In-situ UV-Vis Spectroscopy of Trisulfur Radicals in Lithium-Sulfur Batteries

Here we employ in-situ UV-Vis spectroscopy to monitor the sulfur redox reaction with oxygen-containing molecules as an additive, for example, biphenyl-4,4′-dicarboxylic acid (BDC). Furthermore, Raman spectrum, electron paramagnetic resonance (EPR), and electrospray ionization-mass spectrometry (ESI-MS) measurements reveal that the formation of BDC-S ₃ ^•‒ complexes can establish the long-term stability of polysulfide radicals, change the kinetics of sulfur redox reaction, and then generate decent capacity retention and rate capability. According to the density functional theory (DFT) analysis, S ₃ ^•‒ radicals are the underlying product of S ₆ ^2‒ cleavage, owing to the decreased chemical energy and the increased stability of S ₃ ^•‒ radicals through Lewis acid-base interaction. The assembled Li-S batteries with BDC additive deliver a high reversible capacity of 420 mA·h·g⁻¹ over 200 cycles with over 98% Coulombic efficiency, under the current density of 0.2 C.