Identifying Key Parameters for Mixed Organic Electrolytes for Lithium–Sulfur Battery

Engineered electrolytes are critical for high-performance lithium–sulfur batteries (LSBs). Present electrolyte selection for simultaneously forming a stable bilateral solid–electrolyte interface (SEI) on both electrodes is largely heuristic. Although the dielectric constant, viscosity, dipole moment, donor number, and orbital energy levels have all been used for electrolyte screening, their effectiveness has not been systematically studied. Here, the effectiveness of these parameters was investigated using a key metric of battery performance. Based on 51 mixed electrolytes investigated in this study, the enhanced stability of LSBs is attributed to the mixed electrolytes' high dielectric constant (ε > 35), which ensures the separation of the LiTFSI salt ions and potentially reduces dendrite growth. However, 3 other high dielectric (ε > 35) mixed electrolytes based on diglyme exhibited a % drop of > ± 1.4%, which is ∼2 times larger than the % drop exhibited by batteries with high dielectric (ε > 35) compositions devoid of diglyme. Classical molecular dynamics indicated the presence of large diglyme molecules in the solvation shell, causing a ∼30% reduction in diffusivity and adversely affecting battery performance. This study indicates that a high dielectric constant (ε > 35) along with the absence of large molecules in the solvation shell are good criteria for LSB mixed electrolyte selection.