Recent electrode and electrolyte developments for Lithium-Sulfur prototype cells and their present and future applications

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Lithium-Sulfur battery cells already achieve high specific energies up to 470 Wh/kg, but still suffer from a low cycle life. Several components need further development in order to tackle the obstacles still limiting volumetric and gravimetric energy as well as power density:
With the state of the art electrolyte (SOTA) concept, polysulfides (PS) as reactive intermediates emerge during discharge. This contributes not only to the electrolyte depletion – it also leads to active material loss, cell dry out, and a low Coulombic efficiency. Hence, adjusting the polysulfide solubility in that way that the conversion mechanism still works efficiently, but the diffusion to the anode is minimized, is an effective approach to intrinsically prevent the so-called shuttle mechanism. In addition, sulfidic solid electrolytes are currently discussed to avoid polysulfide dissolution completely. For both electrolytes, the cathode porosity and carbon-electrolyte interface design need to be adapted again to enable an electrochemical sulfur/lithium sulfide utilization both in sparingly polysulfide solvating and sulfidic solid-state electrolyte concepts. With the latter, promising data on almost theoretical sulfur utilization (> 1600 mAh/g) have been already gained for carbon/sulfur-composites in combination with argyrodite-based solid electrolyte with low electrolyte:sulfur mass ratio (E:S < 2). The DRYtraec® process developed by Fraunhofer IWS is a promising candidate to enable highly densified cathodes for liquid electrolyte as well as processable carbon/sulfur/solid state electrolyte films for prototype manufacturing. It has been turned out that the sulfur utilization strongly depends on the electrolyte type when different pressures are applied on Li-S pouch cells with cathodes prepared by the DRYtraec® process. As for increasing the power density of Li-S cells, the carbon material in the cathode needed to be adapted from porous carbon blacks to a carbon-nanotube (CNT) scaffold. A new sulfur infiltration process was developed, and especially under lean electrolyte conditions, the combination of CNT-based cathodes with sparingly PS solvating electrolytes outperforms the SOTA electrolyte concept. Thus, energy density, cycle life, rate capability and temperature dependence of the resulting cells will be evaluated and their present and future applications will be discussed. This work has received funding from the Federal Ministry of Education and Research (BMBF), support code HiPoLiS (03XP0178A), LISA (GA814471), SoLiS (03XP0395A), SkaLiS (03XP0398c).

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