Polymeric solid electrolytes: Dynamic bond percolation and free volume models for diffusion

Polymeric solid electrolytes offer a difficult problem from the viewpoint of understanding the charge transport mechanism. While quasithermodynamic theories (configurational entropy, free volume) are useful for rationalizing the behavior of these materials, they do not really amount to a microscopic picture. We have developed a dynamic bond percolation (DBP) model to describe ionic conductivity in these materials. The DBP model is based on a master equation describing ion hops among sites. The percolation aspects are included by making the bonds between sites randomly open or closed. The dynamical aspect is due to the configurational motions of the polymer, and results in the variation of the bond assignments as open or closed. The relationship of DBP to free volume theory is sketched; this involves a specific consideration of kinetic effects on free-volume motion.