Prompted by the observation that recent literature displays marked disagreement as to the proper treatment of data on the refractive indices of binary liquid mixtures, this paper clarifies the relationships among refractive index, molar volume and molar refraction and the corresponding mixing properties. It is shown that the molar refraction deviation function must be calculated on a mole fraction basis and the refractive index deviation function on a volume fraction basis, which makes it directly interpretable as a sign-reversed measure of the deviation of reduced free volume from ideality.

Refractive indices, molar volumes and molar refractions of binary liquid mixtures: concepts and correlations

DOI: 10.1002/aenm.201902116 batteries with higher energy densities.[5,6] However, lithium metal has been studied for over five decades and many challenges persist.[5–7] Lithium metal is highly reactive—a consequence of its low reduction potential—and continuous reactivity with the electrolyte leads to electrolyte loss and severe capacity fade.[5,6] Several approaches have been pursued to limit the side reactions that lithium may undergo with the electrolyte.[5,7] First, carbonate solvents primarily used in Li-ion batteries have been eschewed for ethers (1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME)) that show much higher Coulombic efficiencies upon lithium deposition and stripping.[7,8] Second, strategies such as the use of high salt content with the so-called “solvent-in-salt” systems have shown promise despite the high cost and high viscosity of the electrolyte solutions.[9–12] Furthermore, polymer coatings have been shown to support smooth lithium deposition.[13,14] Despite all—and in combination with—these strategies, the use of additives is paramount.[7] Additives are chosen that can preferentially react with lithium metal and passivate the surface, thereby limiting further reaction with the electrolyte and extending cycle life.[5,7,15] A plethora of additives such as inorganic salts (e.g., LiNO3), phosphates,[17] water,[18] solvents,[19] and fluorinated compounds,[20,21] among others have been explored.[7] Their working principle is as follows: these additives react with lithium metal to create a modified solid electrolyte interface (SEI) that is better able to stabilize the lithium metal surface. However, these additives continually react with any fresh lithium metal surface that is generated during cycling. Upon complete consumption of the additive, little is known about the lithium metal behavior. A recent study by Gasteiger et al.[22] provides important insight. They show that the cell Coulombic efficiency drops by over 60% upon consumption of the electrolyte additive (the widely used fluoroethylene carbonate) in Si-C composite anodes (which upon lithiation also reacts with and consumes the electrolyte like lithium metal). Increasing the additive concentration is often not a solution because of cost, decreased ionic conductivities, and possible hindrance of the desired redox kinetics. Hence, reactive additives may have limited utility because their continuous consumption will lead to a regime where the additive is no longer present and there is significant degradation in performance. Therefore, it is important to explore an alternative strategy: the development of nonreactive additives.[23,24] These nonreactive Lithium metal batteries have been plagued by the high reactivity of lithium. Reactive additives that can passivate the lithium metal surface and limit electrolyte accessibility to a fresh lithium surface have been widely explored, but can have limited utility with continuous consumption of the additive. In this work, an alternative strategy is explored. The use of nonreactive cosolvents such as nonpolar alkanes is studied and its is shown that hexane and cyclohexane addition to ether solvents (1,3-dioxolane and 1,2-dimethoxyethane) halves the nucleation and growth overpotentials for lithium deposition, increases the cell coulombic efficiency, improves the lithium deposition morphology, increases the electrolyte oxidative stability (>0.2 V), and doubles the cycle life—even when compared to a widely used fluorinated ether. The nonpolar alkanes modify the lithium-ion solvation environment and reduce the solvation free energy; hence reducing the reaction barrier for lithium deposition. Exploration of nonpolar alkanes as part of the electrolyte mixture is a promising strategy for controlling metal deposition.

Nonpolar Alkanes Modify Lithium-Ion Solvation for Improved Lithium Deposition and Stripping

In the present study, the simultaneous and accurate determination of liquid viscosity and surface tension of the n-alkanes n-hexane (n-C6H14), n-octane (n-C8H18), n-decane (n-C10H22), and n-hexadecane (n-C16H34) by surface light scattering (SLS) in thermodynamic equilibrium is demonstrated. Measurements have been performed over a wide temperature range from 283.15 K up to 473.15 K for n-C6H14, 523.15 K for n-C8H18, and 573.15 K for n-C10H22 as well as n-C16H34. The liquid dynamic viscosity and surface tension data with average total measurement uncertainties (k = 2) of (2.0 and 1.7) % agree with the available literature and contribute to a new database at high temperatures. Over the entire temperature range, a Vogel-type equation for the dynamic viscosity and a modified van der Waals equation for the surface tension represent the measured data for the four n-alkanes within experimental uncertainties. By also considering our former SLS data for n-dodecane (n-C12H26) and n-octacosane (n-C28H58), empirical models for the liquid viscosity and surface tension of n-alkanes were developed as a function of temperature and carbon number covering values between 6 and 28. Agreement between these models and reference correlations for further selected n-alkanes which were not included in the development procedure was found.

Liquid Viscosity and Surface Tension of n-Hexane, n-Octane, n-Decane, and n-Hexadecane up to 573 K by Surface Light Scattering (SLS).

New experimental densities and surface tensions for binary mixtures of {heptane + 1-butanol, + 1-hexanol, and + 1-octanol} in a temperature range of (288.15, 293.15, 298.15, 303.15, and 308.15) K are reported for the whole composition range. Densities were measured with an Anton Paar DMA 4500 densimeter. Surface tensions were measured using a Lauda TVT2 automated tensiometer, which uses the principle of the pending drop volume. The experimental data of pure liquids and mixtures have been used to calculate excess molar volumes and surface tension deviations of the mixtures as a function of mole fractions.

Density and Surface Tension Variation with Temperature for Heptane + 1-Alkanol

Densities and volumetric properties of 1,4-dioxane with ethanol, 3-methyl-1-butanol, 3-amino-1-propanol and 2-propanol binary mixtures at various temperatures

Refractive indices (n) for {1,4-dioxane + 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol, or 1-decanol} and surface tensions (σ) for {1,4-dioxane + 1-hexanol, 1-heptanol, 1-octanol, or 1-decanol} at the temperature 298.15 K and normal atmospheric pressure have been determined as a function of mole fractions. The experimental data have been used to calculate refractive index and surface tension deviations (Δn, Δσ), over the entire composition range. The results obtained have been discussed and interpreted in terms of the type and nature of the specific intermolecular interactions between the components.

Refractive indices and surface tensions of binary mixtures of 1,4-dioxane + n-alkanes at 298.15 K

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Refractive indices and surface tensions of binary mixtures of 1,4-dioxane + n-alkanes at 298.15 K