Bulk nanostructure of the prototypical ‘good’ and ‘poor’ solvate ionic liquids [Li(G4)][TFSI] and [Li(G4)][NO3]

Abstract

The bulk nanostructures of a prototypical ‘good’ solvate ionic liquid (SIL) and ‘poor’ SIL have been examined using neutron diffraction and empirical potential structure refinement (EPSR) simulated fits. The good SIL formed by a 1 : 1 mixture of lithium bis(trifluoromethylsulfonyl)imide (Li[TFSI]) in tetraglyme (G4), denoted [Li(G4)][TFSI], and the poor SIL formed from a 1 : 1 mixture of lithium nitrate (Li[NO₃]) in G4, denoted [Li(G4)][NO₃], have been studied. In both SILs there are strong Lewis acid–base interactions between Li⁺ and ligating O atoms. However, the O atoms coordinated to Li⁺ depend strongly on the counter anion present. Li⋯O coordination numbers with G4 are 2–3 times higher for [Li(G4)][TFSI] than [Li(G4)][NO₃], and conversely the Li⋯O anion coordination number is 2–3 times higher in [Li(G4)][NO₃]. In both solvates the local packing of Li around G4 O atoms are identical but these interactions are less frequent in [Li(G4)][NO₃]. In both SILs, Li⁺ has a distribution of coordination numbers and a wide variety of different complex structures are present. For [Li(G4)][NO₃], there is a significant proportion uncoordinated G4 in the bulk; ∼37% of glyme molecules have no Li⋯O contacts and each G4 molecule coordinates to an average of 0.5 Li⁺ cations. Conversely, in [Li(G4)][TFSI] only ∼5% of G4 molecules lack Li⋯O contacts and G4 molecules coordinates to an average of 1.3 Li⁺ cations. Li⁺ and G4 form polynuclear complexes, of the form [Li_x(G4)_y]^x+, in both solvates. For [Li(G4)][TFSI] ∼35% of Li⁺ and G4 form 1 polynuclear complexes, while only ∼10% of Li⁺ and G4 form polynuclear complexes in [Li(G4)][NO₃].