A Study on the Decomposition of Lithium-Ion Battery Electrolytes from Different Field-Tested Hybrid Vehicles

In times of the climate change due to global warming partly caused by consumption of fossil fuels, mankind should focus on the prevention or at least reduction of exhaust gases. Emissions of cars powered by internal combustion engines (ICEs) are one source for greenhouse gases. These emissions could be decreased by electric vehicles (EVs) which are charged with regenerative energy. To evade disadvantages of EVs like range or charge time hybrid vehicles (HVs) could be an interim solution.

Lithium-ion batteries (LIBs) are the energy storage system of choice for HVs due to high energy density and high reversibility.^[1] However, also LIBs are limited in lifetime because of internal decomposition reactions of the electrolyte and other battery parts, but mechanisms are yet not fully understood. Another challenge in this context is the development, application and combination of suitable analytical methods due to the complex composition of a LIB electrolyte. In recent years, several investigation methods have been applied regarding electrolyte characterization and decomposition.^[2-8]However, to the author´s best knowledge all investigations were realized on the laboratory scale. Herein, we report the analysis of electrolyte decomposition of battery cells from field-tested HVs.

Besides the inquisitiveness about internal decomposition reactions we are also interested in safety aspects of HVs. We wanted to know how the electrolyte behaves when it escapes due to an accident. Thus, cells were opened in a laboratory hood under exposure of air. The electrolyte was collected through a small drilled hole. The solution was analyzed with gas chromatography coupled to a mass spectrometer (GC‑MS) to identify organic carbonates, additives and volatile decomposition products. Quantification was done with a flame ionization detector (GC‑FID). Ion chromatography (IC) was applied to gain information about the used conducting salt and its amount. Each cell contained LiPF₆. Further IC experiments were carried out by hyphenation to an electrospray ionization mass spectrometer (IC‑ESI‑MS). Besides hydrofluoric and phosphoric acid, several fluorinated phosphates and organo phosphates could be identified. This advanced hydrolysis is partly caused by the reaction with humidity during the cell opening process.

LiBF₄ could be detected with IC via retention time and verified with IC-ESI-MS. It was further proven with hyphenation of IC to an inductive coupled plasma -  mass spectrometer (IC-ICP-MS), inductive coupled plasma - optical emission spectroscopy (ICP-OES) and nuclear magnetic resonance (¹¹B-NMR).

This work is a demonstration of the logical combination of different (hyphenated) investigation methods for the characterization of unknown LIB electrolytes from large scale cells which were applied in field-tested HVs.

We kindly thank the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety for funding of the project LithoRec II (project grant number: 16EM1025) and also the project partners for support and cooperation. Verena Naber and Constantin Lürenbaum are acknowledged for their help during sample preparation and investigations with IC and GC.

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*to whom correspondence should be addressed:

sascha.nowak@uni-muenster.de