Li+ Local Structure in Li-Tetraglyme Solvate Ionic Liquid Revealed by Neutron Total Scattering Experiments with the 6/7Li Isotopic Substitution Technique

Equimolar mixtures of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and tetraglyme (G4: CH3O (CH2CH2O)(4)-CH3) yield the solvate (or chelate) ionic liquid [Li(G4)][TFSA], which is a homogeneous transparent solution at room temperature. Solvate ionic liquids (SILs) are currently attracting increasing research interest, especially as new electrolytes for Li sulfur batteries. Here, we performed neutron total scattering experiments with Li-6/7 isotopic substitution to reveal the Li+ solvation/local structure in [Li(G4)][TFSA] SILs. The experimental interference function and radial distribution function around Li+ agree well with predictions from ab initio calculations and MD simulations. The model solvation/local structure was optimized with,nonlinear least-squares analysis to yield structural parameters. The refined Li+ solvation/local structure in the [Li(G4)][TFSA] SIL shows that lithium cations are not coordinated to all five oxygen atoms of the G4 molecule (deficient five-coordination) but only to four of them (actual four-coordination). The solvate cation is thus considerably distorted, which can be ascribed to the limited phase space of the ethylene oxide chain and competition for coordination sites from the TFSA anion.