Experimental and Theoretical Analysis of Products and Reaction Intermediates of Lithium–Sulfur Batteries

We investigated the reduction process of sulfur during cycling in a lithium–sulfur battery, correlating the output of ultraviolet–visible (UV–vis) spectroscopy and further characterization techniques with a theoretical model. The experimental setup allows carrying out UV–vis absorption measurements under argon atmosphere. The characteristic absorption bands (λmax) of sulfur and dilithium sulfide dissolved in tetra-ethylene glycol dimethyl ether (TEGDME) are determined to be at 265 and 255 nm, respectively. Reference solutions of polysulfides diminish the λmax in the UV region with decrease of polysulfide order. The same tendency is observed in the range between 25–75% depth of discharge (DOD), caused by a progressive reduction of polysulfides in the electrolyte. At 425 and 615 nm, absorption bands are identified in the reference polysulfide solutions and also in the electrolyte at different DOD. These bands are interpreted as the characteristic bands of S42– and S3•–, and concentration changes of these species are determined semiquantitatively. The highest concentration of polysulfides is found at around 37% DOD (450 Ah·kgS–1). This was confirmed by the results of electrochemical impedance spectroscopy and computer simulations.