EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Lithium-Sulfur batteries are among promising candidates for next-generation energy-storage devices. Huge efforts have been put into the design of new cathode materials for these batteries. In this regard, sulfur copolymers have recently attracted considerable attention, due to their flexible structure and their ability to provide reversible capacity. Here, the focus is set on sulfur-diisopropylbenzene (S/DIB) co-polymers, which recently have been shown to deliver stable performance. However, their redox mechanism is still not well known. To understand this mechanism, first, the lowest-energy structures of the cathode should be found. For this, we focus on the local structure of two DIB moieties interconnected via a sulfur chain (DIB–S$_x$–DIB, $x$ = 1-8), with two connection possibilities: where (A) S chain connects to quaternary carbon, and (B) it connects via a CH$_2$ group. We aim at finding the most favorable structure based on an optimal S chain length $x$ and its connection $c$ to DIB molecules (AA, AB, BB). Here, we adopt a temperature-assisted minimum-energy structure search, where for each $(x,c)$, classical MD simulations have been performed for 10ns. Out of each trajectory, 10 uncorrelated snapshots have been taken, on which simulated quenching has been carried out at DFT level of theory. Formation energy per S is calculated afterward for all 240 samples. Our results show that S/DIB co-polymers favor short S chain lengths in their local structure. In particular, $x$=3 shows the lowest formation energy for all connection possibilities, implying that the connection type could only have minor effects. Moreover, we present the probability for each $(x,c)$ based on the Boltzmann distribution of formation energies. As such, based on the local insights, we are able to propose a candidate structure for S/DIB co-polymer.