Witali Beichel

Temperature Dependence of the Viscosity and Conductivity of Mildly Functionalized and Non‐Functionalized [Tf2N]− Ionic Liquids

A series of bis(trifluoromethylsulfonyl)imide ionic liquids (ILs) with classical as well as mildly functionalized cations was prepared and their viscosities and conductivities were determined as a function of the temperature. Both were analyzed with respect to Arrhenius, Litovitz and Vogel-Fulcher-Tammann (VFT) behaviors, as well as in the context of their molecular volume (V(m)). Their viscosity and conductivity are highly correlated with V(m)/T or related expressions (R(2) ≥0.94). With the knowledge of V(m) of new cations, these correlations allow the temperature-dependent prediction of the viscosity and conductivity of hitherto unknown, non- or mildly functionalized ILs with low error bars (0.05 and 0.04 log units, respectively). The influence of the cation structure and mild functionalization on the physical properties was studied with systematically altered cations, in which V(m) remained similar. The T(o) parameter obtained from the VFT fits was compared to the experimental glass temperature (T(g)) and the T(g)/T(o) ratio for each IL was calculated using both experimental values and Angells relationship. With Walden plots we investigated the IL ionicity and interpreted it in relation to the cation effects on the physical IL properties. We checked the validity of these V(m)/T relations by also including the recently published variable temperature viscosity and conductivity data of the [Al(OR(F))(4)](-) ILs with R(F) =C(H)(CF(3))(2) (error bars for the prediction: 0.09 and 0.10 log units, respectively).

Size matters! On the way to ionic liquid systems without ion pairing.

Several, partly new, ionic liquids (ILs) containing imidazolium and ammonium cations as well as the medium-sized [NTf2 ](-) (0.230 nm(3) ; Tf=CF3 SO3 (-) ) and the large [Al(hfip)4 ](-) (0.581 nm(3) ; hfip=OC(H)(CF3 )2 ) anions were synthesized and characterized. Their temperature-dependent viscosities and conductivities between 25 and 80 °C showed typical Vogel-Fulcher-Tammann (VFT) behavior. Ion-specific self-diffusion constants were measured at room temperature by pulsed-gradient stimulated-echo (PGSTE) NMR experiments. In general, self-diffusion constants of both cations and anions in [Al(hfip)4 ](-) -based ILs were higher than in [NTf2 ](-) -based ILs. Ionicities were calculated from self-diffusion constants and measured bulk conductivities, and showed that [Al(hfip)4 ](-) -based ILs yield higher ionicities than their [NTf2 ](-) analogues, the former of which reach values of virtually 100 % in some cases.From these observations it was concluded that [Al(hfip)4 ](-) -based ILs come close to systems without any interactions, and this hypothesis is underlined with a Hirshfeld analysis. Additionally, a robust, modified Marcus theory quantitatively accounted for the differences between the two anions and yielded a minimum of the activation energy for ion movement at an anion diameter of slightly greater than 1 nm, which fits almost perfectly the size of [Al(hfip)4 ](-) . Shallow Coulomb potential wells are responsible for the high mobility of ILs with such anions.

Establishing consistent van der Waals volumes of polyatomic ions from crystal structures.

Based on temperature (T) dependent crystal structure data of seven organic salts, a radii-based scheme for the calculation of the van der Waals volume (V(vdw)) is analyzed. The obtained volumes (V(vdw,r), r=radius-based) are nearly T independent. An ion volume partitioning scheme is proposed by fixing the anion volumes of [Cl](-), [Br](-), [I](-), [BF(4)](-), [PF(6)](-), [OTf](-) and [NTf(2)](-). The van der Waals volumes (V(vdw,r) (+/-)) of 48 ions are established, with low standard deviations (0.2-3.6 Å(3), 0.1-4.5 % of V(vdw,r) (+/-)). The ion volumes are independent of the counterion and one crystal structure already suffices for their derivation. Correlations of the viscosity with V(vdw,r) via a Litovitz ansatz and our recently derived Arrhenius-type approach prove that these volumes are suitable for the volume-based description and prediction of IL properties. The corresponding correlation coefficient for the latter is R(2)=0.86 for 40 ILs (354 data points) in the T range of 253-373 K.

Inhibiting Polysulfide Shuttle in Lithium–Sulfur Batteries through Low-Ion-Pairing Salts and a Triflamide Solvent

The step-change in gravimetric energy density needed for electrochemical energy storage devices to power unmanned autonomous vehicles, electric vehicles, and enable low-cost clean grid storage is unlikely to be provided by conventional lithium ion batteries. Lithium-sulfur batteries comprising lightweight elements provide a promising alternative, but the associated polysulfide shuttle in typical ether-based electrolytes generates loss in capacity and low coulombic efficiency. The first new electrolyte based on a unique combination of a relatively hydrophobic sulfonamide solvent and a low ion-pairing salt, which inhibits the polysulfide shuttle, is presented. This system behaves as a sparingly solvating electrolyte at slightly elevated temperatures, where it sustains reversible capacities as high as 1200-1500 mAh g-1 over a wide range of current density (2C-C/5, respectively) when paired with a lithium metal anode, with a coulombic efficiency of >99.7 % in the absence of LiNO3 additive.

Straightforward Synthesis of the Brønsted Acid hfipOSO3H and its Application for the Synthesis of Protic Ionic Liquids

The easily accessible hexafluoroisopropoxysulfuric acid (1, hfipOSO3H; hfip = C(H)(CF3)2) was synthesized by the reaction of hexafluoroisopropanol and chlorosulfonic acid on the kilogram scale and isolated in 98 % yield. The calculated gas-phase acidity (GA) value of 1 is 58 kJ mol(-1) lower in ΔG° than that of sulfuric acid (GA value determined by a CCSD(T)-MP2 compound method). Considering the gas-phase dissociation constant as a measure for the intrinsic molecular acid strength, a hfipOSO3H molecule is more than ten orders of magnitude more acidic than a H2SO4 molecule. The acid is a liquid at room temperature, distillable at reduced pressure, stable for more than one year in a closed vessel, reactive towards common solvents, and decomposes above 180 °C. It is a versatile compound for further applications, such as the synthesis of ammonium- and imidazolium-based air- and moisture-stable protic ionic liquids (pILs). Among the six synthesized ionic compounds, five are pILs with melting points below 100 °C and three of them are liquids at nearly room temperature. The conductivities and viscosities of two representative ILs were investigated in terms of Walden plots, and the pILs were found to be little associated ILs, comparable to conventional aprotic ILs.

Estimation of Lattice Enthalpies of Ionic Liquids Supported by Hirshfeld Analysis

New measurements of vaporization enthalpies for 15 1:1 ionic liquids are performed by using a quartz-crystal microbalance. Collection and analysis of 33 available crystal structures of organic salts, which comprise 13 different cations and 12 anions, is performed. Their dissociation lattice enthalpies are calculated by a combination of experimental and quantum chemical quantities and are divided into the relaxation and Coulomb components to give an insight into elusive short-range interaction enthalpies. An empirical equation is developed, based on interaction-specific Hirshfeld surfaces and solvation enthalpies, which enables the estimation of the lattice enthalpy by using only the crystal-structure data.