Structure and ion transport in glyme–lithium solvate ionic liquids with asymmetric imide anions

• Quantum chemistry has guided the design of Li salts with intermediate Lewis basicity. • Asymmetric imide anions were synthesized to develop glyme-based solvate ionic liquids. • Systems with intermediate Li + -anion association improve Li⁺ transport under anion blocking conditions. Solvate ionic liquids (SILs), a subclass of ionic liquids formed from equimolar mixtures of glymes and Li salts, have emerged as promising electrolytes with high thermal and electrochemical stabilities. However, their performance is limited by the conductivity–Li + transference number trade-off arising from the competing effects of cation–anion and cation–solvent interactions. In this study, we introduce Li salts bearing asymmetric bulky imide anions as new design axes for SILs. Guided by quantum chemical calculations of the Li + -anion stabilization energies, Li salts with intermediate Lewis basicities, including lithium N -acetyltrifluoromethanesulfonylimide (Li[TfNAc]) and lithium N -methyltrifluoromethanesulfonylimide (Li[TfNMe]), were evaluated in equimolar mixtures with tetraglyme (G4). G4-Li salt equimolar mixtures, [Li(G4)][TfNAc] and [Li(G4)][TfNMe], formed homogeneous liquids at room temperature, exhibiting relatively high Li + transference number under anion-blocking conditions. The ionicity, solvent-to-Li + diffusivity ratio, Onsager coefficients, electrophoretic NMR, and Raman spectra collectively demonstrated a coexistence of dissociated and associated ion species, reflecting mixed Li + –anion and Li + –solvent coordination environments. This behavior is distinct from the dissociative systems with weakly coordinating anions (e.g., bis(trifluoromethanesulfonyl)amide, [Tf 2 N] − ) and the strongly associated systems with highly basic anions (e.g., trifluoroacetate, [TFA] − ) reported in previous studies. These findings indicate asymmetric imide anions with intermediate Lewis basicity as a promising strategy within the present framework for improving Li + transport while balancing the conductivity–transference number trade-off in SILs.