Understanding the Concurrent Influence of Electrolyte System and Cathode Microstructure on Lithium-Sulfur Battery Performance

Kurzfassung

Lithium-sulfur batteries have an extraordinarily high theoretical capacity of 1.672 mAh/g(S). Sulfur is inexpensive, non-toxic, environmentally friendly and available in large quantities. However, the cycle stability of lithium-sulfur batteries has so far been low, as it is limited by the solubility of the polysulfides, that are formed in the during discharge, in the electrolyte and the associated morphological changes in the cathode. In order to obtain the microstructure of the cathode after discharging and charging, in this work, composites were produced. These composites with a defined microstructure consisted of sulfur and so-called ultramicroporous (UMP), microporous (MP) and micro/mesoporous carbons as host materials. Among other, the infiltration method was varied in this work. Due to industrially relevant production with lower costs compared to gas phase infiltration, melt infiltration might find its application in the industry and is therefore relevant for successful commercialization. In addition, the influence of different electrolytes on the performance of the resulting Li-S batteries was investigated. The electrophilic carbonates react irreversibly with, in particular, short-chain and therefore more nucleophilic polysulfides to form thiocarbonates. Although this has the negative effect of reducing the active material and forming an insulating cover layer on the electrode surface, especially with excessive thiocarbonate formation, the pores can be closed and the encapsulated sulfur can be protected from further reaction with solvent, if thiocarbonate formation takes place at the pores entrance in a controlled way. Thiocarbonate is thus part of the cathode-electrolyte interphase (CEI) which, depending on the structure of the substrate material, enables the combination of sulfur cathodes with carbonate solvents for Li-S batteries. Cathodes were prepared from the composites produced in this work and their cycle stability was compared on the basis of their discharge capacity over the number of cycles. The cells were cycled until their practical discharge capacity was lower than 800 mAh/g(S). In addition, the morphology of the carbon host was characterized by porosimetry and the resulting cathodes were characterized via scanning electron microscopy. It was shown, that the so-called quasi-solid-state behavior known from literature could be reproduced with this type of infiltrated cathodes in combination with carbonate electrolyte. The influence of the solvent dependent pore penetration depth was also investigated. In addition, special attention was paid to the pore structure dependent comparison of the carbon host materials. For this, it was shown that the larger pore volume of the MP and micro/mesoporous carbon made it possible to increase the sulfur content in the composite, but this was associated with considerable losses in the achievable capacity. However, by selecting special electrolytes and reducing the discharge potential, a capacity of > 800 mAh/g(S) for 100 cycles could still be achieved with a doubled sulfur content in comparison to the UMP carbon material. Furthermore, significant influences of the grinding process of the composite production and the so-called formation were identified.