Tuning structure and transport properties in triglyme-based solvate ionic liquids via controlled water addition.

Abstract

Solvate ionic liquids (SILs) are characterised by high thermal and electrochemical stability but suffer from high viscosities and limited ionic conductivities, restricting their application as battery electrolytes. Recently, we introduced water-in-solvate-ionic-liquid (WISIL) electrolytes [Philipp et al., ChemPhysChem, 2026, 27, e70353], demonstrating that the addition of one equivalent of water per [Li]+ substantially improves transport properties while preserving the characteristic chelate structure and broad electrochemical stability window of the parent SIL. Here, we investigate how further addition of water affects the local lithium solvation environment, the stability of the cationic chelate complexes and the resulting transport properties of ternary lithium bis(trifluoromethanesulfonyl)imide:triglyme:water ([Li][NTf2]:G3:H2O) mixtures containing up to eight water equivalents per [Li][NTf2]. Combining molecular dynamics simulations with PFG-NMR, eNMR, viscosity, conductivity and cyclic voltammetry measurements, we show that water gradually replaces G3 within the first lithium coordination sphere, thereby increasingly destabilising and eventually disintegrating the characteristic chelate complexes. These structural changes are directly reflected in the transport properties, leading to higher self-diffusion coefficients and ionic conductivities together with a pronounced decrease in viscosity. While the characteristic SIL coordination motif and concerted [Li]+-G3 transport persist up to approximately two water equivalents per lithium ion, further dilution leads to increasingly hydrated lithium species and a gradual transition towards a concentrated aqueous electrolyte. These results establish a direct link between microscopic lithium solvation, coordination dynamics and macroscopic transport behaviour, providing valuable design guides for future electrolytes.