(Invited) Isotopic Labeling of Carbonate Solvents for Mechanistic Insights into Battery Gassing and Gas Reduction By Fluorinated Organosilanes

Abstract

During cycling and high temperature storage, conventional lithium-ion electrolytes degrade through mechanisms that can lead to insoluble electrode surface films, solution-phase products, and gaseous species. Gas degradation products can cause significant cell swelling, leading to performance and safety problems. Innovations in electrolyte materials that can reduce gassing are highly desired, and gaining a greater understanding of the mechanisms of gas generation can further inform electrolyte development and optimization for reduced gassing.Fluorinated organosilicon (OS) compounds developed by Silatronix® have been studied in lithium-ion batteries, and their inclusion in the electrolyte has been shown to dramatically reduce gassing, as well as increasing cycling stability in nickel-rich battery systems.1 In this prior study, the gas species were identified and quantified, but the mechanistic origin of each gas remained unknown, Therefore, the mechanism of gas reduction by OS, as well as the effects of varying the OS molecule structure, required more investigation.In this study, isotopic labeling of multiple electrolyte components is used to provide insight into the origin of gas products, and the nature of gas reduction by several different OS materials. In a previous study, Silatronix® used 13C-labeled ethylene carbonate (EC) to identify the gases produced from EC during the battery’s first charge, as well as during high temperature storage.2 This follow-up investigation expands the 13C-labeled electrolyte components to include diethyl carbonate (DEC) and dimethyl carbonate (DMC) in addition to EC. Tertiary carbonate blends are used, each containing one 13C-labeled carbonate solvent blended with two unlabeled carbonates. Carbonate-only electrolytes are also compared to electrolytes with several different OS compounds added to the labeled carbonate formulations. NMC811/Gr pouch cells are charged and stored at high temperature, followed by gas generation measurements. GC-MS is used to analyze the gas components, labeled and unlabeled, generated in the battery pouch cells. Gases coming from EC, DEC, and DMC are each identified and quantified. The results show that all OS-containing cells have significant reductions in gas volume (-63%) compared to the carbonate-only cells, primarily through decreasing CO2. The quantification of gases shows that the largest fraction of solvent-generated CO2 comes from EC. All labeled gas species and quantities originating from each labeled carbonate are presented, and the effects of OS on the gases and their source carbonate components are shown, as well as the effect of different OS structures on gas reduction. Overall, this study illuminates in detail the origins of gassing from each of three common carbonate solvents when combined in a complex electrolyte formulation, and demonstrates that organosilicon materials are effective gas-reducing electrolyte components.