Dany Brouillette

Stable solvates in solution of lithium bis(trifluoromethylsulfone)imide in glymes and other aprotic solvents: Phase diagrams, crystallography and Raman spectroscopy

Lithium bis(trifluoromethylsulfone)imide (LiTFSI), a promising electrolyte for high energy lithium batteries, forms several stable solvates having low melting points in aprotic solvents. In a previous study (D. Brouillette, G. Perron and J. E. Desnoyers, J. Solution Chem., 1998, 27, 151), it was suggested, based on thermodynamic studies, that such stable solvates may persist in solution and influence their properties. To verify this hypothesis, phase diagrams and Raman spectra have been measured for solutions of LiTFSI in acetonitrile, propylene carbonate and glymes (n(ethyleneglycol) dimethyl ether or Gn), which have the chemical structure CH3–O–(CH2–CH2–O)n–CH3 for n = 1 to 4 and 10. The relative intensities of the LiTFSI and solvent Raman bands are proportional to the concentration for systems without solvates. The systems for which stable solvates were identified in the phase diagram show important changes in the relative intensities for both the LiTFSI and the solvent Raman bands at concentrations corresponding to particular stoichiometries and support the conclusion that stable solvates are present in the solutions. The structure of the crystalline G1:LiTFSI solvate was determined by X-ray crystallography. Structures for (G2)2:LiTFSI and (G1)3:LiTFSI solvates are proposed.

Apparent Molar Volume, Heat Capacity, and Conductance of Lithium Bis(trifluoromethylsulfone)imide in Glymes and Other Aprotic Solvents

Lithium bis(trifluoromethylsulfone)imide (LiTFSI) is a promising electrolyte for high-energy lithium batteries due to its high solubility in most solvents and electrochemical stability. To characterize this electrolyte in solution, its conductance and apparent molar volume and heat capacity were measured over a wide range of concentration in glymes, tetraethylsulfamide (TESA), acetonitrile, γ-butyrolactone, and propylene carbonate at 25°C and were compared with those of LiClO4 in the same solvents. The glymes or n(ethylene glycol) dimethyl ethers (nEGDME), which have the chemical structure CH3−O−(CH2−CH2−O)n−CH3 for n = 1 to 4, are particularly interesting since they are electrochemically stable, have a good redox window, and are analogs of the polyethylene oxides used in polymer-electrolyte batteries. TESA is a good plasticizer for polymer-electrolyte batteries. Whenever required, the following properties of the pure solvents were measured: compressibilities, expansibilities, temperature and pressure dependences of the dielectric constant, acceptor number, and donor number. These data were used in particular to calculate the limiting Debye-Hückel parameters for volumes and heat capacities. The infinite dilution properties of LiTFSI are quite similar to those of other lithium salts. At low concentrations, LiTFSI is strongly associated in the glymes and moderately associated in TESA. At intermediate concentrations, the thermodynamic data suggests that a stable solvate of LiTFSI in EGDME exists in the solution state. At high concentrations, the thermodynamic properties of the two lithium salts approach those of the molten salts. These salts have a reasonably high specific conductivity in most of the solvents. This suggests that the conductance of ions at high concentration in solvents of low dielectric constant is due to a charge transfer process rather than to the migration of free ions.

Sulfamides and Glymes as Aprotic Solvents for Lithium Batteries

In view of the high reactivity of the lithium metal, lithium batteries must operate in an aprotic environment, which can either be a conducting polymer, a liquid solvent, or a mixture of them. Two families of aprotic liquids were considered as solvents for lithium bis(trifluoromethylsulfone)imide (LiCF{sub 3}SO{sub 2}NSO{sub 2}CF{sub 3} or LiTFSI). The first one is the substituted sulfamides, R{sub 1}R{sub 2}NSO{sub 2}NR{sub 3}R{sub 4}, where the R groups are either methyl, ethyl, or methoxyethyl (CH{sub 2}CH{sub 2}OCH{sub 3}), and the second one is the glymes, CH{sub 3}O(CH{sub 2}CH{sub 2}O){sub n}CH{sub 3}, for n up to 10. The phase diagrams, potential windows, conductivities, and the lithium interfacial resistances of the solutions were investigated, often as a function of temperature. The potential use of these solvents for different types of batteries is discussed.

Effect of viscosity and volume on the specific conductivity of lithium salts in solvent mixtures

As part of a study on the optimization of the electrolyte for high energy lithium batteries, the conductivity, viscosity and density of LiAsF6, LiBr, and LiClO4 were measured in aprotic solvent mixtures. The conductivity of lithium bis(trifluoromethylsulfone)imide (LiTFSI) was also obtained in a large number of mixed aprotic solvents. The solvents were chosen to verify the effect of various parameters such as viscosity, permittivity, volume, acceptor number and donor number on the conductivity. These results were used to develop a simple model for excess conductivities based on the viscosity and volume of the pure solvents. Without adjustable parameters, this model predicts the correct sign of the excess conductivities in ≈90% of the cases and the magnitude of the conductivity of the ternary mixtures within an average of 15%. Deviations from the predictions are mostly observed with solvents of low permittivity and this supports the hypothesis that a different conduction mechanism is in operation at high concentration in these solvents, and the solvating power of these solvents plays an important role in this mechanism.

Dielectric constants of acetonitrile, γ-butyrolactone, propylene carbonate, and 1,2-dimethoxyethane as a function of pressure and temperature

The dielectric constants ∈ of 1,2-dimethoxyethane, acetonitrile, γ-butyrolactone, and propylene carbonate were determined from capacitance measurements extrapolated to infinite frequency; ln ∈ are reported as a function of pressure up to 80 MPa at 15, 25, 35, 45°C and as a function of temperature in the range 10 to 50°C at 0.10133 MPa. The variation of ln ∈ with temperature or pressure can be expressed by a second order polynomial expression. The isothermal compressibilities β of the solvents were determined at 25°C from sound velocities, densities, and heat capacities. A simple correlation can be established between ∂ ln ∈/∂P and β for most aprotic solvent.