A Boron-Based Anion-Receptor Additive for High-Loading Lithium–Sulfur Batteries under Lean Electrolyte Conditions

Abstract

Although Li–S batteries offer significant promise as replacements for conventional lithium-ion batteries in terms of sustainability and performance, their practical implementation remains challenging. Designing cathodes with high sulfur loading (≥5 mg/cm²) under lean electrolyte conditions (E/S ≤ 5 μL/mg) presents specific obstacles, including sluggish reaction kinetics, poor sulfur utilization, and increased interfacial resistance, often driven by the saturation of lithium polysulfide species in the electrolyte. To overcome these issues, the development of solvents and additives that can enhance sulfur utilization/dissolution while mitigating the polysulfide shuttle effect has become crucial. To address this, we introduce a boron-based anion receptor, tris(trimethyl)silyl borate (TMSB), as an electrolyte additive for Li–S batteries to improve the dissolution of polysulfide species in the electrolyte. Due to its Lewis-acid ability, TMSB was found to dissolve more anionic polysulfide species (Lewis bases) in DOL/DME-based electrolytes. Surface analysis of cycled cathodes revealed that TMSB also aids in scavenging decomposition products at the cathode surface by coordinating with anions, including sulfates and thiosulfates. At high-loading (5 mg/cm²) and lean electrolyte (E/S = 8 μL/mg) conditions, results show that 5 wt % TMSB in the electrolyte reduced the overpotential by 850 mV. Further, the addition of TMSB also enabled stable cycling at a 0.2C rate for 100 cycles compared to the intermittent cycling in the blank electrolyte. Hence, this study shows that TMSB as an electrolyte additive helps in improving sulfur utilization and reducing interfacial resistance for high-loading lithium–sulfur batteries under lean electrolyte conditions.