A team led by Prof. Yan Lu, HZB, and Prof. Arne Thomas, Technical University of Berlin, has developed a material that enhances the capacity and stability of lithium-sulphur 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 polysulphides, which would shorten the battery life. Some of the experimental analyses were conducted at the BAMline at BESSY II.
Crystalline framework structures made of organic polymers are a particularly interesting class of materials. They are characterised by their high porosity, comparable to a sponge, but with pores measuring only a few micrometres at most. These materials can exhibit special functionalities, which make them interesting for certain applications in electrochemical energy storage devices. For example, they could act as 'hosts' for sulphur compounds such as polysulphides in the electrodes of lithium-sulphur batteries. The idea is that the polysulphides could bind to the inner surfaces of pores in the COF structures and react there to generate elemental sulphur again. However, this has not yet worked properly.
Newly developed COF
A team led by Prof. Yan Lu (HZB) and Prof. Arne Thomas (Technical University of Berlin) 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 trisulphide radical anions (S3•-) 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,' explains Yan Lu: 'They contribute to delocalised π orbitals, which facilitates charge transfer between the layers and thus improves the catalytic properties.'
Combination of experiments
In a highly complex study, the team has elucidated the central role of radical motifs in catalysing the sulphur 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 at BESSY II to characterise the pores inside 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 centres that bind LiPSs and facilitate the elongation and cleavage of the S−S bonds,' says Sijia Cao, a PhD student in Yan Lu's team.
Significant improvement
The result is amazing: 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 thus increases to over 1,500 cycles with a capacity loss of only 0.027% 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 1,000 cycles, according to reports from the past few years.
'Integrating such radical scaffold structures into lithium-sulphur batteries shows great promise,' says Yan Lu. In addition, there is a wide range of possibilities for further optimisation. 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 catalysing sulphur reduction reactions.
