Proposal for an environmental protection directive has been requested from the Commission.
In a groundbreaking development, a team of researchers, led by Prof. Yan Lu from the Helmholtz-Zentrum Berlin (HZB) and Prof. Arne Thomas from the Technical University of Berlin, have created a new material that significantly improves the performance of lithium-sulfur (Li-S) batteries.
The new material, known as radical cationic covalent organic frameworks (COFs), is designed to catalytically accelerate reactions in the pores of the COF. It consists of tetrathiafulvalene units (TTF) and trisulfide radical anions (S) connected via benzothiazole (R-TTF-COF).
The reactions in the pores of the new COF material capture polysulfides that would otherwise shorten the lifespan of Li-S batteries. This capture mechanism, coupled with the material's increased catalytic activity and electrical conductivity, has resulted in a remarkable enhancement in the battery's durability.
Li-S batteries, known for their high energy density, typically have a lifespan of less than 1,000 cycles, according to recent reports. However, the new R-TTF-COF material has increased the lifespan of these batteries to over 1,500 cycles with a capacity loss of only 0.027% per cycle. This durability has not yet been achieved with COF materials or other purely organic catalysts.
The team used solid-state nuclear magnetic resonance spectroscopy (ssNMR) and electron spin resonance spectroscopy (EPR) to investigate the role of radical motifs in the catalysis of sulfur reduction reactions within the new R-TTF-COF material. In-situ X-ray tomography at the BAMline at BESSY II was used to characterise the pores inside the new R-TTF-COF material in more detail.
Some of the experimental analyses were conducted at the BAMline at BESSY II. The theoretical calculations combined with experimental results showed that the radical cations [TTF] act as catalytic centres that bind LiPS and facilitate the elongation and splitting of S−S bonds within the new R-TTF-COF material.
The new COF material is based on polymers that form an open-pored scaffold, making it suitable for applications in electrochemical energy storage, particularly as a host for sulfur compounds in the electrodes of Li-S batteries.
Research into COFs with stable radical building blocks specifically tailored for the catalysis of sulfur reduction reactions will likely remain a productive field of study. The use of the new R-TTF-COF material in Li-S batteries marks a significant step forward in the quest for more efficient and longer-lasting energy storage solutions.
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