Rational Design of Weakly-Solvating Molecules for Salt-In-Pre-Ionic-Liquid Electrolytes for Li Metal Batteries

Abstract

Lithium metal batteries (LMBs) promise step-changes in energy densities but suffer from poor cycle life due to unstable electrolyte-lithium interfaces. Conventional carbonate electrolytes exhibit excessive lithium-ion solvation and low oxidative stability, leading to rapid capacity loss. Herein, we report a rationally designed weakly-solvating cyclic sulfonamide, 1-trifluoromethanesulfonyl)amide pyrrolidine (TFMSPyr), which integrates an electron-withdrawing trifluoromethanesulfonyl functional group at pyrrolidinic-N. TFMSPyr acts as a pre-ionic-liquid solvent that forms intrinsically localized, anion-dominated solvation, coupling molecular architecture, solvation topology, and transport dynamics. As a result, LiFSI based salt-in-pre-ionic-liquid (SIPIL) electrolytes exhibit high lithium-ion transference, oxidative stability > 5 V vs. Li/Li+ and anion-derived solid electrolyte interphases (SEI). Li||Cu cells with SIPIL deliver a first cycle Coulombic efficiency (CE) of ~ 99 % with average CE of 99.2 % for 100 cycles, and lithium half-cells with lithium iron phosphate (LFP) cathode exhibit 82% capacity retention after 400 cycles with CE of 99.98 %. In anode-free full cells, 95 % of initial capacity is retained after 63 cycles with an average CE of 99.5 %. These results demonstrate that molecular engineering of solvents offers a powerful pathway to stabilize lithium metal interfaces and enable practical Anodeless LMBs at low salt concentrations.