Curled cation structures accelerate the dynamics of ionic liquids

Abstract

Ionic liquids are modern liquid materials with potential and actual implementation in many advanced technologies. They combine many favourable and modifiable properties but have a major inherent drawback compared to...

Novel Phosphonium-Based Ionic Liquid Electrolytes for Battery Applications

“…In Walden analysis, exponents smaller than unity which are found for the phosphonium ILs and phosphonium IL lithium salt mixtures here, are a common finding for ionic liquids [ 27 , 47 , 58 , 59 ] as molar conductivity and viscosity do not show an ideal reverse relationship, which also manifests in the slightly different activation energies for conductivity and viscosity [ 60 ]. All samples with the [FSI] − anions are closer to the bisection of the Walden plot, which is sometimes referred to as the ‘ideal KC line’, although the assumption is somewhat arbitrary and has been criticized [ 48 , 61 ].…”

“…Additionally, the ionic liquid [2O2a] with and without additional [Li][TFSI] salt was the only IL where ordered solids could not be observed. Due to the altered cation conformation of the ether side chains in the 2-ethoxy-ethyl derivatives, the effect of the additional conducting salt on the melting point might be limited [ 46 , 47 ]. Additionally, the influence and disturbance of additional ions in the IL matrix affect the well-known supercooling effect, which is more pronounced in salt mixtures.…”

“…With similar argumentation, the ionicity of the alkylated samples is higher than the ether substituted and is decreased upon side chain elongation, while adding lithium salt influences the ionicity only to a minor extent. Slightly lower viscosity values for ether-containing ionic liquids in comparison to their isostructural alkylated counterparts are also reported for other cation classes, such as phosphonium [ 40 ] or ammonium [ 27 , 47 ].…”

“…The markedly lower viscosities of the ether-containing samples compared to the hydrocarbon analogues can be explained by altered cation conformation for the ether samples. While alkyl side chains are reported to prefer linear geometries, samples with 2-alkoxy-ethyl side chains often show curling of the side chain towards the positively charged cation center [ 46 , 47 , 57 ]. This altered cation conformation of the ether ILs causes more spherical cations and reduces the cation–anion interaction by shielding the cation charge and blocking coordination sites for the anion [ 47 ].…”

“…While alkyl side chains are reported to prefer linear geometries, samples with 2-alkoxy-ethyl side chains often show curling of the side chain towards the positively charged cation center [ 46 , 47 , 57 ]. This altered cation conformation of the ether ILs causes more spherical cations and reduces the cation–anion interaction by shielding the cation charge and blocking coordination sites for the anion [ 47 ]. The curling of the ether side chain as a reason for the reduced ionic interactions also explains why the viscosity of the [2O2a] sample is slightly lower than for [2O1a], an effect also observed for ammonium-based ionic liquids [ 27 ].…”