Viscosity of Molten ZnCl2and Supercritical Behavior in Its Binary Solutions

Dynamics of an Excess Electron in Molten LiF, KF, MgF2, and BeF2

Ab initio molecular dynamics simulations suggest that the dynamics of an excess electron in different types of molten salts are not always the same. In molten LiF, KF, and MgF2, the excess electron localizes in the cavity as a solvated electron for 10 ps, which agrees with the widely accepted theory of Pikaev. In molten BeF2, the excess electron shows a different localization pattern: it mostly exists in localized states but also occurs in many delocalized states. This "localize-delocalize" pattern originates from the high viscosity of BeF2 (16 000 cP at 900 °C), which will lead to slow ionic motion and finally result in slow solvent relaxation. Besides, the species formed by the localization of the excess electron in these four melts are also different. The spectral feature (broad peak in the vis-IR region) of the localized electron in molten alkaline halides was also observed in LiF, KF, MgF2, and BeF2. Both an excess electron and electrons in the bulk liquid could contribute to the spectra, but the excitation of the excess electron makes a bigger contribution to the broad vis-IR peak. Our predicted spectrum of molten LiF/KF qualitatively reproduces the major feature of the experimental spectrum, which partially validates our simulations.

Amide-Based Ionic Liquid Electrolytes for Alkali-Metal-Ion Rechargeable Batteries

Ionic liquids (ILs) have a wide variety of applications in energy storage and material production. ILs are composed of only cations and anions, without any molecular solvents, and are generally known as "designer liquids (solvents)" because their physicochemical properties can be tuned by the combination of ionic species. In recent several decades, research and development activities of rechargeable batteries have garnered considerable attention because certain groups of ILs exhibit high electrochemical stability and moderate ionic conductivity, rendering them suitable for application in high-voltage batteries. ILs with amide anions are representative electrolytes and are extensively researched by many research groups, including our group. This paper focuses on amide-based ILs as electrolytes for alkali-metal-ion rechargeable batteries, introducing their history, characteristics, and existing challenges to be addressed.

Why the Brillouin Light Scattering Relaxation Times of Molten Zinc Chloride Are Shorter and Weaker in Temperature Dependence than the Structural Relaxation Times from Depolarized Light and Neutron Spin Echo Spectroscopy

A longstanding problem in the Brillouin light scattering (BLS) study of polymers is the relaxation times τ_BLS(T) being more than an order of magnitude shorter than the α-relaxation times τ_α(T) determined by dielectric, depolarized light scattering (DLS), and molecular dynamics simulations. In tackling the problem, τ_BLS(T) was identified with the relaxation time τ₀(T) of the primitive relaxation in the coupling model, which can be calculated from τ_α(T) and the stretch exponent β_K of the Kohlrausch correlation function for the α-relaxation.. The problem was solved by finding that indeed τ₀(T) is in good agreement with τ_BLS(T). A recent work performed the neutron spin echo study of the structural α-relaxation of the network ionic liquid ZnCl₂ and found the same anomaly as polymers. The α-relaxation time τ_NSE(T) from neutron spin echo (NSE) as well as the α-relaxation time τ_DLS(T) from DLS of ZnCl₂ are much longer than τ_BLS(T) from BLS obtained before by several research groups. The finding of the same anomaly in ZnCl₂ and polymers with very different chemical and physical structures offers an opportunity to critically test the explanation given before. The test was carried out by calculating the primitive relaxation times τ_0,DLS(T) and τ_0,NSE(T) from τ_DLS(T) and τ_NSE(T), respectively, in zinc chloride. Good agreements of τ_BLS(T) from BLS with τ_0,DLS(T) and τ_0,NSE(T) were found and thus the explanation given for polymers remains valid for ZnCl₂. The test was extended to glycerol by comparing τ_BLS(T) with τ_0,ICNS(T) and τ_0,CNS(T) calculated from the α-relaxation time τ_ICNS(T) and τ_CNS(T) from incoherent and coherent neutron scattering, respectively. There is good agreement between τ_BLS(T) and τ_0,ICNS(T) in glycerol. There is also semiquantitative agreement of τ_BLS(T) with τ_0,DS(T) from dielectric spectroscopy as well as τ_0,CNS(T). Thus, the explanation for polymers is verified in the two very different glass formers, ZnCl₂ and glycerol, and it is an advancement in the application of BLS to study the dynamics of glass formers.

