The dependence of lithium transference numbers on temperature, salt concentration and anion type in poly (vinylidene fluoride)-hexafluoropropylene copolymer-based gel electrolytes

The electrophoretic NMR method was used to measure lithium transference num bers in gel electrolytes consisting of various salts in propylene carbonate -swollen poly (vinylidene fluoride)-hexafluoropropylene copolymer. We compa re the concentration dependence of the lithium transference numbers obtaine d for a series of anions. The anions considered are CF3SO3- (Tf), N(CF3SO2) (2)(-) (TFSI), N(C2F5SO2)(2)(-) (BETI) and (C(CF3SO2)(3)(-) (TFSM). Larger anions were generally found to result in higher lithium transference number s. t(Li) in LiTf containing electrolytes decreases steadily with the salt c oncentration. However, in LiTFSI-containing gel electrolytes, t(Li) is almo st invariant over a large concentration range. The results are discussed in the context of the effect of ion-ion interaction, the extent of which is d ependent on the degree of anion charge delocalization. (C) 1998 Published b y Elsevier Science S.A. All rights reserved.