Larger cations can move faster in solid polymer electrolytes

Poly[(ω-carboxy)-oligo(oxyethylene)methacrylate] alkali metal salts (P(CME_nM)s) are synthesized as solid polymer electrolytes which can transfer only cations. The conductivity of a series of alkali metal ions such as Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺ is measured, and the results are summarized with the reduced temperature (T-T_{g:Tg=glass transition temperature}) to neutralize the effect of segmental motion on the relative conductivity. They show a linear relation between the logarithm of conductivity at T-T_g=60°C and the logarithm of radius of the carrier cations. The ion-dipole interaction is suggested to be the most effective factor to control the mobility in the solid polymer electrolytes. Cations having larger ionic radius such as Rb⁺ or Cs⁺ are suggested to have weaker interaction force with ether oxygens, reflecting the faster ion exchange between possible adjacent sites in the polymer matrix.