A team led by Professor Yan Lu, Helmholtz-Zentrum Berlin, and Professor Arne Thomas, Technical University of Berlin, has developed a material that enhances the capacity and stability of lithium-sulfur batteries. The material is based on polymers that form a framework with open pores (known as radical-cationic covalent organic frameworks or COFs). Catalytically accelerated reactions take place in these pores, firmly trapping polysulfides, which would shorten the battery life.
Crystalline framework structures made of organic polymers are a particularly interesting class of materials. They are characterized by their high porosity, comparable to a sponge, but with pores measuring only a few micrometers at most. These materials can exhibit special functionalities that make them interesting for certain applications in electrochemical energy storage devices. For example, they could act as ‘hosts’ for sulfur compounds such as polysulfides in the electrodes of lithium-sulfur batteries. The idea is that the polysulfides could bind to the inner surfaces of pores in the COF structures and react there to generate elemental sulfur again. However, this has not yet worked properly.
The team has now demonstrated a major advance with a newly developed COF material. By incorporating certain radicals, the team achieved a catalytic acceleration of the desired reaction in the pores. The material consists of tetrathiafulvalene units ([TTF] 2 •+) and trisulfide radical anions (S 3 •-) connected via benzothiazole (R-TTF•+-COF). This significantly improves the catalytic activity and electrical conductivity of the COF. “Unpaired electrons play an important role in the micro/mesopores of COFs,” explained Yan Lu. “They contribute to delocalized π orbitals, which facilitates charge transfer between the layers and thus improves the catalytic properties.”
In a highly complex study, the team has elucidated the central role of radical motifs in catalyzing the sulfur reduction reactions. For the study, the researchers investigated the COF materials in Li-S battery cells using solid-state nuclear magnetic resonance (ssNMR) spectroscopy, electron spin resonance (EPR) spectroscopy, and also performed in situ X-ray tomography at the BAMline hard x-ray beamline at the third-generation synchrotron radiation source BESSY II, to characterize the pores more precisely. They combined these experimental results with theoretical calculations to interpret the results. “This allowed us to show that the radical cations [TTF] 2 •+ act as catalytic centers that bind LiPSs and facilitate the elongation and cleavage of the S-S bonds,” said Sijia Cao, a Ph.D. student in Yan Lu’s team.
The result is that the performance of the Li-S battery improves significantly with the use of the new R-TTF•+-COF material. The service life of Li-S batteries will thus increase to over 1500 cycles with a capacity loss of only 0.027 percent per cycle. This durability of Li-S batteries has not yet been achieved with COF materials or other purely organic catalysts. Typically, Li–S batteries exhibit less than 1000 cycles, according to reports from the past few years.
“Integrating such radical scaffold structures into lithium-sulfur batteries shows great promise,” said Yan Lu. In addition, there is a wide range of possibilities for further optimization. The electronic properties of the scaffold and the catalytic activity change depending on which molecules are used as radicals. Nevertheless, further research is needed into COFs with stable radical building blocks that are specifically tailored for catalyzing sulfur reduction reactions.
For more information, contact Arne Thomas at
