% ethyl methyl carbonate electrolyte\n(LP58), as well as formulations using only the individual carbonates.\nOur results indicate that all electrolyte formulations characterized\noxidize through multiple processes that are detected at distinct voltages.\nSome processes generate soluble and reducible products that are detected\nby the SECM tip electrode and may be consistent with deprotonation\nof carbonate solvents. However, several other oxidation processes\ndo not appear to generate soluble and reducible species and may be\nconnected to the formation of either nonelectroactive products or\nof processes only occurring on the substrate electrode surface. This\nwork provides information about the electrochemical oxidation of commonly\nused carbonate electrolytes for comparison to studies involving potentially\nmore complex processes in the presence of LIB cathode materials.