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Gas analysis of ether-based electrolytes in lithium-oxygen cells

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Summary:

Low-cost cell chemistries like metal-oxygen batteries are an essential component of future energy storage systems. Due to its very high theoretical energy density the system lithium-oxygen (Li-O2) is an interesting candidate.

Gas analytical studies of Li-O2 cells with ether-based electrolytes are presented. The electrolytes used consist of 1M LiTFSI in DEGDME respectively TEGDME. The focus is on investigations of non-linear ageing processes such as electrolyte decomposition during cycling. All measurements were carried out using specially developed multifunctional test setups and accordingly modified test cells. Li-O2 measurements at different O2-flow rates were examined by GC-MS and in-operando MS.

Next-generation battery systems typically suffer from severe gassing, which causes a loss of electrolyte and finally the cell to dry out. Consequently, the cycling stability is strongly limited. Gas analysis is a suitable method to identify decomposition and ageing reactions, to benchmark and to define operating parameters. With the GC-MS, a post-mortem analysis could be performed to identify the individual substances qualitatively. In addition, in-operando MS could be used to detect gaseous substances produced by (electro)chemical processes as a function of the state of charge.

As major results, the cyclic formation of several degradation products could be determined. CO2, hydrogen as well as methanol, methyl formate, methylal and 1,3-dioxolane were identified as characteristic decomposition products of DEGDME. In addition, the stepwise decomposition of DEGDME can be demonstrated as a function of the state of charge. These analytical studies make an important contribution to the understanding of the reaction mechanism and ageing reactions in Li-O2 cells with ether-based electrolyte. As a consequence, the shown results help to develop appropriate countermeasures in order to reduce the negative effects mentioned above and thus to ensure a higher cycling stability.

This work is funded by the German Federal Ministry of Education and Research (BMBF) in the project “Osaban” (03XP0227B) in cooperation with University of Kyoto (Japan) and Justus-Liebig-University Gießen.

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