Towards 3D-lithium ion microbatteries based on silicon/graphite blend anodes using a dispenser printing technique.

In this work we present for the first time high capacity silicon/carbon-graphite blend slurries designed for application in 3D-printed lithium ion microbatteries (3D-MLIBs). The correlation between electrochemical and rheological properties of the corresponding slurries was systematically investigated with the prospect of production by an automated dispensing process. A variation of the binder content (carboxymethyl cellulose/styrene-butadiene rubber, CMC/SBR) between 6 wt%, 12 wt%, 18 wt% and 24 wt% in the anode slurry proved to be crucial for the printing process. Regarding the rheological properties increasing binder content leads to increased viscosity and yield stress values promising printed structures with high aspect ratios. Consequently, interdigital 3D-printed micro anode structures with increasing aspect ratios were printed with increasing binder content. For printed 6-layer structures aspect ratios of 6.5 were achieved with anode slurries containing 24 wt% binder. Electrochemical results from planar coin cell measurements showed that anodes containing 12 wt% CMC/SBR binder content exhibited stable cycling at the highest charge capacities of 484 mA h g-1 at a current rate of C/4. Furthermore, at 4C the cells showed high capacity retention of 89% compared to cycling at C/4. Based on this study and the given material formulation we recommend 18 wt% CMC/SBR as the best trade-off between electrochemical and rheological properties for future work with fully 3D-printed MLIBs.

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.

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.

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.

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.

Charge-scaling effect in ionic liquids from the charge-density analysis of N,N'-dimethylimidazolium methylsulfate.

The charge scaling effect in ionic liquids was explored on the basis of experimental and theoretical chargedensity analyses of [C1MIM][C1SO4] employing the quantum theory of atoms in molecules (QTAIM) approach. Integrated QTAIM charges of the experimental (calculated) charge density of the cation and anion resulted in non-integer values of ±0.90 (±0.87) e. Efficient charge transfer along the bond paths of the hydrogen bonds between the imidazolium ring and the anion was considered as the origin of these reduced charges. In addition, a detailed QTAIM analysis of the bonding situation in the [C1SO4]- anion revealed the presence of negative πO→σ*S-O hyperconjugation.

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.

Free volume in ionic liquids: a connection of experimentally accessible observables from PALS and PVT experiments with the molecular structure from XRD data.

In the current work, free volume concepts, primarily applied to glass formers in the literature, were transferred to ionic liquids (ILs). A series of 1-butyl-3-methylimidazolium ([C4MIM](+)) based ILs was investigated by Positron Annihilation Lifetime Spectroscopy (PALS). The phase transition and dynamic properties of the ILs [C4MIM][X] with [X](-) = [Cl](-), [BF4](-), [PF6](-), [OTf](-), [NTf2](-) and [B(hfip)4](-) were reported recently (Yu et al., Phys. Chem. Chem. Phys., 2012, 14, 6856-6868). In this subsequent work, attention was paid to the connection of the free volume from PALS (here the mean hole volume, ) with the molecular structure, represented by volumes derived from X-ray diffraction (XRD) data. These were the scaled molecular volume Vm,scaled and the van der Waals volume V(vdw). Linear correlations of at the "knee" temperature ((T(k))) with V(m,scaled) and V(vdw) gave good results for the [C4MIM](+) series. Further relationships between volumes from XRD data with the occupied volume Vocc determined from PALS/PVT (Pressure Volume Temperature) measurements and from Sanchez-Lacombe Equation of State (SL-EOS) fits were elaborated (V(occ)(SL-EOS) ≈ 1.63 V(vdw), R(2) = 0.981 and V(occ)(SL-EOS) ≈ 1.12 V(m,scaled), R(2) = 0.980). Finally, the usability of V(m,scaled) was justified in terms of the Cohen-Turnbull (CT) free volume theory. Empirical CT type plots of viscosity and electrical conductivity showed a systematic increase in the critical free volume with molecular size. Such correlations allow descriptions of IL properties with the easily accessible quantity V(m,scaled) within the context of the free volume.

Free volume and phase transitions of 1-butyl-3-methylimidazolium based ionic liquids from positron lifetime spectroscopy.

