Decomposition of Electrolyte Solutions at Positive Electrodes

Abstract

Preventing the decomposition reactions of electrolyte solutions is essential for extending the lifetime of lithium-ion batteries. However, the high voltage (>4.5 V) reactivity of commonly used electrolyte solutions is still poorly understood; electrolyte decomposition at the positive electrode is generally believed to proceed via electrochemical oxidation of the organic carbonate solvent,1 while more recent work has revealed an alternative pathway involving reactive lattice oxygen species originating from the positive electrode (i.e., singlet oxygen).2 Unfortunately, we still have a limited understanding of these reactions, the products that form, and which chemical structural units in the electrolyte solution make them reactive - all of which make it challenging to assess and mitigate the impact of electrolyte decomposition on the battery’s lifetime.In this work, operando gas measurements and solution NMR were combined to study the electrochemical and chemical decomposition of widely used electrolyte salts, solvents, and additives. First, the electrochemical stability of the electrolyte components was reviewed against C65 electrodes in a standard H-cell setup: all solvents reveal little decomposition up to 5.5 V (vs Li/Li+), consistent with previous electrochemical measurements.3,4 Based on the experimentally detected soluble and gaseous products, the electrochemical decomposition mechanism is inferred. Subsequently, the chemical stability against reactive oxygen species (i.e., singlet oxygen; 1O2, peroxide anion; O2 2 −, and superoxide radical anion, O2 −) was explored. All carbonate solvents demonstrated instability against reactive oxygen species. As before, the experimentally detected gaseous and soluble products were used to infer the decomposition mechanisms. The proposed mechanisms were verified using 17O and 18O isotopic labeling for NMR spectroscopy and mass spectrometry measurements, respectively. The findings on the reactivity between the electrolyte components and reactive oxygen species provide a deeper understanding of electrolyte decomposition processes occurring at high voltage positive electrodes (e.g., Li-rich layered and disordered rocksalt oxides, high-voltage spinels, and polyanionic materials). Moreover, these insights will support the development of more stable electrolyte formulations, further extending the lifetime of lithium-ion batteries.(1) Imhof, R.; Novak, P. Oxidative Electrolyte Solvent Degradation in Lithium-Ion Batteries: An In Situ Differential Electrochemical Mass Spectrometry Investigation. J Electrochem Soc 1999, 146 (5), 1702–1706.(2) Jung, R.; Metzger, M.; Maglia, F.; Stinner, C.; Gasteiger, H. A. Oxygen Release and Its Effect on the Cycling Stability of LiNi x Mn y Co z O 2 (NMC) Cathode Materials for Li-Ion Batteries. J Electrochem Soc 2017, 164 (7), A1361–A1377. https://doi.org/10.1149/2.0021707jes.(3) Zhang, X.; Soc, J. E.; Zhang, X.; Kostecki, R.; Richardson, T. J.; Pugh, J. K.; Ross, P. N. Electrochemical and Infrared Studies of the Reduction of Organic Carbonates Electrochemical and Infrared Studies of the Reduction of Organic Carbonates. Journal of The Electrochemical Society 2001, 148 (12), A1341–A1345. https://doi.org/10.1149/1.1415547.(4) Moshkovich, M.; Cojocaru, M.; Gottlieb, H. E.; Aurbach, D. The Study of the Anodic Stability of Alkyl Carbonate Solutions by in Situ FTIR Spectroscopy, EQCM, NMR and MS. Journal of Electroanalytical Chemistry 2001, 497, 84–96. https://doi.org/http://dx.doi.org/10.1016/S0022-0728(00)00457-5.