Abstract
Physical and electrochemical properties of triethylsulfonium bis(trifluoromethylsulfonyl)imide (S222TFSI) ionic liquids with highly concentrated lithium bis(fluorosulfonyl)imide (LiFSI) were studied for application as stable electrolytes over a wide temperature range for Li metal batteries. xLiFSI-(1-x)S222TFSI ionic liquids exhibited a wide liquid-phase temperature range between x = 0.1 and 0.7. The ionic conductivity (σ) at 25 °C decreased from 5.6 to 0.27 mS cm−1 as x increased from 0.1 to 0.6, while the lithium transfer number (tLi) significantly increased with increasing x. The product of σ and tLi took a maximum value at x = 0.3. Impedance spectra of Li/Li symmetric cells using the ionic liquid electrolytes showed that the interface resistance (Ri) of the Li electrode decreased with increasing x up to 0.5. An increase in Ri with aging time caused by the formation of a solid electrolyte interphase layer was suppressed by increasing LiFSI concentration. Highly concentrated LiFSI ionic liquid at x = 0.6 significantly enhanced the cycling stability of Li deposition-dissolution, indicating that highly concentrated LiFSI ionic liquids from x = 0.3 to 0.6 are promising stable electrolytes with a wide liquid-phase temperature range for Li metal batteries.
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