Ionic transport in polymer electrolytes: The essential role of associated ionic species

Polymer electrolytes based on poly(bis(methoxyethoxyethoxy)phosphazene) (MEEP) exhibit some of the highest room temperature ionic conductivities reported in a single-phase system. The mechanism of ionic transport in polymer electrolytes, particularly the nature of the charge-carrying species, has been poorly understood at the molecular level. Vibrational spectroscopy is used to determine the nature and relative amounts of ionic species present in MEEP-LiCF ₃SO ₃ as a function of salt concentration. The composition corresponding to the conductivity maximum is completely dominated by neutral cation - anion contact ion pairs and the triple anion species, {Li(CF ₃SO ₃) ₂} ^-. Furthermore, the conductivity changes only slightly at higher salt concentrations where triple cations dominate. These data require a dissociation - reassociation mechanism of ion transport in which dynamic equilibrium is established between the various ionically associated species by exchange of individual Li ⁺ and CF ₃SO ₃ ^- ions. The individual ions migrate between associated ionic species through the liquidlike domains defined by the entangled ethylene oxide side chains.