New Material Concepts for Lihium-Sulfur-Batteries
Authors: Dörfler Susanne , Strubel Patrick , Althues Holger , Thieme Sören , Piwko Markus , Tschöcke Sebastian , Kaskel Stefan 
Published: February 2020

Lithium-Sulfur (Li-S) batteries have great potential to replace the presently used Lithium-intercalation (Lithium Ion) technology as they have a remarkably higher energy density and lower material costs. However, for the commercialization of this cell type, several challenges have to be addressed that are mainly attributed to the complex chemical reactions during electrochemical cycling of these batteries.
The cycling stability of realistic Li-S cells comprising reasonable sulfur loadings, low amounts of electrolyte and a lithium metal anode is usually limited to less than 200 cycles. This is not only a result of dendrite formation and mossy lithium growth leading to degradation of the anode structure and safety issues, but also caused by continuous electrolyte depletion at the instable anode/electrolyte interface (battery dry out).
Moreover, low volumetric energy is still a further disadvantage limiting the application areas for Li-S batteries. For that reason, new concepts have to be developed and transferred to prototype demonstration in order to overcome today’s limitations.
In this work, the major limiting factors for energy density and cycle life in Li-S batteries are discussed and new material concepts are introduced significantly improving the performance of Li-S cells. The development of specifically tailored cathode materials / process techniques, electrolytes, modified separators and alternative anodes is focused in order to establish Li-S batteries as future energy storage systems.

For stable cathode performance and high sulfur utilization, porous carbons without and with (2 %) nitrogen doping as rigid, conductive host structure for S8/Li2S deposition were systematically developed and investigated. The pore geometry can be precisely controlled (hierarchical micro-macroporosity) by an efficient, scalable and and eco-friendly salt-templating process. Additionally, high values for total pore volume (up to 3.8 cm³/g) as well as for the internal surface area (up to 3000 m2/g) were realized [1]. The resulting carbon /sulfur composite showed stable capacities of > 1000 mAh g-1
sulfur, and using the nitrogen-doped carbon, up to 240 cycles were achieved.

Innovative up-scaled solvent-free process equipment for electrode production was designed and constructed. With this machine, moisture being detrimental for the cathode performance in the resulting electrode is avoided [2]. Another benefit of the presented solvent-free process is the increase in areal capacity to more than 4 mAh cm-2. This is about twice as much as slurry based coating techniques offer

By tailoring the electrolyte formulation, the anode and cathode chemistry is significantly influenced. Reduced Lithium surface roughness after cycling and high sulfur utilization at low electrolyte contents (2-3 µl/mg-S) were observed.

Semipermeable Nafion-impregnated separators were generate to suppress the polysulfide shuttle due to repulsive interaction between polysulfides anions and the negatively charged sulfonic acid groups linked to the perfluorinated polymer backbone. Since the Li ion conductivity of commercial Nafion membranes was found insufficient for direct application, dense and smooth Nafion thin films were applied onto a microporous polypropylene backbone. Diffusion tests as well as electrochemical measurements and self-discharge analysis revealed the effectiveness of the approach [3].

Introducing a pure silicon thin film anode potentially results in strongly enhanced volumetric energy densities. Here, we demonstrate silicon thin film anodes with areal capacities > 3 mAh/cm² for SLS (Sulfur-Lithium-Silicon) full cells. Estimations for achievable energy density on cell level will be derived from these results.

In conclusion, the proposed material concepts open new opportunities for improving Li-S batteries by adressing the major challenges for commercialization of this cell chemistry. The work also suggests that the Li-S research is still at a very early stage. Although many improvements and innovative concepts have been described in literature, the transfer of those into prototype cells and demonstration of their performance will be important next steps in order to achieve the technology breakthrough.
[1] P. Strubel , S. Thieme , T. Biemelt , A. Helmer , M. Oschatz , J. Brückner , H. Althues S. Kaskel Adv.
Funct. Mater.
2015, 25, 287–297 DOI: 10.1002/adfm.201402768
[2] S. Thieme , H. Althues, S. Kaskel, et al J. Mater. Chem. A 2013, 1(32), 9225-9234.DOI: 10.1039/c3ta10641a
[3] I. Bauer, S. Thieme, J. Brückner, H. Althues, S. Kaskel, J. Power Sources, 2014, DOI: 10.1016/j.jpowsour.2013.11.090