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

Abstract

Lithium-Sulfur batteries offer five times more energy density compared to conventional Li-ion batteries. However, there are challenges related to 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 differen sulfur/DIB wt% were synthesized. Figure 1 shows the cyclic voltammetry (CV) obtained using the three different cathodes. As it can be seen from this fugure 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 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. Our results show that the C-S bond located at ~695 cm-1 in sample 1 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 which originates from having Li, with lower electronegativity (0.98 VS. 2.58), instead of S. However, when we tested the sample 2 cell, the C-S cyclic shift, happens between ~704 to ~706 cm-1, only. The smaller C-S peak shift in sample 2 compared to sample 1 is attributed to the existence of the short chain length sulfur. Figure 1