Diluting Pyrrolidinium FSI Ionic Liquid Electrolytes for Lithium Metal Batteries: From Inert Fluorinated Ethers to Weakly Solvating Fluorinated Esters

Abstract

Ionic liquid electrolytes (ILEs) based on LiFSI in N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (Pyr13FSI) are among the most promising electrolytes for safe lithium metal batteries (LMBs). They are non-volatile and thermally stable, and their anion-rich Li⁺ solvation yields a LiF-rich solid electrolyte interphase (SEI) on lithium. Their weakness is transport: a [LiFSI]2[Pyr13FSI]3 electrolyte has a viscosity of about 167 cP and a conductivity of only 2.2 mS cm⁻¹ at room temperature. Locally concentrated ionic liquid electrolytes (LCILEs) address this by adding a low-viscosity, weakly or non-solvating fluorinated co-solvent that thins the bulk liquid while keeping the FSI⁻-dominated Li⁺ shell. This short review summarizes how the concept evolved from hydrofluoroether diluents (BTFE, TTE, TfTHF) to aromatic and partially solvating co-solvents. It compares reported viscosity, conductivity, Li Coulombic efficiency (CE) and full-cell cycling, which reach 98–99.6% CE and stable cycling with NCM811 and LiNiO2 cathodes. We then examine 2,2-difluoroethyl acetate (DFEA) as an emerging candidate. DFEA is oxidatively robust (to about 5.5 V) and highly conductive, but it binds Li⁺ more strongly than classic diluents, so it may enter the solvation shell. We argue that a DFEA-modified Pyr13FSI electrolyte, paired with a small amount of fluoroethylene carbonate (FEC), should be treated as a partially solvating hybrid rather than a true LCILE. Its design therefore requires quantitative solvation analysis, SEI chemistry tracking, and testing under practical conditions (thin Li, lean electrolyte, high loading).