(75b) Porous Composites As Host Materials for Lithium-Sulfur-Batteries (Invited)

Scalability, efficiency and economic feasibility are important aspects for the fabrication of cathode materials for lithium-sulfur batteries. For the application of lithium-sulfur batteries in electric vehicles and stationary energy storage systems, different challenges need to be overcome to achieve sufficient energy and power density within weight and volume constraints. Further safety requirements and the reproducible fabrication of large active material quantities at low costs have also to be guaranteed for a wide range of applications [1]. Sulfur allows high theoretical capacities of 1675 mA h g^-1 as cathode material in comparison to lithium, resulting in a theoretical energy density of around 2600 W h kg^-1 for lithium-sulfur batteries [2-5]. However, using sulfur as active material leads to compositional and structural changes and large volumetric expansion up to 80 % during cycling, aspects which must undoubtedly be considered. Besides, reducing the formation of polysulfides as adverse byproducts and insulating Li₂S is still a challenge to be solved [1, 6, 7]. As a consequence of these insufficient cycle stabilities, low coulomb efficiencies [3] and high self-discharge rates [4, 8], a barrier has been built against commercialization [2, 7, 9-11]. Porous host materials with defined pore structures which are able to incorporate certain contents of sulfur have been postulated as a possible solution to compensate the drastic volumetric expansion during cycling. A widely-used method to incorporate sulfur within host materials is the melt-diffusion process.

As published in Zellmer and Titscher et al. [12], different aspects, such as the used host material and its specific properties, the parameters of the comminution process as well as the formulation strategy to prepare the electrodes were investigated to evaluate their influence on the electrochemical performance of the lithium-sulfur-battery. Concerning the host material, the content of sulfur, the pore volume, the particle size distribution, the crystallinity and the conductivity of the material were characterized. Subsequently, the host materials, prepared via hydrothermal carbonization or purchased, were used for the comminution process in different mills. As published in [13 -16] planetary ball mills can be used to incorporate sulfur into host materials. The process can be optimized by adjusting the process time and including thermal treatments. Based on the specific energy input, sulfur can be embedded within the material but at the same time pores within the material can be removed [13-16]. Experimental studies and DEM simulation were performed to characterize the operation of different mills with regard to the incorporation of sulfur and to manufacture homogeneous composites without the combustion of filled pores. A planetary ball mill was chosen in this study in comparison to the literature and additionally, a vibration mill was used to prepare the sulfur carbon composites under lower stress energies than the ones arisen during the process in the planetary ball mill. The vibration mill enables the preservation of the pores while assuring simultaneous homogenization of the product and shows considerable potential for the scale-up [17-19]. For the evaluation of the influence of the mechanical treatment in the vibration mill on the preparation of homogeneous composites, different carbon host materials with various pore size distributions were processed and characterized after each process step regarding the content of incorporated sulfur and the resulting particle size and pore size distribution. Different aspects were found in comparison to the investigations dealing with the planetary ball mill. On the one hand, smaller particle sizes and thus, lower contents of incorporated sulfur within the carbon host material were detected due to the higher specific energy within the planetary ball mill. On the other hand, more specific power is transferred into friction events, and thus heat can be dissipated leading to an improved incorporation of sulfur in the pore structure. This fact can also be discerned and determined via DEM simulations. Further on, only a homogenization and not an elimination of the composite (sulfur and host material) take place because of the lower frequency of events in normal direction [20]. Subsequently, electrodes were produced in order to characterize the material, understand the fabrication process and determine the electrochemical performance of the manufactured composites. To this aim, water was used as environment-friendly solvent and PEDOT:PSS as conductive binder to fabricate cathodes for such lithium sulfur batteries [6, 21]. The influence of the composite structure after the manufacturing within the different mills was also discerned via the electrochemical characterization. After analyzing the electrochemical performance, it was clearly observed that higher specific capacities at low cycle numbers were reached using the composites prepared via the planetary ball mill process. Nevertheless, a constant decrease of the specific capacity was also detected over the lifetime. In the case of the composites manufactured in the vibration mill, lower specific capacities for the first cycles were measured, but constant capacities over the whole lifetime were achieved. Additionally, significant influences of the different host materials on the electrochemical performance were also investigated.

Various particle sizes and thus, different formulation strategies and electrochemical performances result from the specific energy input based on the different comminution devices, namely vibration and planetary ball mill. In current studies, the focus is set on the usage of a continuous process (vertical stirred media mill) starting with the homogenization of different porous, conductive host materials combined with sulfur as active material. Furthermore, the addition of different solid and liquid electrolytes can be experimentally fulfilled in the mill. Besides, DEM simulations were coupled with the experiments in order to support the outcomes, compare previous investigations and give insight into the process.

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