Positron annihilation lifetime spectroscopy (PALS) was used to study a series of ionic liquids (ILs) with the 1-butyl-3-methylimidazolium cation ([C4MIM](+)) but different anions [Cl](-), [BF4](-), [PF6](-), [OTf](-), [NTf2](-), and [B(hfip)4](-) with increasing anion volumes. Changes of the ortho-positronium (o-Ps) lifetime parameters with temperature were observed for crystalline and amorphous (glass, supercooled, and normal liquid) states. Evidence for distinct phase transitions, e.g. melting, crystallization and solid-solid transitions, was observed in several PALS experiments. The o-Ps mean lifetime τ3 showed smaller values in the crystalline phase due to dense packing of the material compared to the amorphous phase. The o-Ps lifetime intensity I3 in the liquid state is clearly smaller than in the crystallized state. This behaviour can be attributed to a solvation of e(+) by the anions, which reduces the Ps formation probability in the normal and supercooled liquid. These phenomena were observed for the first time when applying the PALS technique to ionic liquids by us in one preliminary and in this work. Four of the ionic liquids investigated in this work ([BF4](-), [NTf2](-), [PF6](-) and [Cl](-) ILs) exhibit supercooled phases. The specific hole densities and occupied volumes of those ILs were obtained by comparing the local free volume with the specific volume from pressure-volume-temperature (PVT) experiments. From the o-Ps lifetime, the mean size vh of free volume holes of the four samples was calculated and compared with that calculated according to Fürth's hole theory. The hole volumes from both methods agree well. From the Cohen-Turnbull fitting of viscosity and conductivity against PALS/PVT results, the influence of the free volume on molecular transport properties was investigated.

Is universal, simple melting point prediction possible?

An investigation of the melting points of 520 organic 1:1 salts is presented with the aim of developing a universal, simple, physically well-founded prediction scheme. The general reliability and reproducibility of the recorded experimental data are discussed with respect to purity, phase behavior, disorder and thermal history of a given substance. Additionally, mistakes, systematic errors, or lack of conventions can lead to considerable differences in the experimental measurements. A rough error bar for the reproducibility of the melting points of organic salts of ±5 to ±15 °C can be assigned. With this restraint, we developed two simple, semiempirical, five- and nine-parameter schemes with easy-to-calculate quantities. With these, we could predict the melting temperature of a given organic salt in the temperature range of -25 to +300 °C with an average error of 33.5 °C and a relative error of 9.3%. All calculated quantities are assessed with the help of conventional DFT, COSMO and COSMO-RS calculations, and are currently implemented into the IL-Prop module of the upcoming version of COSMOtherm. These prediction schemes are suitable for high-throughput computational screening of substances in the context of "computer-aided synthesis". Therefore, they are valuable tools to find a compound with a suitable melting point before its first synthesis.

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 Angell's 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).

Rotational dependence of the proton-transfer reaction HBr+ + CO2-->HOCO+ + Br. I. Energy versus angular momentum effects.

Cross sections for the endothermic proton-transfer reactions of rotationally state-selected HBr(+) and DBr(+) ions with CO(2) were measured in a guided ion beam apparatus in order to determine the influence of rotational excitation and collision energy in the center of mass (c.m.) system on the cross section. Ab initio calculations were performed to obtain energetic information about reactants, intermediates, and products. In the experiment HBr(+) and DBr(+) ions were prepared with the same mean rotational quantum number but different mean rotational energies as the rotational constants differ by about a factor of two. The mean rotational energy was varied from 1.4 to 66.3 meV for HBr(+) and from 0.7 to 43.0 meV for DBr(+). Collision energies (E(c.m.)) ranged from 0.32 to 1.00 eV. Under all conditions considered, an increase in the rotational excitation leads to a decrease in the cross section for both reactions. However, the effect is more pronounced for the higher collision energies. For E(c.m.)=1.00 and 0.85 eV; a comparison between the results for HBr(+) and DBr(+) indicates that the cross section is dominated by effects of rotational energy rather than angular momentum. For lower collision energies the cross sections for the deuteron transfer and the proton transfer are in best agreement if not compared for the same c.m. collision energy but for the same value of the difference between the collision energy and the reaction enthalpy.