Neutron Spin-Echo Studies of the Structural Relaxation of Network Liquid ZnCl2 at the Structure Factor Primary Peak and Prepeak

Using neutron spin-echo spectroscopy, we studied the microscopic structural relaxation of a prototypical network ionic liquid ZnCl₂ at the structure factor primary peak and prepeak. The results show that the relaxation at the primary peak is faster than the prepeak and that the activation energy is ∼33% higher. A stretched exponential relaxation is observed even at temperatures well-above the melting point T_m. Surprisingly, the stretching exponent shows a rapid increase upon cooling, especially at the primary peak, where it changes from a stretched exponential to a simple exponential on approaching the T_m. These results suggest that the appearance of glassy dynamics typical of the supercooled state even in the equilibrium liquid state of ZnCl₂ as well as the difference of activation energy at the two investigated length scales are related to the formation of a network structure on cooling.

Universal correlations between the fragility and interparticle repulsion of glass-forming liquids

A recently published analytical model describing and predicting elasticity, viscosity, and fragility of metallic melts is applied for the analysis of about 30 nonmetallic glassy systems, ranging from oxide network glasses to alcohols, low-molecular-weight liquids, polymers, plastic crystals, and even ionic glass formers. The model is based on the power-law exponent λ representing the steepness parameter of the repulsive part of the inter-atomic or inter-molecular potential and the thermal-expansion parameter α_T determined by the attractive anharmonic part of the effective interaction. It allows fitting the typical super-Arrhenius temperature variation of the viscosity or dielectric relaxation time for various classes of glass-forming matter, over many decades. We discuss the relation of the model parameters found for all these different glass-forming systems to the fragility parameter m and detect a correlation of λ and m for the non-metallic glass formers, in accord with the model predictions. Within the framework of this model, the fragility of glass formers can be traced back to microscopic model parameters characterizing the intermolecular interactions.

Comparative Study of M[N(SO2F)(SO2CF3)]-[N-Butyl-N-methylpyrroridinium][N(SO2F)(SO2CF3)] (M = Li, Na, K, Rb, Cs) Ionic Liquid Electrolytes

We systematically evaluated the physicochemical properties of a series of M[FTA]-[C₄C₁pyrr][FTA] ionic liquids (ILs) (M = alkali metal, FTA = (fluorosulfonyl)(trifluoromethylsulfonyl)amide, C₄C₁pyrr = N-butyl-N-methylpyrrolidinium) as electrolytes for alkali metal-ion batteries. First, the viscosity (η), ionic conductivity (σ), and density (ρ) of the M[FTA]-[C₄C₁pyrr][FTA] ILs at x(M[FTA]) = 0.20 (x(M[FTA]) = molar fraction of M[FTA]) were measured. The σ values ranged from 1-3 mS cm^-1 at 298 K and increased as follows: Na < Li < K < Rb < Cs, which indicated that the Li-based IL did not obey the trend predicted by the charge densities of alkali metal cations. Second, the Li-based IL exhibited slightly lower vertical intercept values than the other FTA-based ILs in the Walden plots obtained using the results of η, σ, and ρ measurements. Third, the electrochemical stability of the ILs was investigated by cyclic voltammetry, and the redox potentials of the alkali metals (E(M⁺/M)) were determined. The E(M⁺/M) values of the FTA-based ILs increased as follows: Cs < Rb < K < Li < Na. Subsequently, we compared the obtained E(M⁺/M) values with those of other general electrolytes, such as propylene carbonate (PC)-based electrolytes and aqueous solutions. The trend in E(M⁺/M) values of the FTA-based ILs was similar to that of PC-based electrolytes and was significantly different from that of aqueous solutions. In particular, the FTA- and FSA-based ILs (FSA = bis(fluorosulfonyl)amide) presented the most negative E(Na⁺/Na) and E(K⁺/K) values among various electrolytes, which indicated that utilization of these IL electrolytes for the development of Na- and K-ion batteries would present significant advantages.

Brillouin scattering study of molten zinc chloride

Polarized and depolarized Brillouin scattering experiments on molten ZnCl2 were performed between 300 and 600 degrees C in different geometries. VV spectra measured in backscattering and small angle scattering were analyzed with conventional viscoelastic theory using either a Debye or a Cole-Davidson model for the memory function. We also analyzed in the same way the temperature dependence of the transverse Brillouin lines detected in a 90 degrees VH geometry. We show that the Cole-Davidson memory function yields a consistent interpretation of all the spectra. The resulting shear and longitudinal relaxation times are equal within their error bars, and are about 2.5 times smaller than the alpha relaxation time previously determined. The static shear viscosity values deduced from the analysis of the propagating transverse waves agree, at all temperatures, with the measured viscosity values.