Tetrabutylammonium-Based Organosulfur Counter-Anion Additives on Li-Sulfur Battery Performance

Lithium-sulfur (Li-S) batteries have attracted considerable interest but are limited by the intrinsic insulating nature of sulfur, lithium-polysulfide (LiP) migration to the anode and parasitic anode-electrolyte reactions. To mitigate these challenges, we investigate low-concentration electrolyte additives based on tetrabutylammonium (TBA+). Contrary to prior reports emphasizing the cation, our results indicate that in TBA+-based salts the anions play the dominant role owing to the weak Lewis acidity of TBA+. By investigating anions with a shared sulfonate backbone, methanesulfonate (MS-) and trifluoromethanesulfonate (TFMS-), we elucidate their roles in reshaping the Li+ solvation structure via molecular dynamics simulations and ⁷Li NMR. The anions’ presence in the solvation shell enhances the sulfur reduction reaction (SRR) and Li2S nucleation, while reducing concentration polarization and activation energy. Operando Raman spectroscopy coupled with in situ distribution of relaxation times (DRT) analysis further corroborates an improved Li+ and LiPs diffusion as well as SRR kinetics. Notably, TFMS- also spontaneously initiates DOL polymerization, forming an anode coating that benefits Li plating. These factors culminate in a noteworthy 397.48 mAh/g and 395.74 mAh/g after 300 cycles at 1C by adding only 0.01 M of MS- and TFMS-, respectively. This work not only underscores the mechanistic importance of anion additives to Li-S cells but also provides a template for further targeted anion design using the TBA+ family.