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3.0 2.5 2.0 1.5 I formation of PS II reduction of PS precipitation of Li2 S oxidation of PS IV and formation of S8 depletion of dissolved PS V I IV II charge discharge III III V 0 250 500 750 1000 Capacity / Ah/kgS Figure 3.14: Representative discharge/charge profile of a liquid-electrolyte Li/S cell. Characteristic stages are labeled and described in the text. When the concentration of dissolved S8 becomes high enough, solid S8 can be formed again [155, 156]. The average length of the dissolved polysulfides increases steadily and therefore, plateaus are hardly discernible (IV). Since the precipitating S8 occupies a smaller volume than the corresponding amount of Li2S, pore clogging is usually not an issue during charging. Instead, the end of charge is triggered by the depletion of the electrolyte of polysulfides, whether all material is used up [148] or the dissolution of Li2S cannot keep up with the oxidation reactions [157]. Either way, this causes a steep rise of the cell voltage. In order to fully recharge the cell, a constant voltage phase is applied at the end of charge (V), during which the current density drops steadily, but not immediately, until it reaches the order of magnitude of the leak cur- rent. This technique is known as CCCV charging and also used for many other types of batteries, cf. [158]. This way, additional capacity is recovered without the need for hazardously high voltages or excessively long charging times. Whether or not Li/S cells are also suitable for boost charging [159] has not yet been investigated. Of course, the specific shape of the profile does depend on the composition of the cell. For dissimilar systems, e.g. with solid electrolytes, the charge/discharge profiles look rather different. For liquid-electrolyte Li/S cells, however, the profile shown in Fig. 3.14 is very typical and was observed for all cells in this work. 52 Cell voltage / VPDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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