An Unwanted Guest in the Electrochemical Oxidation of High-Voltage Li-Ion Battery Electrolytes: The Life of a Superacidic Proton

Lithium-ion batteries (LIBs) are central to the urgent societal need to decarbonize both transportation and energy storage on the grid. Unfortunately, despite their attractive energy/power density and high coulombic efficiency, further improvement of this technology – especially their durability – is desperately needed. To support these efforts, our study focuses on fundamental understanding of the decomposition pathways for LIB electrolytes at the cathode-electrolyte interface (CEI), as the nature of these reactions directly controls the extent to which cell capacity and voltage decays in these systems. In this study, we employ electrochemical methods, coupled with product analysis using NMR spectroscopy and mass spectrometry, to determine the decomposition mechanisms for a model electrolyte (1.3 M LiClO 4 in EC) and demonstrate the impact of these mechanisms on technologically relevant systems (1.0 M LiPF 6 in EC/EMC). Remarkably, we discover the electrochemical formation of superacidic protons, which provides a unified origin for a myriad of side reactions in every individual component of the CEI. In particular, electrochemically generated protons chemically catalyze EC decomposition into CO 2 and other short and long chain ethers. They also react with LiPF 6 to form the weaker acid HF and are involved in a direct acid-base reaction with the cathode active material, causing the leaching of transition metals into the electrolyte. Collectively, the results of this study all point to the urgent need to either mitigate this proton formation or introduce benign harvesting additives via new electrolyte design strategies. ___________________ The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan. http://energy.gov/downloads/doe-public-access-plan