Fabrication and Characterization of Lithium-Sulfur Batteries

Kurzfassung

The lithium–sulfur battery is a promising system for the future generation of rechargeable batteries. Its main advantages are the high theoretical capacity (1675 Ah kgS−1), high energy density (2500 Wh kgS−1), and low cost of sulfur. So far, the commercial application of this battery has been hindered by the reduced cycle-life. The isolating properties of sulfur as well as the formation of polysulfides in a complex reaction mechanism, which is not completely understood, are mainly causes for battery degradation. This work is focused on the characterization of the Li–S battery by application of several characterization techniques under in situ and ex situ conditions. Using X–ray diffraction, the reaction of sulfur was monitored during discharge and charge, and the formation of nano–crystalline lithium sulfide as end product of discharge was identified for the first time in operando. The structural changes of sulfur and its partial amorphization were observed after charge and analyzed using the Rietveld method. Furthermore, electrochemical impedance spectroscopy was applied during cycling to measure the impedance characteristics of the cell. For this, an electrical equivalent circuit was designed to describe specific physical and electrochemical process. Thus, the resistance of the electrolyte, the charge transfer resistance in the electrodes, as well as the reaction and dissolution of isolating products were simulated and quantified. The polysulfides, as well as S8 and Li2S, were investigated in an organic electrolyte using UV–vis spectroscopy. Here, the species S62− and S3•− were identified and semi–quantified at several states of discharge. Further characterization methods, like scanning electron microscopy, atomic force microscopy, and thermal analysis coupled with mass spectroscopy were used to understand the degradation processes that caused morphological changes in the cathode. The output obtained through the application of the different characterization techniques was compared with a physico–chemical model in order to obtain a deeper knowledge in the reaction mechanisms occurring in the battery. Moreover, through further developments on the fabrication process of the battery, main factors influencing the battery capacity were identified. Thereby, the capacity of the battery was increased from 275 Ah kgS−1 to 800 Ah kgS−1 (after 50 cycles, at a discharge rate of 0.18 C).