Ionic conduction mechanisms of polyvinylidenefluoride-hexafluoropropylene type polymer electrolytes with LiN(CF3SO2)2

rue ionic conductivity and self-diffusion coefficient of gel electrolyte composed of the lithium salt of LiN(CF3SO2)(2) dissolved in ethylene carbonate/diethylcarbonate (2/3) and the polyvinylidenefluoride-hexafluoropropylene copolymer have been measured to understand the ionic conduction properties and the ionic condition in the gel electrolytes by changing the mixing ratio of polymer to solution. The conductivity and the self-diffusion coefficient increased with an increase in the solution ratio in the gel electrolytes. The change in the diffusion coefficient is due to the change in steric hindrance and the solution viscosity caused by the interaction between the dissociated ions and/or ion pairs and the surrounding polymer. The conductivity change from 50 to 80 wt % of solution fraction in the eel electrolyte reached to three orders of magnitude. In that range, the diffusion coefficient which is associated with the mobility from the Nernst-Einstein equation changed about one order of magnitude. Therefore, a two-order difference between the conductivity and the diffusion coefficient changes is ascribed to the change in carrier concentration. The effective carrier concentration would be affected by the interaction between the electrolyte solution and the polymer in the gel. It is possible that the polymer chains act on the electrolyte solution to influence the dissociation of the salt and solvate the dissociated ions, resulting in the reduction of carrier content. (C) 2000 The Electrochemical Society. S0013-4651(98)12-060-8. All rights reserved.