Organosulfur/conducting polymer composite cathodes: II. Spectroscopic determination of the protonation and oxidation states of 2,5-dimercapto-1,3,4-thiadiazole

The dimercaptan 2,5-dimercapto-1,3,4-thiadiazole (DMcT) has been employed as a component in secondary lithium battery cathodes. However, spectroscopic studies of its behavior have been impeded by general uncertainty regarding its vibrational band character. The purpose of this study is to provide a spectroscopic basis for the unambiguous determination of protonation and oxidation state of DMcT. The first part of this paper is concerned with structural and spectroscopic assignment of DMcT and some derivatives. Several structural and spectroscopic assignments are intrinsically interesting, including (i) the suggestion that the thioamide-containing compounds adopt the thione tautomer structure whenever the opposite nitrogen is unsubstituted and (ii) assignment of an A₁ ring mode for the first time for these compounds. The second part of this paper uses that resource to examine the in situ Raman spectroelectrochemistry of DMcT in a nonaqueous solvent. The results confirm, in part, the redox wave assignments made previously in the literature. Specifically, Raman spectroelectrochemistry of solutions of DMcT and the lithium salt of its first conjugate base (LiDMcT) show that at ca. 0.8 V LiDMcT is oxidized while DMcT is not oxidized, and that at +1.9 V both species were oxidized. Results regarding oxidation-initiated precipitation of oligomers are also reported.