Enhancement of ionic conductivity by the addition of plasticizers in cationic monoconducting polymer electrolytes

Temperature-dependent Li-7 nuclear magnetic resonance (NMR) spin-lattice relaxation time (T-1) data of a lithium monoconducting polymer electrolyte based on maleic anhydride-styrene copolymers are examined in an effort to understand the mechanism of how plasticizers affect significant changes in the observed ionic conductivity (sigma). The use of strong polar organic plasticizers, such as propylene carbonate, is believed to induce changes in the charge-carrier separation, which is expected to facilitate the transport of ions in these materials. Cationic monoconducting gel electrolytes consisting of polyelectrolytes with immobilized carboxylic anions, charge compensating mobile Li+ ions, and various combinations of poly(ethylene glycol) dimethyl ether, dimethylsulfoxide, propylene carbonate, and BF3 exhibit ambient sigma values from 10(-3) to 10(-6) S cm(-1). For systems not containing BF3 the T-1 data as a function of inverse temperature and the value of the T-1 at its minimum are found to be independent of the electrolyte composition in spite of the fact that the delta values are observed to increase by 1 to 3 orders of magnitude in these samples. For these samples it is suggested that the Li+ ions find comparable sites possibly in the form of contact ion pairs with the carboxylic anions and that they are dynamically coupled to the motion of the polymer host. The observed increase in conductivity upon addition of the plasticizers is assumed to arise from a weakening of the ion-polymer interactions and not from a fundamental change in the ionic conductivity mechanism. Upon addition of BF3, however, a shift in the T-1 minimum to lower temperatures and a decrease in the depth of the T-1 minimum are observed. These observations suggest that the Li+ ions interact preferentially with -O-(BF3)-F-circle minus moieties and are consistent with the existence of an alternate conduction path such that the ion motions are decoupled from the local motions of the polymer host. (C) 1998 Elsevier Science BN. All rights reserved.