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Tuesday, 09 February 2021

A cathode based on 2D organic structures loaded with iron single atoms improves lithium-sulfur batteries performance

by Virginia Greco

Lithium-sulfur batteries (LSBs) are considered promising candidates for the next-generation energy storage devices, thanks to their high energy density and cost effectiveness. Despite these appealing properties, the practical application and commercialization of LSBs are still hindered by a number of issues strictly related to their electrochemistry. Many approaches have been used to address these problems but, even though many steps forward have been made, improvements are still required to harness the potentiality of this technology.

A team of researchers led by ICREA Prof. Jordi Arbiol, head of the ICN2 Advanced Electron Nanoscopy group, and ICREA Prof. Andreu Cabot, from the Catalonia Institute for Energy Research (IREC, Barcelona), has developed a novel type of cathode in which the use of a carbon nitride-based 2D layered nanostructure allows mitigating some of the drawbacks of LSBs. As described in a paper published in this month’s edition of Advanced Energy Materials and featured on its inside back cover, a structure made of iron atoms dispersed in a carbon nitride framework (Fe/C₂N) acts as a host material for the sulfur, providing excellent electrochemical performance.

Discovered in the 1960s, lithium-sulfur batteries were abandoned in the 1990s in favour of those based on lithium-ions (LIBs), which established themselves as the go-to systems for powering electronic devices, because of their stable electrochemistry and longer lifetime. The interest in Li-S batteries rose again at the beginning of the new millennium, due to the rapidly increasing demand of a technology that could provide higher energy densities. This requirement can indeed be met by LSBs, which overcome commercial Li-ion batteries, allowing for up to five times greater energy densities. In addition, the fact that sulfur is non-toxic and very abundant in the earth crust makes LBSs cheaper and environmental friendly alternatives.

On the other hand, though, Li-S batteries show poor energy efficiency because sulfur and lithium sulfide (Li₂S) have low ionic and electronic conductivities. Their lifespan is also strongly reduced by the solubility in most electrolytes of the polysulfides (Li₂S_x -type compounds with x higher than 1) generated in the intermediate steps of the charge and discharge processes. In fact, the transition from molecules of lithium and sulfur to lithium sulfide happens in consecutive reduction stages — oxidation stages in the reverse process — involving the formation of various lithium polysulfides. Not only these intermediate molecules can dissolve into the electrolyte leading to irreversible loss of active sulfur, but they can also migrate towards the lithium anode, by a so-called “shuttle effect”, causing additional deterioration of the battery performance. Furthermore, the cathode undergoes extreme volume variations along the charge/discharge cycle: the molecules produced when the sulfur in the cathode combines with the lithium ions flowing from the anode (in the discharge phase) occupy a volume nearly 80% bigger than that of the original sulfur. This causes large mechanical stresses on the cathode, which is a major cause of rapid degradation.

These drawbacks of LSBs can be mitigated by replacing the sulfur cathode with a more complex structure. First of all, sulfur can be combined with a substrate exhibiting high electron conductivity, such as a carbon-based one. Then, a porous structure can adsorb the polysulfides formed at the cathode, limiting their diffusion in the electrolyte. Finally, an elastic substrate can better endure volume variations. As a consequence, high surface area and high porosity carbon-based materials characterized by good elasticity (such as mesocarbon, graphene, carbon nanotubes, etc.) are used as sulfur “hosts” to build more resistant and efficient cathodes. Among others, various catalyst substrates based on carbon nitride (C₂N), a bidimensional organic material having a periodic porous structure, have been developed for this application. Nevertheless, their preparation and characterization is still quite challenging.

The authors of the here-highlighted paper propose the use of a C₂N framework loaded with atomically and uniformly dispersed iron (Fe/C₂N) as a host material for the sulfur. This catalyst presents several advantages, since C₂N shows excellent electrical conductivity and is a highly porous and high surface area framework. The iron atoms trapped in the pores — up to two atoms can fit in one hole — improve the ability of the cathode material to immobilize the soluble polysulfides and promote the reaction kinetics between sulfur, polysulfides and lithium sulfide. In this work.

A detailed characterization of the synthesized 2D layered nanostructures, down to the atomic level, is provided in the article, as well as theoretical simulations elucidating the complex electrocatalytic mechanisms involved. The authors also performed extensive tests on cathodes based on these Fe/C₂N frameworks hosting sulfur, demonstrating their excellent electrochemical performance and long-term cycling stability for application in lithium-sulfur batteries.

 

Reference article:

Zhifu Liang, Dawei Yang, Pengyi Tang, Chaoqi Zhang, Jordi Jacas Biendicho, Yi Zhang, Jordi Llorca, Xiang Wang, Junshan Li, Marc Heggen, Jeremy David, Rafal E. Dunin‐Borkowski, Yingtang Zhou, Joan Ramon Morante, Andreu Cabot, and Jordi Arbiol, Atomically dispersed Fe in a C2N Based Catalyst as a Sulfur Host for Efficient Lithium–Sulfur Batteries. Advanced Energy Materials, 2020, Volume 11, Issue 5, 2003507. DOI: 10.1002/aenm.202003507

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