Lithium (fluorosulfonyl)(n-nonafluorobutanesulfonyl)imide for stabilizing cathode–electrolyte interface in sulfonamide electrolytes

Rechargeable lithium metal batteries (RLMBs) have been regarded as promising successors for contemporary lithium-ion batteries, in view of their high gravimetric and volumetric energy densities. Conventional non-aqueous liquid electrolytes containing organic carbonate solvents possess high chemical reactivities with metallic lithium anode and high flammability, which induces considerable safety threats under extreme conditions (e.g., overcharging and thermal runaway). Herein, we propose the utilization of fluorinated sulfonamide (i.e., N,N-dimethyl fluorosulfonamide (DMFSA)) as solvent, together with lithium (fluorosulfonyl)(n-nonafluorobutanesulfonyl)imide (LiFNFSI) as co-salt and/or electrolyte additive for accessing safer and high-performing RLMBs. Comprehensive physical (e.g., thermal transition, viscosity, and ionic conductivity) and electrochemical (e.g., anodic stability on different electrodes) characterizations have been performed, aiming to reveal the inherent characteristics of the sulfonamide-based electrolytes and the particular role of LiFNFSI on the stabilization of LiCoO₂ cathode. It has been demonstrated that the sulfonamide-based electrolytes exhibit superior flame-retardant abilities and decent ionic conductivities (> 1 mS·cm⁻¹ at room temperature). The incorporation of LiFNFSI as co-salt and/or electrolyte additive could significantly suppress the side reactions occurring at the cathode compartment, through the preferential decompositions of the FNFSI⁻ anion. This work is anticipated to give an in-depth understanding on the working mechanism of LiFNFSI in the sulfonamide-based electrolytes, and also spurs the development of high-energy and safer RLMBs.