(719b) Understanding the Reduction Reaction Mechanisms of Sulfur-Based Cathodes: A Theoretical Approach from DFT and Reaxff Molecular Dynamics

The next generation of batteries must help to do the jump from portable consumer electronics to high demanding energy applications like electric vehicles (EVs) and smart-grids. In this regard, as the energy density of the Li-ion batteries is approaching to its theoretical limit, Lithium/Sulfur (Li/S) batteries come up as one of the most promising technologies able to close the gap between the capacity of the current electrode materials and the future energy requirements. Besides the natural abundance, non-toxicity and low-cost of sulfur, the sulfur-based cathode materials are theoretically able to deliver a specific capacity of 1675 Ah/kg and an energy density of 2600 Wh/kg, which happens to be up to 10 times higher than the commonly-used LiCoO₂ cathodes.

The successful commercialization of Li/S batteries requires, however, critical improvements regarding cycle-life, stability, and electrochemical activity. Since its initial introduction around 30 years ago, the sulfur-based cathode still suffers from the insulating nature of sulfur and the formation of soluble lithium sulfide intermediates. The most followed strategy to overcome these two issues is the sulfur trapping within porous conductive carbon-based materials. This way, the carbonaceous structure serves as the electrical conductor increasing the cathode's conductivity and also as the container diminishing the loss of active material out of the cathode. So far, there is a vast and always growing list of structured sulfur-carbon composites proposed by different authors: hollow carbon spheres, carbon nanotubes, carbon nanofibers, and graphene, are just a few examples of them.

Complete trapping/isolation of sulfur inside the carbonaceous structures is not the key for sulfur-based cathodes though. It is also known the dissolution of the polysulfides (PS) created during sulfur reduction is essential for the performance of a Li/S battery. The non-conductive nature of sulfur and its reduction products implies the sulfur reduction can only take place on the surfaces of conductive carbon. The dissolution of PS guarantees then the remaining sulfur is exposed so the reduction process can progressively go on. The incomplete understanding of these processes is still a bottleneck for the commercial development of Li/S batteries. The exclusive use of experimental techniques does not allow to get this deeper understanding, but now the emergence of the computational methods opens a new window at the very atomic level for the description of the reduction processes taking place during the discharge/charge of Li/S batteries.

In this work, we describe at the atomic level the reduction reactions taking place during discharging. Insights about the different structural arrangements during reduction along with the sulfur/polysulfide-carbon interaction were studied. For doing so, we used in conjunction state-of-the-art density functional theory (DFT), ab initio molecular dynamics (AIMD), and reactive molecular dynamics (ReaxFF). The sulfur/carbon composite cathode is represented with different arrangements of slabs of orthorhombic sulfur along the 110 plane coupled with graphene layers (~60 %-wt S). These structures were set in contact with liquid DOL and liquid mixtures of DOL/DME with Lithium (Li⁺) ions embedded in it for studying the early stages of sulfur reduction with the ReaxFF method. The diffusion of Li⁺ ions from the bulk liquid towards the sulfur slabs was observed along with the formation of Li/S bonds breaking the outer sulfur rings. We also observed the further formation of Li-S bonds, which gave us insights on the solid-liquid two-phase reaction mechanisms happening at the onset of cathode reduction. Formation of lithium/sulfur bulk structures around the graphene layers also gave us insights on the sulfur/carbon interactions and the role of carbon hindering the massive detachment/diffusion of the formed PS into the bulk liquid. DFT and AIMD calculations were also performed to corroborate, at a smaller scale, the mechanisms of polysulfide formation upon sulfur contact with the liquid electrolyte. Hints on the Li solvation mechanisms and its effects on the polysulfide diffusion into the bulk liquid electrolyte were drawn.

The studying of the liquid-liquid single-phase reduction from the dissolved Li₂S₈ to low-order PS is also addressed. The studying of the liquid-liquid single-phase reduction from the dissolved Li₂S₈ to low-order PS is also addressed. The molecular dynamics calculations have shown that the Li₂S₈ reduction is influenced by the graphene layers present. When the graphene layers are present, the insoluble reduction products of Li₂S₈ tend to agglomerate around them. This way, we observed that carbon not only hinders the diffusion of polysulfides into the bulk liquid but also influences in the precipitation of Li₂S₈ into insoluble Li₂S₂/Li₂S like structures, helping in the complete reduction of sulfur. These theoretical calculations greatly contribute to the understanding of the reduction reactions of sulfur-based, helping in the engineering of Li/S batteries with longer cycle-life, better stability, and higher electrochemical activity.