title: 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. LiO coordination numbers with G4 are 2-3 times higher for [Li(G4)][TFSI] than [Li(G4)][NO3], and conversely the LiO 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 LiO 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 LiO contacts and G4 molecules coordinates to an average of 1.3 Li(+) cations. Li(+) and G4 form polynuclear complexes, of the form [Lix(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].
authors: Priority Research Centre for Advanced Fluids and Interfaces, Newcastle Institute for Energy & Resources, University of Newcastle, Callaghan, NSW 2308, Australia. rob.atkin@newcastle.edu.au.
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