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].

Boundary layer friction of solvate ionic liquids as a function of potential

“…11 Conversely, Li(G4) NO 3 is a poor SIL, due to the highly favourable coordination of Li + with the strongly basic NO 3 À anion leading to high concentration of free glymes in the mixture and reduced the ionic properties of the obtained SIL. 8 Recently, the bulk structure of SILs has been investigated; 14,15 however, ion arrangements of SILs at electrode interfaces have rarely been investigated despite their importance for electrochemical applications. 16 Conventional IL ions interact strongly with solid surfaces and present pronounced interfacial structures, which can be divided into three distinct regions: the boundary (surface-adsorbed) layer, the transition zone (near-surface layers) and the bulk phase.…”

Kamlet–Taft Solvation Parameters of Solvate Ionic Liquids

“…Recent studies of the nanostructure of the good SIL [Li(G4)]NTf 2 (bis(trifluoromethylsulfonyl)imide) and poor SIL [Li(G4)]NO 3 shows that the coordination around the lithium changes significantly between SILs. In the good SIL the lithium is coordinated on average by 2.27 G4 oxygen atoms and 1.73 NTf 2 oxygen atoms whereas in the poor SIL the lithium is coordinated by only 0.93 G4 oxygen atoms and 4.39 nitrate oxygen atoms …”

“…Selection criteria for good SILs exist, based on empirical data. [5] The nanostructure [7,8] and dynamics [8] in SILs is complex. A lithium ion's preferred coordination number is 4o r5 , [9] which leads to the expectation that G3 or G4, with 4a nd 5c oordination sites, respectively,a re the most appropriateo ligoethers for use in SILs, and this is indeed what was found; [10,11] The chelate effect provides significant additional stabilityo ver similar systemsm ade of smaller monodentate speciesp resent as 4equivalences (per lithium).…”

“…In the good SIL the lithium is coordinated on averageb y 2.27 G4 oxygen atoms and 1.73 NTf 2 oxygen atoms whereas in the poor SIL the lithium is coordinated by only 0.93 G4 oxygen atoms and 4.39 nitrateo xygen atoms. [7] SILs offer the usual benefits and limitations as ionic liquids, however the presenceo fl ithium in high concentrations provides additional opportunities for electrochemical processes, such as lithium secondaryb atteries. [12] Additionally investigating the SILs as pure compounds can provide insight into the behaviour of more dilute solutionso fs alts in glymes for other applications, like air [13] and sulfur [14] batteries.…”

Categorizing Molten Salt Complexes as Ionic Liquids and Their Applications to Battery Electrolytes

“…Studies of the fundamentals and applications are further required to establish the concept of solvate ILs. The bulk liquid structure of the glyme-Li salt solvate IL was revealed by high-energy X-ray diffraction measurements, 31 neutron diffraction measurements, 32 and MD simulations. 16,33 Studies of the solid-liquid interface are also quite important to understand the electrode reactions in batteries, especially when desolvation/solvation of the complex cations occurs.…”