Mass and Charge Transport in a Cross-Linked Polyether-Based Electrolyte. The Role of Ion Pairs

Abstract

In an amorphous solid polymer electrolyte the positive and negative ions are found to move much faster in the form of neutral pairs than individually. Although the fraction of ion pairs turns out to be small (less than 10%), the ion pairs provide the most significant contribution to the transport of cations being crucial for battery operation. This is a striking new result of a detailed comparison of tracer diffusion experiments and electrical conductivity measurements made on a cross-linked random copolymer of ethylene oxide and propylene oxide complexed with sodium iodide. The study utilizes radioactive isotopes of sodium and iodine in tiny amounts to trace the diffusivity of both ion species individually. The overall charge diffusivity due to the joint effect of cations and anions is monitored by impedance spectroscopy. A thorough evaluation of the experimental data shows that the diffusivity of all mobile species, although different in magnitude, shows the same temperature dependence. This is a conseq...