Novel Tailored 3D Carbon Nanotube Cathodes for Effective Trapping of Polysulfides in Lithium Sulfur Batteries
Authors: Krüger Helge , Nöhren Sandra , Schütt Fabian , Mishra Yogendra Kumar , Adelung Rainer 
Published: February 2020

Lithium-sulfur batteries are one of the most promising Li-Ion batteries. Sulfur, with the highest gravimetric capacity of all cathode materials, allows a theoretical energy density that is five times higher than state of the art rechargeable batteries. Furthermore, sulfur is a cheap and earth abundant material, which makes it possible to be used in large-scale applications. Hereby, progress in the field of mobile energy storage can be achieved.[1]
However, rechargeable lithium-sulfur batteries still suffer from the intrinsic problems of sulfur in the battery environment. The insulating character, the expansion of sulfur during cycling as well as the well-known polysulfide shuttle effect are still a challenge and are studied with great effort. The phenomena described lead to lowered sulfur utilization and loss of active material during cycling, resulting in an overall reduced energy density and a continuous fading of the battery capacity, unsuitable for practical applications. The shuttle effect, characterized by the dissolution of sulfur into the electrolyte during cycling, is still a major concern, but can be reduced by using for example nanomaterials.[2][3]
Much of the research is done on the electrolyte or electrode composition, but the structural design of the cathode plays major role. The use of nanomaterials based on carbon, such as graphene, graphene oxide or carbon nanotubes, have shown already huge potential in the use as a cathode scaffold material.³ However, these materials need to be arranged into a macroscopic 3D structure to make use of their extraordinary electrical and mechanical properties.
The approach presented here is based on the fabrication process established in a previous work[4], for the preparation of an open porous 3D self-organized hierarchical carbon nanotube tube network, used to minimize the shuttle effect. A highly porous 3D network based on tetrapodal zinc oxide[5] is coated with carbon nanotubes using wet chemical techniques. The tetrapodal zinc oxide is used as a removable scaffold material, giving the cathode a defined 3D structure. After zinc oxide removal, a conductive, mechanically stable, 3D scaffold of carbon nanotubes remains possessing a hierarchical porous structure. The structure has a large pore to volume ratio and therefore a large reachable surface inside the cathode material.
By employing these open porous network structures in combination with sulfur vapour deposition methods, the sulfur is homogenously distributed throughout the structure, improving the sulphur utilization and improving the overall contact to the highly conductive carbon host scaffold. In addition the structure of the network acts as a trap for polysulfide due to tailorable micro and macro pores, reducing capacity fading of the battery from the shuttle effect.
The performance of these sulphur cathodes was shown to be almost constant from the 5th to the 50th cycle with a capacity of 950mAh/g and sulfur loadings up to 50wt.%. This novel electrode design is able to counteract the polysulfide-shuttle effect, withstand the expansion of sulfur during cycling, and offers promising results in the battery performance.
[1] Manthiram, Arumugam; Fu, Yongzhu; Chung, Sheng-Heng; Zu, Chenxi; Su, Yu-Sheng (2014): Rechargeable lithium-sulfur batteries. In: Chemical reviews 114 (23), S. 11751–11787. DOI: 10.1021/cr500062v.
[2] Liquid electrolyte lithium/sulfur battery. Fundamental chemistry, problems, and solutions (2013). In: Journal of Power Sources 231, S. 153–162.
[3] Wang, Hailiang; Yang, Yuan; Liang, Yongye; Robinson, Joshua Tucker; Li, Yanguang; Jackson, Ariel et al. (2011): Graphene-Wrapped Sulfur Particles as a Rechargeable Lithium–Sulfur Battery Cathode Material with High Capacity and Cycling Stability. American Chemical Society
[4]Schütt, Fabian; Signetti, Stefano; Krüger, Helge; Röder, Sarah; Smazna, Daria; Kaps, Sören et al. (2017): Hierarchical self-entangled carbon nanotube tube networks. In: Nature Communications 8 (1), S. 1215. DOI: 10.1038/s41467-017-01324-7.
[5] Mishra, Yogendra Kumar; Adelung, Rainer (2018): ZnO tetrapod materials for functional applications. In: Materials Today 21 (6), S. 631–651. DOI: 10.1016/j.mattod.2017.11.003.