In-Operando FTIR Study on The Redox Behavior of Sulfurized Polymers as Cathode Material for Li-S Batteries

Sulfurized polymers are an attractive cathode active material, promising to overcome the intransigent polysulfide shuttle effect sulfur cathodes face via the anchoring of sulfur by a carbon-sulfur (C-S) bond. Both sulfur-diisopropenylbenzene copolymers (SDIB) and sulfurized polyacrylonitrile (SPAN) contain C-S bonds which should function in this way, however, their performance in ether electrolytes still exhibit capacity fade associated with the polysulfide shuttle. We investigate this anchoring effect using in-operando ATR-FTIR spectroscopy to develop a molecular level understanding of the polysulfide speciation reaction in sulfurized polymers and cathode level molecular changes. We find that in SDIB copolymers with sulfur wt % of 80 and 30 wt %, the C–S bond is not active in the voltage window of Li–S batteries (1.8-2.6V vs Li/Li+). In SPAN, however, we found that the C-S bond is active in the typical voltage window (1-3V vs Li/Li+) contributing to the evolution of polysulfide species in electrolyte with low concentrations of lithium nitrate (LiNO3), however this effect is mitigated when higher concentrations of LiNO3 are used. We attributed the mitigation of the polysulfide shuttle to the formation of a cathode electrolyte interface (CEI), composed of lithium fluoride, established via ex-situ XPS studies on cycled cathodes. Additionally, we observed the reversible behavior of the C-S bond in IR at ~680 cm-1 in electrolytes with high concentration of LiNO3, as opposed to the irreversible behavior of the bond in electrolyte with low concentrations of LiNO3. Moreover, we observed the lithiation of the cyclized PAN backbone, identified by the elimination of the aromatic region of the IR spectrum after the first cycle.