Lithium-Sulfur batteries offer five times more energy density compared to conventional Li-ion batteries. However, there are challenges related to the long term cycling of these batteries. 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 the formation of organo-lithium polysulfides (OPs), instead of lithium polysulfides (PS). In the present work, we demonstrate in-situ FTIR spectroscopy as a tool to investigate the C-S bond and lithium polysulfide evolution in sulfur-rich copolymer-based Li-S batteries. The copolymer is synthesized using inverse vulcanization reaction, where sulfur powder reacts with a monomer, here 1,3-diisopropenylbenzene (DIB), at high temperatures. In this study, we varied the DIB monomer concentration, and three samples with varied sulfur/DIB wt% were synthesized. Figure 1a shows the cyclic voltammetry (CV) obtained using the 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 a 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 decrease in sulfur chain length with increasing the DIB concentration. To investigate the hypothesis, cells consisted of two different cathodes, one cathode with a low (S1) and another with a high DIB wt% (S2), were built on the FTIR to collect IR spectra and CVs simultaneously. Our results show that the C-S bond located at ~695 cm-1 in S1 shifts to higher wavenumbers (~705cm-1) as we discharge the cell. The shift to higher wavenumbers shows that the formation of OPs strengthens the C-S bonds. This shift originates from having Li, with lower electronegativity (0.98 VS 2.58), instead of S. However, when we tested the S2 cell, the C-S cyclic shift, happens between ~704 to ~706 cm-1, only. The smaller C-S peak shift in S2 compared to S1 is attributed to the existence of the short-chain length sulfur.

In situ FTIR study on organo-polysulfide evolution in CNF/sulfur-copolymer cathodes for Li-S batteries: Effect of sulfur chain length on the electrochemical performance

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.