Electrolyte decomposition and gas evolution in a lithium-sulfur cell upon long-term cycling

Experimental data elucidating the time-dependent composition of the electrolyte within a standard lithium-sulfur battery cell are presented. The electrolyte employed consisted of a solution of LiTFSI in a 1:1 mixture of dimethoxyethane and dioxolane, including nitrate salts. In order to track the decomposition reactions of the electrolyte components, the cells were run for a fixed number of cycles, after which they were opened and the remaining electrolyte was extracted with an excess amount of 1,4-dioxane. The amount of each of the components (namely LiTFSI, dimethoxyethane and dioxolane) within these mixtures was determined by means of gas chromatography and 19F-NMR. It was found that the amount of electrolyte accessible to extraction remains relatively constant during the first 25 cycles, but then continuously decreases within the subsequent 40 investigated cycles. The ratio of the organic constituents of the electrolyte does not change considerably, which means there is no preferred decomposition pathway for either of the two components dioxolane and dimethoxyethane, respectively. These results demonstrate that the capacity fading of a lithium-sulfur cell coincides with the loss of the electrolyte within the cell and there is a strong correlation between the failure of a lithium-sulfur cell and the decomposition reactions of its electrolyte. These findings are supported by a detailed analysis of the gaseous decomposition products after specified cycle numbers. Realistic pouch bag cells are used as test vehicles. Such cells do resemble industrially viable, commercial cells much closer as e.g. coin-type cells: Much lower electrolyte/active mass ratios can be used, thus allowing for a more realistic picture of the failure mechanisms involved.