Effect of Sulfur Chain Length on the Electrochemical Performance of Sulfur-Rich Copolymers in Li-S Batteries

Abstract

Lithium-Sulfur battery is considered to be a promising next-generation candidate because of its high theoretical capacity of ~1675 mAh/g compared to current commercial batteries. However, there are challenges related to the long-term cycling of these batteries, originating from the formation of soluble lithium polysulfides species and their subsequent shuttling. Recently, we synthesized a sulfur-rich copolymer/ carbon nanofiber cathode material for Li-S batteries. The formation of a C-S bond in these sulfur-rich copolymers results in formation of organo-lithium polysulfides (OPs) instead of lithium polysulfides (PS). The copolymer is synthesized using inverse vulcanization reaction, where sulfur powder reacts with a monomer, here 1,3-di-isopropenyl benzene (DIB), at high temperatures. In this study, we varied the DIB monomer concentration, and three samples with different sulfur/DIB wt% were synthesized. We characterized these three different samples using XRD, DSC, and FTIR. The XRD results show that the sulfur-rich copolymer transitions from a crystalline material to a completely amorphous composite. Moreover, FTIR results show a redshift in the C-S peak of the sulfur-rich copolymer with increasing the monomer wt%. These results confirm that the molecular structure of the sulfur-rich copolymer is changed as a result of changing S/DIB wt ratio in the synthesis of sulfur-rich copolymers. We believe that by increasing the monomer to sulfur ratio in such copolymers, the sulfur chain length becomes shorter. Figure 1 shows the cyclic voltammetry (CV) obtained using these three different cathodes. As can be seen from this figure, two reduction peaks at ~2.3 and ~2 V represent the formation of higher and lower order Ps and OPs. However, with further increase in DIB concentration, the first reduction peak at ~2.3V disappears. We hypothesize that the disappearance of the first reduction peak can be attributed to the existence of a short sulfur chain length. To investigate the hypothesis, an in situ cell consisted of two different cathodes, one cathode with a low (sample 1) and another with a high DIB wt% (sample 2), were built on the FTIR to simultaneously collect IR spectra and CVs. The FTIR results show a redshift from 695 cm-1 to 705 cm-1 in the C-S bond for sample 1 with higher sulfur chain length (lower monomer wt%). This shift is attributed to the formation of organopolysulfides. Moreover, as the cell was discharged, multiple peaks between the lithium polysulfide region (510 to 480 cm-1) appeared, which confirms the formation of high and low order lithium polysulfides. On the other hand, when sample 2 with lower sulfur chain length (high monomer wt%) was tested, the C-S peak shifted from 704 cm-1 to 706 cm-1 only. The smaller C-S peak shift in sample 2 compared to sample 1 confirms the existence of short-chain polysulfides. Moreover, there was no lithium polysulfide peaks observed in the polysulfide region for sample 2, which confirms that the formation of loose lithium polysulfide is avoided. Figure 1