Phase Equilibria and Ionic Solvation in the Lithium Tetrafluoroborate–Dimethylsulfoxide System

Abstract

The phase diagram and electrical conductivity isotherms for the lithium tetrafl uoroborate (LiBF 4 )-dimethylsulfoxide (DMSO) system and Raman spectra of DMSO and the LiBF 4 -DMSO solution were studied. Spectroscopic signatures of a H-bond between DMSO and BF 4 -ions were found. The bonds of Li + ions to the solvent were stronger than the bonds in DMSO dimers because formation of the solvate destroyed dimeric DMSO molecules. The τ ω values for DMSO molecules in the Li + -ion solvate shell of the LiBF 4 -DMSO system were similar to those for associated solvent molecules.Introduction. Solvation is a complicated phenomenon that accompanies the dissolution of both electrolytes and nonelectrolytes [1, 2]. It is described by a combination of energetic and structural changes in the dissolved compound-solvent system and forms a solution of a defi nite composition. In order to understand if a lithium salt-solvent system satisfi es the criteria for battery applications, its phase diagram and concentration dependence of the conductivity are of utmost signifi cance. Vibrational spectroscopy plays an important role among methods for studying equilibria in non-aqueous solvents. Thus, the use of spectroscopic methods can with varying degrees of certainty characterize the solvation of cations and anions, identify ion pairs separated by solvent and more complicated aggregates, and estimate the equilibrium between the various particles in the system [3,4].The structure and properties of LiBF 4 in aprotic solvents have become interesting in the last decade because of its use as a salt for lithium batteries. We used DMSO [(CH 3 ) 2 SO] as the solvent. Equilibria in lithium-salt solutions have been characterized rather well by spectroscopic methods [5,6]. Spectra in the region of SO stretching vibrations [ν 7 (A′), ~1050 cm -1 ] [7] led to the conclusion that individual unassociated DMSO molecules, cyclic dimers, and chain-like associates were present [8,9]. The presence of the last was disputed [10]. Furthermore, the ν 7 vibration is known to be subject to a so-called noncoincidence effect and was split into several components in the pure liquid and concentrated solutions [11][12][13]. Therefore, spectra in the region ~1050 cm -1 were diffi cult to interpret unambiguously. Nevertheless, association constants of DMSO were determined only a few times [14][15][16][17]. It was hypothesized that the anions were solvated through H-bonds between DMSO methyls and F atoms in the anion. However, issues regarding the composition of ionic associates in DMSO and other solvents were treated differently [5,6,18]. We investigated previously electrolytes for lithium-ion current sources based on dimethylsulfone [19][20][21]. Because DMSO and dimethylsulfone are close analogs, it seemed interesting to compare the properties of solutions of lithium salts in these solvents.Experimental. DMSO (Aldrich, ≥98%) was stored over molecular sieves and vacuum distilled. The completeness of the purifi cation was monitored using the melting and boiling points. An...

Novel Non-Aqueous Electrolytes Based on Coordination Boron Compounds

“…It was found that for such solvents as DMSO, propylene carbonate, and other aprotic dipolar solvents in the case of lithium salts and organic cations, the electrolyte has a specific structure, which does not depend on dissociation level. [18][19][20][21][22]. Ions in solution form supramolecular structures -ion pairs separated by a solvent and contact ion pairs, which exist in a certain equilibrium depending on the concentration [18][19][20][21][22].…”

Specialties of the structure and conductivity of the non-aqueous electrolytes based on alkali metal bis (salicyl) borates and bis (oxalate) borates

“…Another option of increasing the electrolyte ion conductivity is the using of lithium salts solutions with organic anions in dimethyl sulfoxide (DMSO), propylene carbonate or other aprotic dipolar solvents. However, the modern study have clarified that the main role of the conductivity formation had a specific cluster-based supra-molecular architecture of the electrolyte in case of full salt dissociation [6][7][8][9][10]. Such clusters stabilize due to the equilibrium between the processes of interconversion, and their production rate (including salvato splited or contact pairs) depends on the salt-solvent ratio [6][7][8][9][10].…”

“…It is well know that the R-enantiomer of such salts take part in the formation of the contact ion pairs in DMSO, that means it could has tendency to form supramolecular structures [6][7][8][9][10]. In the dimethylformamide solution of tetramethylammonium salt, as well as for alkali metal salts, two signals of carbon atoms of the carboxyl group are observed, while the value of chemical shifts of the carbon atoms 4, 5 and 3, 6 signals are significantly separated, which can be explained by the ability to form the salt-solvent hydrogen bonds due to steric effect [6][7][8][9][10] (table 1).…”

Speciation in Solutions of Lithium Salts in Dimethyl Sulfoxide, Propylene Carbonate, and Dimethyl Carbonate from Raman Data: A Minireview

“…In our recent papers [3][4][5][6][7] Journal of Spectroscopy readily available on the market as the solvents for commercial lithium-ion batteries. DMSO is a prospective solvent for lithium-oxygen batteries [8][9][10].…”

“…In [3][4][5][6][7], speciation in DMSO, PC, and DMC solutions of lithium salts has been first described explicitly, with no arbitrary assumptions, in a single experiment. Solvated cations, anions, SSIPs, and CIPs have been detected, and their concentrations have been determined with due regard for equilibria existing in the solvents.…”