Revisiting Coulombic Efficiency Paradigm: Electrolyte Additive Design for Lithium Metal Batteries

Small-dose electrolyte additives are widely used to enhance battery performance, yet rational additive selection for the stabilization of lithium metal (Li°) electrodes remains challenging. Coulombic efficiency (CE) measurements in asymmetric Li°||Cu° cells serve as critical evaluative metrics, but the resulting CE values often deviate from expectations based on additive structure alone. Herein, we delve into the design of fluorinated electrolyte additives by combining statistical analysis of Li°||Cu° cycling data with compositional characterization as well as atomistic and chemical simulations. Our results demonstrate that nonafluorobutanesulfonyl fluoride, as an electrolyte additive, induces only a marginal effect on the Aurbach CE values during short-term cycling tests of Li°||Cu° cells. Intriguingly, during extended-cycling, statistical analysis reveals that the same additive exhibits divergent effects in electrolyte families based on bis(trifluoromethanesulfonyl)imide (TFSI^-) and bis(fluorosulfonyl)imide (FSI^-) anions. These differences arise from the interplay between the film-forming chemistry of the fluorinated additive and the active involvement of TFSI^- and FSI^- anions, which collectively modulate the chemical and electrochemical features of the resulting solid-electrolyte interphases (SEI) on Li°. This work elucidates how additive functionality translates into interphase chemistry and provides a statistically robust framework for screening electrolyte additives for practical lithium metal batteries.