Engineering a Stable Solid–Electrolyte Interphase through a Novel Trifluoromethyl-Free Lithium Salt for Lithium Metal Polymer Batteries

Abstract

The deployment of safe and high-energy density lithium metal polymer batteries (LMPBs) still requires further advances in the quest for new solid polymer electrolytes (SPEs). In this regard, salt anions have a decisive role in the overall SPE performance. While lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was chosen earlier to have a highly flexible sulfonimide center and an extensively delocalized negative charge, it still suffers from several drawbacks ascribed to its poor interfacial compatibility with the lithium metal (LiM) anode and the fact that it is a PFAS. In this work, a novel lithium salt is cunningly designed, aiming to combine the advantages of previously reported lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(difluoromethanesulfonyl)imide (LiDFSI) to overcome the limitations of the state-of-the-art SPE based on LiTFSI/poly(ethylene oxide) (PEO). The SPE containing the developed (difluoromethanesulfonyl)(fluorosulfonyl)imide (LiDFFSI) salt presented reduced interfacial resistance and improved compatibility with the lithium metal (LiM) anode compared with LiTFSI/PEO, enabled by the formation of a stable, uniform, and ionically conductive solid–electrolyte interphase (SEI). In addition, LiDFFSI-based SPEs demonstrated a prolonged cycling stability, achieving over 125 cycles at C/10 with minimal capacity fading in LiM||LiFePO₄ cell configuration. These findings evidence how a rational design of the lithium salt chemistry allows tuning the formed SEI, directly impacting the overall SPE performance. Thus, LiDFFSI is presented as a promising alternative lithium salt to improve electrochemical performance and interfacial stability in next-generation LiM batteries.