(335h) Lithium Ion Solvation and Electrodeposition in Ternary Ionic Liquid Electrolytes for Lithium Metal Batteries

Ionic Liquids (ILs) are promising electrolytes to realize rechargeable lithium-metal batteries due to their lack of flammability and high electrochemical stability. The two key properties of IL based electrolytes relevant to energy storage applications and specifically to Li-metal batteries are the ion transport in the bulk and the interfacial stability. While the potential of Li⁺ containing IL electrolytes have been recognized, the conductivity and charge-discharge cycling at high current densities remain a challenge. To address these challenges and improve our understanding of Li⁺ solvation, and electrodeposition in IL mixtures, we have studied the bulk and interfacial properties of LiFSI salt in an IL mixture composed of n-methl-n-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [PYR13][TFSI] and n-propyl-n-methylpyrrolidinium bis(fluorosulfonyl)imide, [PYR13][FSI]. The binary mixture of [PYR13][TFSI]/[PYR13][FSI] (v : v = 1 : 1) showed relatively high ionic conductivity (7.23 mS/cm at 30ËšC), low viscosity (39.08 mPaË‘s at 30ËšC) and wide electrochemical window (5.64 V measured with Ag quasi-reference at 27ËšC) with respect to common ILs. The viscosity, conductivity, and Li⁺ solvation in the ternary mixture LiFSI/[PYR13][TFSI]/[PYR13][FSI] were compared with those of binary mixtures of LiFSI/[PYR13][TFSI] and LiFSI/[PYR13][FSI], where the LiFSI concentration ranged from 0 to 1 M. Partial solvation of Li⁺ with [TFSI] and [FSI] in the mixture electrolytes was investigated by Raman spectroscopy as a function of the Li-salt concentration.

The stability of the electrodeposition-stripping process for lithium was examined via Li-Li symmetrical cell cycling. The ternary electrolyte, 0.5 M LiFSI in [PYR13][TFSI]/[PYR13][FSI] (v : v = 1 : 1), formed a more resistive solid electrolyte interphase during the initial chemical passivation as evident from the electrochemical impedance spectroscopy, compared to binary mixtures. Long term cycling of Li-Li symmetrical cells at 1, 10, and 100 mA/cm² were achieved with enhanced stability with the ternary electrolyte compared to the cells with binary and organic electrolytes. This study highlights the advantages of IL mixtures for Li-metal batteries with respect to the stability of the solid-electrolyte interface and the bulk conductivity.