Standard partial molar volumes of some electrolytes in ethylene carbonate based mixtures

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Abstract

Apparent molar volumes V2,φ and standard partial molar volumes V2,φ0 of LiClO4, LiBr and three symmetrical tetraalkylammonium bromides R4NBr (R=ethyl, propyl, butyl) have been determined at 298.15 K from precise density measurements in solvent mixtures of ethylene carbonate with tetrahydrofuran (THF), acetonitrile (AN), ethyl acetate (EA) and dimethoxyethane (DME). It is shown that the V2,φ0 values of LiClO4 and LiBr are dependent strongly on the nature of the solvents, whereas the contribution of CH2 group to the partial molar volume of the tetraalkylammonium salts has nothing to do with the nature of the solvents and the composition of the solvent mixtures. This provided a helpful evidence for the unsolvation of large tetraalkylammonium cations in organic solvents. The results have been discussed from ion–solvent interactions and the dielectric effect of the solvents.

Introduction

The volumetric behavior of solutes has been proved to be very useful in elucidating various interactions occurring in solutions. Studies on the apparent molar volumes of electrolytes have been widely used to examine the ion–ion and ion–solvent interactions. With the increased interest in high energy and low environmental impact batteries in recent years, a great variety of solvents and electrolytes have been employed in an effort to improve battery characteristics [1]. It is more interesting that molar volumes of electrolytes are relevant to developing an understanding of lithium battery behavior because of their effect on solution properties such as conductivity and viscosity, etc. [2].

Molar volumes of some electrolytes have been reported in ethylene carbonate (EC) [3], acetonitrile (AN) [4] and dimethoxyethane (DME) [4]. However, no data are available in the mixed solvents involving EC. In this paper, we report the partial molar volume of lithium salts (LiClO4 and LiBr), tetraethylammonium bromide (Et4NBr), tetrapropylammonium bromide (Pr4NBr) and tetrabutylammonium bromide (Bu4NBr) in solvent mixtures of EC with tetrahydrofuran (THF), AN, ethyl acetate (EA), DME at 298.15 K. Since our final purpose is to understand the nature of ionic solvation and ionic association in practical lithium battery electrolyte solutions, volumetric ratio of EC to the co-solvents was chosen to be 1:1. The results have been discussed from ion–solvent interactions and the dielectric effect of the solvents.

Section snippets

Experimental

LiClO4 · 3H2O (Beijing Baili Chem. Co., A.R.) was recrystallized twice from ethanol or acetonitrile, and dried at 453 K under vacuum for four days. LiBr · 2H2O (Beijing Chem. Co., A.R.) was recrystallized from ethanol, and dried at 423 K under vacuum for three days. Et4NBr (Shanghai Chem. Co., A.R.) was recrystallized from ethanol, and dried at 333 K to 353 K under vacuum for two days. Pr4NBr (Fluka, AT) was recrystallized from acetone or methanol, and dried at 383 K under vacuum for two days. Bu4

Results and discussion

Apparent molar volumes of lithium salts and tetraalkylammonium salts in a given solvent mixture were calculated from the solution densities byV2,φ=M/ρ−103(ρ−ρ0)/(mρρ0),where M and m are, respectively, molar mass and molality of salts, ρ and ρ0 are densities of the ternary solutions and the binary solvents, respectively. Apparent molar volumes of these salts in EC based solvent mixtures are given in TABLE 1, TABLE 2, TABLE 3, TABLE 4 as a function of molality of the electrolytes. Since there is

Acknowledgements

We wish to acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 29973009) and the Research Foundation of Ministry of Education of China.

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    The variations of these properties with solution composition and their derived values at infinite dilution are very useful for providing information regarding ion–solvent and ion–ion interactions in the systems and particularly as regards the modification induced by the ions and electrolytes on the solvent structures [3–5]. At present, ion–ion and ion–solvent interactions have been extensively studied using theoretical and experimental approaches, including scanning electron microscopy (SEM) [6], quantum chemical calculation [7], thermodynamic and transport properties [8–11], NMR spectroscopy [12,13], IR spectroscopy, Raman spectroscopy [14,15] and among others. However, none of the methods can elucidate the solvation effect clearly.

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