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

Area of Science:

• Materials Science
• Electrochemistry
• Chemical Engineering

Background:

• Lithium metal batteries (LMBs) offer high energy density but face challenges with unstable electrolyte-lithium interfaces and poor cycle life.
• Conventional carbonate electrolytes lead to rapid capacity loss due to excessive lithium-ion solvation and low oxidative stability.

Purpose of the Study:

• To design and synthesize a novel weakly-solvating cyclic sulfonamide solvent, 1-trifluoromethanesulfonyl)amide pyrrolidine (TFMSPyr), for stabilizing lithium metal interfaces.
• To investigate the properties of TFMSPyr-based salt-in-pre-ionic-liquid (SIPIL) electrolytes and their performance in LMBs.

Main Methods:

• Molecular design and synthesis of TFMSPyr, integrating an electron-withdrawing trifluoromethanesulfonyl group.
• Characterization of TFMSPyr-based SIPIL electrolytes, including solvation topology and transport dynamics.
• Electrochemical testing of Li||Cu cells, lithium half-cells with LFP cathodes, and anode-free full cells.

Main Results:

• TFMSPyr forms intrinsically localized, anion-dominated solvation, enhancing stability.
• SIPIL electrolytes demonstrate high lithium-ion transference number and oxidative stability (> 5 V vs. Li/Li+).
• Li||Cu cells achieved ≈ 99% first cycle Coulombic efficiency (CE) and 99.2% average CE over 100 cycles.
• Lithium half-cells with LFP cathode retained 82% capacity after 400 cycles (99.98% CE).
• Anode-free full cells maintained 95% capacity after 63 cycles (99.5% average CE).

Conclusions:

• Molecular engineering of solvents, like TFMSPyr, is a powerful strategy for stabilizing lithium metal interfaces.
• TFMSPyr-based SIPIL electrolytes enable practical and high-performance Anodeless LMBs.
• The developed electrolytes significantly improve cycle life and Coulombic efficiency in lithium metal batteries.