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New COF Material Improves Lithium-Sulfur Battery Lifespan
Researchers from the Karlsruhe Institute of Technology (KIT), in collaboration with the Excellence Cluster SE2A, have developed a new COF (Covalent Organic Framework) material that significantly improves the capacity and stability of lithium-sulfur batteries. This breakthrough could revolutionize the battery industry, as Li-S batteries have traditionally faced challenges in achieving long lifespans.
The new COF material, [TTF]-[S]_n-R-TTF-COF, is based on tetrathiafulvalene units and trisulfide radical anions connected via benzothiazole. This unique structure gives the material high porosity, comparable to a sponge, with pores that are at most a few micrometers in size.
The increased catalytic activity and electrical conductivity of the new COF material compared to previous ones is due to the incorporation of certain radicals. The unpaired electrons in the micro-/mesopores of the new COF material contribute to delocalized π orbitals, which facilitates charge transfer between layers and thus improves catalytic properties.
The new COF material, when used in lithium-sulfur batteries, catalytically accelerates reactions that capture polysulfides, improving battery lifespan. Polysulfides in the new COF material bind to the inner surfaces of the pores and generate elemental sulfur there. The radical cations [TTF] act as catalytic centers that bind LiPS and facilitate the elongation and splitting of S−S bonds in the new R-TTF-COF material.
Experimental analyses of the new COF material were conducted at the BAMline at BESSY II. In-situ X-ray tomography was used to characterize the pores inside the COF materials in more detail. The team also used solid-state nuclear magnetic resonance spectroscopy (ssNMR) and electron paramagnetic resonance (EPR) to examine the COF materials in Li-S battery cells.
The use of the new R-TTF-COF material in Li-S batteries significantly improves the battery's performance. Li-S batteries usually have a lifespan of less than 1,000 cycles, according to reports from recent years. However, the new COF material has proven to increase the lifespan of Li-S batteries to over 1,500 cycles with a capacity loss of only 0.027% per cycle.
Research into COFs with stable radical building blocks specifically tailored for the catalysis of sulfur reduction reactions is sure to remain a fertile field of study. This development marks a significant step forward in the quest for durable and high-performing Li-S batteries. The team behind the development of the new COF material includes Prof. Yan Lu from HZB and Prof. Arne Thomas from the Technical University of Berlin, with doctoral student Walter Cistjakov from KIT playing a key role in the research.
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