Transport properties of a high molecular weight poly(propylene oxide)-LiCF3SO3 system

Conductivities (sigma), salt diffusion coefficients (D-S), and cationic transference numbers (t(+)(0)) are reported for a high molecular weight polypropylene oxide (Parel(TM)-LiCF3SO3 polymer electrolyte system at 85 degrees C. Transference numbers were determined as a function of salt concentration using a recently described electrochemical method based on concentrated solution theory. For the Parel-LiCF3SO3 system, t(+)(0) is slightly positive for electrolytes with O:Li ratios of 15 or 12:1 but decreases to negative values for more concentrated solutions. This implies that negatively charged ionic aggregates such as triplets are more mobile than free cations in this concentration range. Such behavior is commonly seen in binary salt/polymer electrolytes, which typically exhibit a high degree of nonideality. The nonunity transference numbers and microphase separation in the Parel-LiCF3SO3 system strongly suggest that salt precipitation or phase separation in operating cells containing these electrolytes due to the development of large concentration gradients during passage of current. (C) 1999 The Electrochemical Society. S0013-4651(98)07-064-5. All rights reserved.