Behavior of lithium polysulfides in different electrolytes for lithium-sulfur batteries

Kolek Martin, Jalkanen Kirsi, Wellmann Julia, Bieker Georg, Winter Martin, Bieker Peter

Zusammenfassung

Lithium-sulfur batteries (LSBs) are promising energy storage systems for applications where the gravimetric energy density is important, such as planes, drones, or satellites. Furthermore, the abundance, the low cost, and the non-toxicity of sulfur make it an attractive choice for the cathode active material.[1] However, despite of intensive research, the commercialization of LSBs is still in its infancy, even if prototype pouch cells are already available and the number of companies in this field is growing. The most important challenges of LSBs are a fast capacity fade and self-discharge. These drawbacks are mainly related to the insulating nature and large volume change of the sulfur cathode and to the formation of polysulfides as soluble intermediate reaction products during charge and discharge [2, 3]. These polysulfide species can be present as di-anions or radical anions with different chain lengths, according to a variety of electrochemical reactions as well as dissociation and disproportionation equilibria [3, 4]. On the one hand, a high solubility of the polysulfide intermediates would be beneficial for enhancing the reaction kinetics. On the other hand, the dissolved polysulfides can migrate through the electrolyte to the lithium anode, causing not only detrimental reactions on the lithium surface but also self-discharge of the cell, if the polysulfide species are reduced and migrate back to the cathode (the so-called shuttle mechanism). To be able to control these phenomena, a further understanding of the complex polysulfide chemistry in different electrolyte media is needed. This work focuses on studying the solubility and behavior of polysulfide species in different organic solvents used in LSBs, such as glycol ethers. Different concentrations of polysulfide species are generated by dissolving a mixture of S8 and Li2S in the studied solvents, after which the solutions are investigated by UV/Vis spectroscopy in order to characterize the formed species. Depending on the physicochemical properties of the solvent, like the donor number or the Li+-coordination ability, polysulfide species with different chain lengths are stabilized [3, 4]. In order to draw conclusions about the effect of the solvent's physicochemical nature on the formation of different polysulfide species, UV/Vis measurements with a reference system (high donor number: DMSO) are performed. The obtained data is combined to give a better understanding of the behavior of polysulfide species in relevant LSB solvents. This information will help to design solvents and solvent mixtures with desired polysulfide dissolving properties for realizing LSBs with enhanced performance.