Experimental
and Theoretical Analysis of Products and Reaction Intermediates of
Lithium–Sulfur Batteries
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Abstract
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 (λ<sub>max</sub>) 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 λ<sub>max</sub> 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 S<sub>4</sub><sup>2–</sup> and
S<sub>3</sub><sup>•–</sup>, and concentration changes
of these species are determined semiquantitatively. The highest concentration
of polysulfides is found at around 37% DOD (450 Ah·kg<sub>S</sub><sup>–1</sup>). This was confirmed by the results of electrochemical
impedance spectroscopy and computer simulations
