Nanostructure of [Li(G4)] TFSI and [Li(G4)] NO₃solvate ionic liquids at HOPG and Au(111) electrode interfaces as a function of potential

Abstract

Atomic force microscopy (AFM) force measurements have been used to study the solvate ionic liquid (IL) double layer nanostructure at highly ordered pyrolytic graphite (HOPG) and Au(111) electrode surfaces as a function of potential. Two solvate ILs are investigated, [Li(G4)] TFSI and [Li(G4)] NO3. Normal force versus apparent separation data indicates that both solvate ILs adopt a multilayered morphology at the electrode interface, similar to conventional ILs. Calculations of adsorption free energies indicate that at 0 V the ion layer in contact with the electrode surface is enriched in the more surface active cations. When a positive or negative surface bias is applied, the concentration of counterions in the innermost layer increases, and higher push-through forces are required to displace near surface layers, indicating a stronger interfacial structure. Generally, [Li(G4)] TFSI has a better defined structure than [Li(G4)] NO3 on both electrode surfaces due to stronger cohesive interactions within layers. Interfacial structure is also better defined for both solvate ILs on HOPG than Au(111) due to the greater surface roughness of Au(111). Further, at all negative potentials on both surfaces, a small final step is observed, consistent with either compression of the complex cation adsorbed structure or desolvation of the glyme from the Li(+).

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

“…The smaller of these repeat lengths, characterised by the weak peak in the d 6 contrast, is consistent with the combined packing dimension of the Li + cation 51 and TFSI À anion 52 while the larger repeat length is slightly greater than the packing dimension expected for an [Li(G4)][TFSI] ion pair calculated using the molecular weight and density of the solvate, assuming a cubic packing geometry. 36,53 This suggests the presence of complexes larger than a simple 1 : 1 lithium : glyme complex cation, i.e. polynuclear complexes, which have been previously suggested.…”

“…Moreover, the solvation and coordination environment of Li + controls bulk transport 23,24,27,34,35 of Li + and its behaviour at electrode interfaces. 18,36 Understanding the coordination environment of Li + in the bulk of SILs is critical for practical applications. Li + has a complex coordination chemistry, forming complexes with coordination numbers between 2 to 8 in solution, and crystalline phases.…”

Ionic Liquid Induced Band Shift of Titanium Dioxide

“…it was clear that the energy levels could be shifted upon the addition of auxiliary components to the electrolyte, such as lithium ions, or pyridines, with the most prominent known as 4‐ tert ‐butylpyridine (4‐TBP) ,,,. The electrolyte itself can play an important role in this regard, especially as ionic liquids were shown to have a very active surface science . In particular, Zhang et al.…”

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

“…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. [34][35][36][37] Given that a large number of combinations of metal salts and ligands are available to form a complex, we can play with different types of complexes, some of which have the potential to form solvate ILs. 38 Moreover, one may introduce a new functionality into the solvate ILs to form, for example, stimuli-responsible ILs by designing the components of the complexes.…”