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Li-S cells have a low voltage (∼ 2.1 V), but their potentially high energy density (200-500 Wh/kg) makes them a promising system for next generation batteries. To obtain high energy densities on cell level, the weight fraction and load of the active material should be as high as possible, while inactive material is reduced to a minimum. Conventionally, sulfur slurry cathodes with an aluminum current collector are used. However, binder-free CNT-coated carbon structures are a promising method of achieving higher loads and higher ratios of active material. Using a specially designed test cell it was demonstrated that sulfur cathodes without a metal current collector can deliver enough power to meet the requirements of consumer electronics at simultaneously high capacities of up to 600 mAh g −1 for the entire electrode and current collector. A literature study compared various equivalent circuits used for Li-S electrochemical impedance spectroscopy (EIS), and enabled the selection of the most suitable one for the system used here. EIS measurements during charge and discharge delivered vital information about the specific resistances of the sulfur cathodes with a carbon current collector.Li-S cells deliver high theoretical capacities of 1672 mAh g −1 at a relatively low average discharge voltage of ∼ 2.1 V, thus providing energy densities of around 200-500 Wh kg −1 on cell level. In most publications, electrodes with sulfur loadings below 2 mg cm −2 are reported, 1 leading to active mass ratios clearly below 40% for the entire electrode and current collector. We believe that this ratio can be improved in some applications with smaller cell size (e.g. smartphones) by replacing the polymer binder and the metal current collector with a carbon current collector in a binder free sulfur cathode.Recent publications also show a growing interest in binder-free electrodes: Elazari et al. demonstrated good sulfur utilizations between 50-60% with a microporous activated carbon fiber cloth and a sulfur load of 6.5 mg cm −2 . 2 Zhou et al. prepared a flexible CNTbased membrane (CNT-S: 2-3 mg cm −2 ) and obtained high capacities of around 700 mAh g −1 S at very high currents of 6 A g −1 . 3 The high rate performance of these CNT-based electrodes is confirmed by Su et al. who achieved around 900 mAh g −1 S at 4C. 4 Kim et al. examined the effects of high temperature conditions on cell capacity, rate capability and cycle durability of a vertically-aligned CNT electrode synthesized on a Ni substrate by a CVD process. 5 With a comparable electrode acceptable capacities of around 700 mAh g −1 S were reported at sulfur loads of 7.1 mg cm −2 and 90 wt% sulfur in the electrode. 6 Higher sulfur loads of up to 20 mg cm −1 with sulfur utilizations around 50% can be obtained with CNT-coated carbon fiber structures .7The electrode we applied also consists of CNTs coated on a carbon fiber structure by a CVD process with subsequent melt infiltration of sulfur. Our belief is that that the optimization of such a cathode can allow active material rat...

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 ...