Electrolyte-Dependent Electrode Potentials in Lithium–Sulfur Batteries

Lithium–sulfur batteries are promising for future energy storage due to their high specific capacity. However, challenges like low cycling stability and the shuttle effect hinder their practical use. A better understanding of the underlying processes is crucial. This study examines the correlation between lithium potential (E_Li) and sulfur reaction potentials, referenced against the ferrocenium/ferrocene couple as an (almost) solvent-independent reference. Sulfur reactions are divided into two main stages during discharge and charge. Most sulfur reactions show a good linear correlation with E_Li (R² up to 0.87), except for the initial reduction of S₈ to Li₂S₈, which shows no significant correlation. This suggests electrolyte design should address individual sulfur reactions rather than treating them collectively. Additionally, the correlation between E_Li and the Raman-active symmetric S–N–S stretching mode v_s(SNS) in the Li[N(SO₂CF₃)₂] electrolyte anion is analyzed across different electrolyte mixtures. A good linear correlation (R² = 0.73) indicates that both the Raman peak position and E_Li reflect the strength of the interaction between Li⁺ and the electrolyte solvent and hence the Li⁺ ion activity. These findings provide valuable insights for optimizing lithium–sulfur battery electrolytes.