Alkali metal ion-poly (ethylene oxide) complexes. II. Effect of cation on conductivity

Complexes of alkali metal salts with various polymers have for some time been recognized as fast ionic conductors. Polymer electrolyte fast ion conductors are currently under consideration for use in high energy density electrochemical cells. In order to aid in our understanding of the mechanism of ionic conductivity we have examined systematically complexes of poly(ethylene oxide) (PEO) with the alkali metal salt series of Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺ with both tetraflouroborate (BF₄^-) and trifluoromethanesulfonate (CF₃SO₄^-) anions. The ratio of monomer to salt was 10:1 in all cases. Complex impedance measurements were made on all samples in the temperature range 40°–125°C. With CF₃SO₄^- as the anion a definite trend was apparent with the smallest cation Li⁺ being the worst conductor and Cs⁺, the largest cation, being the best. When BF₄^- salts are used, the Na⁺ complex is found to be the best conductor and Rb⁺ the worst. This study, in connection with our earlier studies, has shown that synergy between cation and anion in the polymer matrix is an important consideration in determining the ionic conductivity.