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

rue ionic conductivity and self-diffusion coefficient of gel electrolyte co mposed of the lithium salt of LiN(CF3SO2)(2) dissolved in ethylene carbonat e/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 rat io of polymer to solution. The conductivity and the self-diffusion coeffici ent increased with an increase in the solution ratio in the gel electrolyte s. The change in the diffusion coefficient is due to the change in steric h indrance and the solution viscosity caused by the interaction between the d issociated ions and/or ion pairs and the surrounding polymer. The conductiv ity change from 50 to 80 wt % of solution fraction in the eel electrolyte r eached to three orders of magnitude. In that range, the diffusion coefficie nt which is associated with the mobility from the Nernst-Einstein equation changed about one order of magnitude. Therefore, a two-order difference bet ween the conductivity and the diffusion coefficient changes is ascribed to the change in carrier concentration. The effective carrier concentration wo uld 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 elec trolyte solution to influence the dissociation of the salt and solvate the dissociated ions, resulting in the reduction of carrier content. (C) 2000 T he Electrochemical Society. S0013-4651(98)12-060-8. All rights reserved.