When dissolved in poly(ethylene oxide) (PEO), the alkali metal imides LiN(CF3SO2)(2) (LiTFSI), NaN(CF3SO2)(2) (NaTFSI) and KN(CF3SO2)(2) (KTFSI) yield rubbery electrolytes over a wide range of concentrations at T > 50 degrees C. Both the glass transition temperature (T-g) and the dc bulk conductivity of these electrolytes exhibit a weak dependence on the cation size. Although T-g of similar electrolytes involving less delocalized anions leg, thiocyanate and triflate anions) also exhibits a weak dependence on the cation size, their conductivity decreases markedly with increasing cation charge density. To clarify this point, a conductivity study was performed on PEO amorphous electrolytes containing binary salt mixtures (KTFSI-LiTFSI and KSCN-LiSCN) in various molar ratios. Over the range of the examined concentrations (0.6-1.6 mol/kg), the conductivity data of the PEO-KTFSI-LiTFSI electrolytes are comparable to those of the one-salt electrolytes. In turn, the conductivity data of the PEO-KSCN-LiSCN electrolytes exhibit a strong, negative deviation from additivity. By analogy with molten salt binary mixtures, it is proposed that both this deviation and the cation-size effect on the conductivity of the PEO-MSCN one-salt electrolytes are caused by low-energy, short-range interactions associated with anion polarization.