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Li S2− 8 S2− 6 S2− 4 S2− 2 S− Li+ S8 Li2 S Figure 1.3: General layout of a liquid-electrolyte lithium-sulfur cell and proposed re- action mechanism. Assuming pure, solid reactants and products at room temperature, the specific en- ergy of the Li/S cell is ∼ 2600 Wh/kg [36], its energy density ∼ 2800 Wh/l [29, 37], and its specific capacity 1675 Ah/kg, cf. appendix A.2. These values are considerably higher than those of today’s batteries. Taking into consideration that only a fraction of the theoretical capacity can be practically exploited, it is expected that Li/S cells can still achieve 2–3 times the performance of the best cells in use today [30, 33, 37]. Li/S cells have been known for decades; the first patent involving Li/S technology was filed in 1958 [38]. Initially, however, the Li/S system was investigated as a high- temperature battery only [39, 40], i.e. operating at a temperature above 115°C, the melting point of sulfur. In this setup, the positive electrode of the cell is liquid so that material transport and microstructure are of no concern. This concept is widely used in sodium-sulfur (Na/S) batteries [41], but the development of high-temperature Li/S batteries never reached commercialization. In the late 1970s, first attempts were made to build Li/S cells using liquid elec- trolytes at room temperature [42, 43]. With this kind of cell, both sulfur and lithium sulfide are present as solids, but the reduction/oxidation reactions take place among polysulfide species dissolved in the liquid electrolyte. While the precise reaction mech- anism is still discussed controversially [44], it definitely includes several dissolved species of the form Li S and/or S2−. The general layout of such a Li/S battery along 2xx with the reaction mechanism assumed for this study is shown in Fig. 1.3. Despite their very attractive properties, Li/S batteries have not yet been introduced to the battery market, except for some special niches. There are only few companies offering Li/S cells [45–47], but many more are engaged with research and development as well as material supply. Besides issues with the commonly used lithium metal negative electrode [15, 48, 49], there are two main problems causing degradation and other side effects preventing widespread adoption of Li/S batteries so far: First, there is the large mechanic stress induced by the expansion and shrinkage of the electrode due to volume changes of the active material during cycling [50], which eventually 14 Negative electrode Separator Positive ElectrodePDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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