Transport properties and microstructure of gel polymer electrolytes

Gel polymer electrolytes based on a copolymer poly(vinylidene fluoride (VdF)/hexafluoropropylene (HFP)) and a solution of ethylene carbonate (EC), diethyl carbonate (DEC) and as a salt LiN(CF3SO2)(2) were prepared by changing the content of the polymer in the range 20-80 wt%. The effects of changing the salt concentration in the solution were also explored. The conductivity was found to vary in the range 10(-2) ohm(-1) cm(-1) (20 wt% of polymer) to 10(-8) ohm(-1) cm(-1) (80 wt% of polymer). Pulsed field gradient (PFG)-NMR was used to determine the diffusion coefficient of lithium (DLi+)and fluoride species (DF-)and in consequence the transport number. Both DLi+ and DF- decrease with increasing polymer content. Cation transport number (tau(+)) values between 0.49 and 0.60 were found, depending on the solution content. Pores having average diameter of about 0.5 mu m were observed by scanning electron microscopy (SEM). (C) 2000 Elsevier Science Ltd. All rights reserved.