Carrier migration mechanism of physically cross-linked polymer gel electrolytes based on PVDF membranes

The conduction propertes of PVDF polymer gel electrolytes prepared by the phase inversion method were investigated through the diffusion coefficient and conductivity in order to confirm the conduction mechanism. The gel conductivity is associated with the total solution uptake which depends on the gelation process. The solution introduced in the polymer is stored in the pores and then penetrates into the polymer chains for swelling the polymer network. In the case of a highly porous polymer membrane, this process proceeds by repeating the steps to form a homogeneous gel finally. However, the gelation process of a low porosity membrane would be stopped if the swollen polymer collapses the pores because the solution cannot continue to be supplied into the pores from outside even if the polymer chain network has not reached swelling saturation. The microscopic migration feature of the carriers in the gel electrolyte is reflected in the diffusion coefficient of the cation and anion species. The apparent transport number estimated from the observed diffusion coefficients changed steeply for a gel of 70 vol % porosity. This reflects the change in the migration mechanism of the carriers. The mobility of the carriers contributing to the polymer swelling in the vicinity of the polymer would be affected by the polymer chains through Coulombic interaction in addition to a physical barrier effect. The PVDF polymer was found to be effective in enhancing the lithium transport number due to selective interaction with the anion.