Design Guidelines for Sulfonyl/Sulfamoyl Fluoride Additives to Modulate Lithium Anode Coulombic Efficiency

Abstract

The lithium metal anode has a high theoretical capacity (3860 mAh / g) and low electrochemical potential (‐3.04 V vs SHE), making it an ideal anode material for high energy density Li batteries. However, the high reactivity of Li metal with electrolytes results in the formation of a solid electrolyte interphase (SEI). In conventional electrolytes, the SEI is unstable, leading to continuous capacity loss and low Coulombic efficiencies (CE). Successful principles for Li metal electrolyte design to achieve high CE have largely focused on promoting the sacrificial reduction of anions (such as lithium bis(fluorosulfonyl)imide (LiFSI)) believed to be beneficial for the SEI. Alternatively, additive development for Li metal has identified several chemical classes that have been shown to effectively modify either the Li plating morphology or the SEI chemistry, and consequently increase CE. Motivated by the high CE of LiFSI systems and the performance of previously studied additives for Li metal, sulfonyl/sulfamoyl fluorides (R‐SO2F and R‐R’ NSO2F, respectively) are examined as a model class of functional electrolyte additives for Li cycling. This thesis examines what parameters govern the performance of this model class of additives, including the additive chemical structure and baseline electrolyte solvent. The effects of additives on CE were evaluated in select high‐CE electrolytes consisting of LiFSI dissolved in representative organic solvents, as well as the commercially relevant carbonate electrolyte LiPF6 dissolved in EC:DEC (LP40). The observed variations in CE, which suggest competitive reactions among solvents, anions, and additive, are then rationalized by characterization of post‐cycling SEI chemical compositions, gas evolution, and Li plating morphologies. The results identify the function of the nitrogen center unique to sulfamoylfluorides in promoting Li+ coordination and preventing structural fragmentation of the additive during cycling.