Networks of Electrochemical Oxidation of Common Lithium-Ion Battery Solvents Revealed by NMR Spectroscopy

Current lithium-ion batteries (LIB) are being cycled to increasingly higher voltages to meet demands for higher energy density, at the expense of exceeding the anodic stability limit of the electrolyte, eventually contributing to cell failure. The design of electrolytes that are more robust against degradation and, thus, unlock longer-term high voltage cycling would decisively benefit from detailed knowledge of the fundamental mechanisms that commercial solvents can undergo upon oxidation. To this end, simplified single-salt, single-solvent formulations of LiClO4 and LiPF6 in dimethyl carbonate (DMC), ethylene carbonate (EC), or ethyl methyl carbonate (EMC)) were anodically electrolyzed on inert electrodes and monitored for extended periods of time using 1H, 13C, 19F, and 35Cl nuclear magnetic resonance (NMR) spectroscopy. The controlled environment of the experiments coupled to the unique sensitivity of NMR unveiled novel metastable intermediates and the formation of branching networks of products with widely varying amounts and temporal evolution. Oxidation of the pristine solvent primarily proceeds through a radical pathway that also produces highly reactive protons, but faces competition from a second pathway involving a radical carbocation intermediate. In all cases, the intermediates undergo a variety of different chemical degradation pathways that can also intersect with each other. The resulting network of reactions represents a significant increase in complexity of experimental descriptions in the literature, yet, critically, it helps explain the wide range of products typically identified in electrolyte oxidation studies. The results highlight the need for refocusing fundamental research on anodic stability to analysis of the hierarchy of reaction networks that inform efforts to mitigate the detrimental effects on battery performance, including detection and prevention of proton and radical formation, proton harvesting and radical capture.