Probing the Functionality of LiFSI Structural Derivatives as Additives for Li Metal Anodes

Lithium (Li) metal is a compelling replacement for graphite anodes in Li-ion batteries to increase gravimetric energy if the cyclability can be improved. Motivated by high Li Coulombic efficiency (CE) achieved with electrolytes featuring the bis(fluorosulfonyl)imide (FSI–) anion, this work examined chemically related sulfonyl/sulfamoyl fluoride additives to correlate FSI–-relevant structural features with CE. Across three exemplary carbonate- and glyme-based electrolytes, extended solid, liquid, and gas phase characterizations reveal that Li+ coordination is necessary yet insufficient for FSI– derivatives to affect cycling. Beyond coordination, the reactivity of the baseline solvent and the key structural features of the additives are shown to strongly regulate CE, with possession of an N center─common to sulfamoylfluorides and FSI–─consistently leading to higher CE. Some derivatives outperform FSI– in short-term cycling; however, they have difficulty competing with the longevity of FSI–. These results provide insights for developing improved additives in the future through careful consideration of reactant structure and solvent codesign.