Thiol-based electrolyte additives for high-performance lithium-sulfur batteries

The discharge and charge mechanisms of the Li-S battery involve the formation of soluble lithium polysulfide species that can diffuse through the battery and cause issues related to capacity fade and poor Coulombic efficiency. In order to control the behavior of the lithium polysulfides, thiol-based electrolyte additives such as biphenyl-4,4′-dithiol (BPD) were used to enhance capacity retention in lithium-sulfur batteries by controlling polysulfide dissolution. In situ Raman spectroscopy, in situ UV–vis spectroscopy, and electrospray ionization mass spectrometry show that an additional sulfur reduction process observed at ~2.1 V vs. Li/Li+ as a result of BPD addition is associated with the formation of BPD-short chain polysulfide complexes such as BPD-Sn anion (1≤n≤4). The interaction between BPD and short chain polysulfide postpones formation of soluble short chain polysulfides and alters the kinetics of the dissolution process. A smooth SEI/Li entangled phase is found on the Li anode with BPD addition. The BPD additive increases the capacity retention of lithium-sulfur batteries, mainly due to the formation of various BPD-polysulfide complexes which prevents polysulfide dissolution. Comparison with other thiol-based additives shows that the optimal additive balances solubility and polysulfide-additive stabilization.