The effects of cations and anions on the ionic conductivity of poly[bis(2-(2-methoxyethoxy)ethoxy)phosphazene] doped with lithium and magnesium salts of trifluoromethanesulfonate and bis(trifluoromethanesulfonyl)imidate

Lithium trifluoromethanesulfonate (LiTf), lithium bis(trifluoromethanesulfonyl)imidate (LiTFSI), magnesium trifluoromethanesulfonate (MgTf₂) and magnesium bis(trifluoromethane sulfonyl)imidate (MgTFSI₂) were dissolved in poly[bis(2-(2- methoxyethoxy)ethoxy)phosphazene] (MEEP) to compare the effect on solvent-free polymer ionic conductivity of monovalent versus divalent cations, and two anions with different degrees of dissociation. The polymer electrolytes with the bis(trifluoromethanesulfonyl)imidate anion had higher ionic conductivities even though the glass transition temperatures, which reflected polymer molecular motion, were higher than those of their counterparts with the trifluoromethanesulfonate anion. Furthermore, polymer electrolytes with magnesium salts achieved their maximum conductivity at lower salt concentrations than their counterparts with lithium salts. The temperature dependence of the ionic conductivity of the solid solutions was fitted to the Vogel-Tamman-Fulcher (VTF) equation. The pseudo-activation energy term, B, of the VTF equation showed a strong dependence on the anion present. This result suggests that the dominant mobile species is the anion, while the cation remains relatively bound to the polymer.