Balancing Solvation: Stabilizing Lithium Metal Batteries via Optimized Cosolvents in Ionic-liquid Electrolytes

Abstract

In this study, we examined three co-solvents with distinct solvation capabilities for ionic-liquid-based electrolytes, 1-methyl-1-propyl pyrrolidinium bis(fluorosulfonyl)imide (Py13FSI). Among them, we demonstrate that 1,1,1-trifluoro-2-(2-(2-(2,2,2-trifluoroethoxy)ethoxy)ethoxy)ethane (FDG), notably enhances the cycle life of Py13FSI-based electrolytes, outperforming 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropylether (TTE) and diglyme (DG). Electrochemical and surface analyses attribute this improvement to the formation of a favorable cathode interphase, promoting efficient Li+ transport with reduced over potential. Spectroscopic techniques (FTIR, Raman, and NMR spectroscopy) and molecular dynamics simulations reveal that co-solvents with varying solvation abilities influence the solvation structures in Py13FSI-based electrolytes. The mild solvating strength and lithium stability of FDG are key contributors to its effectiveness. Conversely, DG, a strong solvating solvent, destabilizes the Py13FSI-DG electrolyte at the lithium metal anode, while TTE, a non-solvating solvent, fails to enhance lithium transport or form a stable cathode interphase. Our findings highlight that a balanced solvation exerted by co-solvents is critical for forming a stable electrolyte-cathode interface, potentially through FSI decomposition. This study offers valuable insights into the development of durable ionic liquid electrolytes, emphasizing the importance of selecting co-solvents with optimal solvation properties.