Analysis of the Decomposition of Sulfur-Based Electrolyte Additives in Spent LiNi0.6Co0.2Mn0.2O2||AG Cells

Abstract

The development and optimization processes of the last decades in the context of lithium ion batteries (LIBs) have led to a variety of electrode chemistries, which enable application-specific targets such as high energy, high power, high safety or long lifetime. Most of these LIB systems rely on a liquid carbonate-based electrolyte, whose basic composition has changed scarcely during this period.1 In this respect, the usage of electrolyte additives is one of the most economical and effective ways to further improve the overall performance, while also impact the application-tailored characteristics of the battery cells at the elctrolyte level without changing the bulk properties.2 These improvements can range from overcharge protection to lowered flammability or interphase formation, among others. In particular, film-forming additives signifcantly influence the performance and life time of LIBs by preventing excessive decomposition of the electrolyte, which is in contact with the positive and negative electrodes, by the formation of a passivation layer. Investigating the electrochemical decomposition of the electrolyte components, especially electrolyte additives, is key to enable a better understanding of the composition from the electrode||electrolyte interface layer and the effects on LIB cell performance.3 Thus, aging mechanisms and parasitic reactions can be elucidated to improve and design new electrolyte additives.Herein, the irreversible decomposition of sulfur-containing electrolyte additives will be addressed. Sulfur-containing additives represent an attractive option for film-forming additives as these compounds endow lower energies in the lowest unoccupied molecular orbital (LUMO) compared to the organic carbonate analogs and therefore are more susceptible to electrochemical reduction.4 However, these class of electrolyte additives find only a limited use in state-of-the-art electrolyte mixtures due to the often reported or suspected carcinogenicity and toxicity towards the human body. Nevertheless, since sulfur-containing additives still play an important role in spent LIBs of recent decades, identification of these hazardous compounds and formed aging products, which should be also classified as potential dangers, is of great interest.1 This contribution focused on the identification of aging products emerging in electrolytes from LIB cells using sulfur-based electrolyte additives of different chemical classes such as sulfates, sulfites, sultones and sulfonates. Ion chromatography and gas chromatography hyphenated to high-resolution acurrate mass spectrometry (HRAM-MS) were used to obtain information on the formation of ionic and volatile compounds after electrochemical operation and to elucidate the corresponding structures. Thus, specific ionic and volatile aging marker molecules could be defined for target-analysis of the original electrolyte additive in LIB material, despite a potentially complete consumption during cycling. This allows improved reverse-engineering of LIB post-mortem analysis and risk assessment. Furthermore, mechanistic conclusions on the decomposition of the investigated electrolyte additives could be drawn, extending literature reports based on X-Ray methods with respect to species information and ultimately contribute to a better understanding of the interphase constitutions.[1] C. Peschel, S. van Wickeren, A. Bloch, C. Lechtenfeld, M. Winter, S. Nowak, Energy Technol. 2022, xxx.[2] S. S. Zhang, J. Power Sources 2006, 162, 1379–1394.[3] J. Henschel, J. M. Dressler, M. Winter, S. Nowak, Chem. Mater. 2019 31 (24), 9970-9976.[4] B. Tong, Z. Song, H. Wan, W. Feng, M. Armand, J. Liu, H. Zhang, Z. Zhou, InfoMat 2021, 3, 1364–1392.