Systematics of salt precipitation in complexes of polyethylene oxide and alkali metal iodides

Conductivity measurements in PEO30MIPEO30MI polymer electrolytes with M=LiM=Li, Na, K, Rb, or Cs over the temperature range from about 65 to 200 °°C show an increasing tendency for salt precipitation with increasing cation size. The salt precipitation in these complexes upon heating is revealed by the decrease of the dc conductivity starting at a critical temperature TcTc. Whereas LiI and NaI complexes do not show precipitation effects, TcTc monotonically decreases from about 140 to 65 °°C when changing the salt component from KI via RbI to CsI. For the PEO–RbI system, precipitation is further investigated by nuclear magnetic resonance (NMR) and tracer diffusion experiments. NMR analysis unambiguously demonstrates the onset of RbI salt precipitation and the increase of the precipitate fraction with increasing temperature. In diffusion experiments on PEO30RbIPEO30RbI with the radiotracers Rb86 and I125, the precipitation effect is manifested by anomalous features in the penetration profiles, however, without noticeable changes in their depth range. Combining the resulting tracer diffusion coefficients with the dc conductivity data enables us to assess crucial parameters characterizing ionic transport in PEO30RbIPEO30RbI.