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 elect rolyte for high-energy lithium batteries due to its high solubility in most solvents and electrochemical stability. To characterize this ele ctrolyte in solution, its conductance and apparent molar volume and he at capacity were measured over a wide range of concentration in glymes , tetraethylsulfamide (TESA), acetonitrile, gamma-butyrolactone, and p ropylene carbonate at 25 degrees C and were compared with those of LiC lO4 in the same solvents. The glymes or n(ethylene glycol) dimethyl et hers (nEGDME), which have the chemical structure CH3-O-(CH2-CH2-O)(n)- CH3 for n = 1 to 4, are particularly interesting since they are electr ochemically stable, have a good redox window, and are analogs of the p olyethylene oxides used in polymer-electrolyte batteries. TESA is a go od plasticizer for polymer-electrolyte batteries. Whenever required, t he following properties of the pure solvents were measured: compressib ilities, expansibilities, temperature and pressure dependences of the dielectric constant, acceptor number, and donor number. These data wer e used in particular to calculate the limiting Debye-Huckel parameters for volumes and heat capacities. The infinite dilution properties of LiTFSI are quite similar to those of other lithium salts. At low conce ntrations, 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 s olution 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 s olvents of low dielectric constant is due to a charge transfer process rather than to the migration of free ions.