For a mass commercialization of Li-S chemistry the gravimetric energy density must be clearly above that of state-of-theart lithium-ion cells (with the Panasonic NCR18650B as current energy density champion) to compensate for the much lower cycle stability. The number 18650 describes the cell's shape with a diameter of ≈18 mm and a height of ≈65 mm. The NCR18650B provides a capacity of ≈3.3Ah with a nominal voltage of 3.6 V resulting in a gravimetric energy density of ≈240 Wh kg −1 and a volumetric energy density of ≈670 Wh L −1 . Additionally, the corresponding cell type can achieve several hundred cycles until 80% of the initial capacity is reached. By contrast, although high cycle numbers are reported for Li-S cells in the literature, the fl aw is that these high cycle numbers are only obtained because of an excess of lithium, an excess of electrolyte and low sulfur areal loads, [ 7 ] resulting in very poor potential gravimetric energy density. Figure 1 shows the gravimetric and volumetric energy density of various electrochemical energy storage systems. The Li-S cell manufacturers Sion Power and Oxis Energy expect that future Li-S cells will have a volumetric energy density comparable to that of state-of-the-art Li-ion cells (≈700 Wh L −1 ) but more than twice the gravimetric energy density with values of 400-600 Wh kg −1 .The scope of this article can be summarized as follows:• A Li-S review will be provided focusing on statistical information like sulfur load and sulfur electrode fraction which determine the energy density and discussing the state-of-theart of the worldwide Li-S research.• By opening an NCR18650B we obtained information about the passive weight distribution of state-of-the-art high energy 18650 cells. With this information we were able to calculate the possible energy densities and prices of future Li-S cells for various sulfur loads, sulfur utilizations, and electrolyte/ sulfur (E/S) ratios. Keeping in mind that a Li-S cell must have a superior gravimetric energy density to the NCR18650B these results provide insights into which electrode properties and electrochemical results must be obtained. Additionally, they allow an evaluation of the state of the art of international scientifi c Li-S research.• Finally, an electrode that meets important demands for high gravimetric energy densities is introduced. Li-S cells

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To obtain information about the Li-S reaction mechanism through spectroscopy, a Raman literature study, polysulfide vibrational mode calculations and experimental in-situ measurements were performed. A special test cell was constructed to examine in-situ Raman spectra in the spectral range from 100 cm−1 to 600 cm−1 during charge and discharge in the voltage range of 1.5 V to 3.0 V. In order to assign the in-situ Raman data and to support the interpretation of the observed changes in the overall Raman-spectrum, several reference measurements on well-defined substances were conducted. The reference measurements included pure solvents, electrolytes and polysulfide solutions prepared from stoichiometric mixtures of S8 and Li2S powders. The assignment of the observed Raman-spectra was further based on a comparison with purely theoretical data for the vibrational modes of the polysulfide di-anions Sn2− and radical mono-anions Sn− calculated at the B3PW91/6–311G(2df,p) level of density functional theory (DFT). The DFT data for the vibrational spectra, corrected for solvent effects in the framework of the polarizable continuum model (PCM), allowed an identification of several characteristic features in the in-situ Raman spectra.

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