Lithium–Sulfur Cells: The Gap between the State‐of‐the‐Art and the Requirements for High Energy Battery Cells

Abstract

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

Visualising the problems with balancing lithium–sulfur batteries by “mapping” internal resistance

“…The increasing resistance causing cell failure was attributed to electrolyte decomposition. It is important to note that the electrolyte volume used here was 6 mL g S À1 -still in too high excess for commercially viable cells 16,21 -and that lower electrolyte volumes tested resulted in cell failure within just a few cycles. Cycle lifetime of B100 cycles is clearly too short for this system to be considered beyond niche applications, but is the inevitable consequence of optimising the amount of electrolyte and negative electrode to that required for high energy cells; it is therefore imperative that the issues resulting from reduced electrolyte and Li excess receive greater attention in future research.…”

“…Nonetheless, lithium foils of at least of 100 mm and possibly far thicker are standard choices in the laboratory, and most previous studies report sulfur electrodes with lower sulfur loadings and far greater electrolyte excesses than we report here. 16 Consideration of the electrolyte volume and anode excess are of particular importance, since the former is currently the most significant barrier to high practical energy density, and both are likely the main contributors to short cycle life. Full experimental details are given in the ESI.…”

Techno-Economic Modeling and Analysis of Redox Flow Battery Systems

“…Some current lithium ion RFBs are several orders of magnitude below this value [25][26][27][28]. The current densities of commercial lithium ion batteries lie within the single-digit mA range; they only have large electrode areas-and hence acceptable volumetric power densities-because of their comparatively extremely thin construction [29]. The more expensive organic electrolyte means that the power density must be far greater, however, and this could prove to be a major obstacle for all RFBs that are based on organic electrolytes or poorly conductive separators, not least because of the poor conductivity of the electrolytes.…”

Recent development of metal compound applications in lithium–sulphur batteries

“…Most of the reports on metal compoundssulphur-based cathodes displayed an areal sulfur loading of 0.3-1.5 mg/cm 2 , 19 which is far less than the requirement of 5.0 mg/cm 2 for practical cells with high energy density of more than 300 W h/kg. 19,163,164 Although some of the carbon-S cathode with an areal sulfur loading has exceeded 5 mg/cm 2 , 165-167 the cycling stability is not ideal. Therefore discovering how to balance the high tap density of metal compounds with the good conductivity of carbon in the nanostructured sulfur cathode is required to realise the practical applications.…”