Ionic liquids with sulfinyl-functionalized imide anion and their lithium electrolytes: (I) Physical and electrochemical properties

Abstract

Imide-based ionic liquids (ILs) are intriguing candidates for constructing safer electrolytes and better rechargeable batteries. In this work, a sulfinyl-functionalized imide anion, (trifluoromethanesulfinyl) (trifluoromethanesulfonyl)imide anion ([(CF₃SO) (CF₃SO₂)N]⁻, [qTFSI]⁻), is proposed as negative charge for building low-melting ILs and high-performing electrolytes. The physicochemical properties of [qTFSI]-based ILs and their electrolytes are extensively characterized, and the reference systems with the classic sulfonimide anion, bis(trifluoromethanesulfonyl)imide anion ([(CF₃SO₂)₂N]⁻, [TFSI]⁻) are also comparatively investigated. It has been revealed that the [qTFSI]⁻ anion shows lesser extent of negative charge delocalization as compared to the reference [TFSI]⁻ anion, which is responsible for slightly stronger interactions between IL cations and the sulfinyl-functionalized anion. The asymmetric feature of the [qTFSI]⁻ anion contributes to lower glass and melting transitions of the corresponding ILs vs. [TFSI]-based ones, which effectively expands the operational temperature of the rechargeable batteries. Furthermore, the co-utilization of [qTFSI]⁻ with [TFSI]⁻ is found to improve the electrochemical compatibility of Li metal anode with the IL-based electrolytes, sustaining better cycling stability of the Li symmetric cells. The current work offers an elegant approach for the design of new anions for interface-favorable ILs and their electrolytes.