Effects of tris(2,2,2-trifluoroethyl) phosphate as a flame-retarding cosolvent on physicochemical properties of electrolytes of LiPF6 in EC-PC-EMC of 3:3:4 weight ratios

We synthesized tris(2,2,2-trifluoroethyl) phosphate (TFP) as a flame-retarding solvent, measured its properties, and studied its effects on the physicochemical properties of the electrolytes LiPF6 in ethylene carbonate-propylene carbonate-ethyl methyl carbonate (EC-PC-EMC) of 3: 3: 4 weight ratios. These properties included dielectric constant of the solvent, flammability and conductivity of the electrolytes, and viscosity of the solvent and the electrolytes. The viscosity was studied indirectly through measuring glass transition (T-g) and boiling temperatures of the solvents and electrolytes. We found that TFP reduced the flammability and vapor pressure of the carbonate-based electrolytes, but lowered their conductivity by raising their viscosities and by lowering the dielectric constant of their solvents. Furthermore, this fall of conductivity with TFP content was almost linear, the rate of fall being higher at lower temperatures and higher LiPF6 concentrations. We compared the conductivity of this system with that of LiPF6 in EMC-EC, and found a number of important similarities and differences. We also calculated vanishing mobility temperature T-0 from fitting the Vogel-Tamman-Fulcher equation to the conductivity/temperature data and found it changed the same way as T-g did: it rose with TFP content and LiPF6 concentration. (C) 2002 The Electrochemical Society.