MIXED-ALKALI EFFECT AND SHORT-RANGE INTERACTIONS IN AMORPHOUS POLY(ETHYLENE OXIDE) ELECTROLYTES

When dissolved in poly(ethylene oxide) (PEO), the alkali metal imides LiN(CF3SO2)(2) (LiTFSI), NaN(CF3SO2)(2) (NaTFSI) and KN(CF3SO2)(2) (KT FSI) 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 les s delocalized anions leg, thiocyanate and triflate anions) also exhibi ts 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 cont aining binary salt mixtures (KTFSI-LiTFSI and KSCN-LiSCN) in various m olar ratios. Over the range of the examined concentrations (0.6-1.6 mo l/kg), the conductivity data of the PEO-KTFSI-LiTFSI electrolytes are comparable to those of the one-salt electrolytes. In turn, the conduct ivity data of the PEO-KSCN-LiSCN electrolytes exhibit a strong, negati ve deviation from additivity. By analogy with molten salt binary mixtu res, it is proposed that both this deviation and the cation-size effec t on the conductivity of the PEO-MSCN one-salt electrolytes are caused by low-energy, short-range interactions associated with anion polariz ation.