We investigated the decomposition of 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP‐TFSI) based electrolytes on Au and glassy carbon (GC) in the absence and presence of Li(TFSI) and Mg(TFSI)2. Detecting the volatile reaction products via differential electrochemical mass spectrometry (DEMS) measurements allowed us to gain insight into the decomposition mechanisms. In neat ionic liquid (IL) and on Au electrodes both ions are decomposed at reductive potentials. The TFSI− anion mainly decomposes by releasing CF3, while for the BMP+ cation ring opening and scission of either the methyl or the butyl side chain occur in parallel. At oxidative potentials the decomposition of the anion is the predominant process. Changing to the GC electrode increases the fraction of TFSI− decomposition products at both cathodic and anodic potentials. The addition of Li+ largely hinders the formation of volatile BMP‐TFSI decomposition. In contrast, addition of Mg2+ promotes the TFSI− decomposition at the expense of the BMP+ decomposition.

Li-S cells can have high gravimetric energy densities above 300 Wh kg −1 when the electrodes and cell components are optimized. Low electrolyte/sulfur mass ratios or more generallly, the relative amount of electrolyte in a Li-S cell have an especially high impact on the achievable gravimetric energy density. A negative side effect of low electrolyte/sulfur ratios are low cycle numbers due to electrolyte decomposition and the possibility that electrolyte becomes inaccessible at the lithium metal anode when the lithium becomes more and more porous during cycling. Electrode thickness measurements were performed during cycling for various cell chemistries such as lithium-sulfur (Li-S) with different cathode sulfur loadings and porosities, lithium-hard carbon (Li-HC), lithium-silicon (Li-Si), prelithiated HC-sulfur (LiHC-S), prelithiated Si-sulfur (LiSi-S) and Li-ion. The thickness measurements provided information about mechanical stress and irreversible thickness changes. The thickness measurements also helped to explain different electrolyte decomposition behavior and they can be used to discuss the impact of thickness changes on gas analysis. The electrolyte decomposition of the Li-S standard electrolyte based on lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dimethoxyethane (DME): dioxolane (DIOX) with LiNO 3 was examined by online mass spectrometry (MS) within Li-S, Li-HC, Li-Si, LiSi-S and LiHC-S cells. Several electrolyte decomposition products were verified by post-mortem gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) of Li-S cells with cycled electrolyte. Li-S prototypes demonstrate that high gravimetric energy densities of 300 Wh/kg and above can be obtained. Possible strategies to optimize future sulfur cells are the reduction of the weight amount of passive components e.g. by decreasing the thickness of separator and current collectors and/or the application of thin perforated current collectors. Additionally with lithium metal being conductive, the copper current collector can be removed completely providing high energy densities despite low sulfur loaded (1-3 mg cm −2 ) cathodes. By utilizing a copper current collector high sulfur load cathodes (>5 mg cm −2 ) are required to compensate for the copper's passive weight.1 Thick, high load sulfur cathodes reduce the electrode and separator coating length in a cell and therefore save costs. However high load sulfur cathodes also have the drawback that high transported capacities during cycling stress the lithium metal anode and increase the chance of lithium induced shorts 2 (next to the costs of the copper). Despite all this, the electrolyte is a major weight source in Li-S cells even if the electrolyte/sulfur weight ratio (E/S) is low. With the Li-S standard electrolyte based on lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dimethoxyethane (DME): dioxolane (DOL) with LiNO 3 additive, the lowest obtainable E/S ratio is likely >3:1. This is due to the high porosity of the sulfur cathode (usually ∼60-80%) which has ...

The Cover Feature shows the decomposition of an ionic liquid in the presence of Li+ and Mg2+ ions on an Au working electrode. The resulting volatile fragments are detected in the mass spectrometer. More information can be found in the Article by D. Alwast et al. on page 3009 in Issue 12, 2019 (DOI: 10.1002/celc.201900371).