Decoupling Ion and Proton Transport from Structural Relaxation in Polymers and Ionic Liquids

Classical theory relates the rate of ion transport to viscosity or rate of structural relaxation in a liquid electrolyte. We present here an overview of the relationship between ion and proton transport and structural relaxation in ionic liquids (IL) and polymer electrolyte. We demonstrate that in contrast to most of ILs, there is a strong decoupling of ionic conductivity from structural dynamics in many polymers, including polymerized ionic liquids (polyIL). This decoupling is caused by frustration in packing of relatively rigid polymer chains. As a result, ions retain high mobility even when structural dynamics is very slow or completely frozen. To provide a systematic picture of ionic conductivity we employ the so-called Walden plot. Application of this analysis to different classes of polymer electrolytes and polyILs helps to identify ‘superionic’ behavior for many polymers. We present several examples of promising polymers with conductivity approaching 10^-4 S/cm at room temperature. Similar ideas can be applied to proton conductivity. We demonstrate significant decoupling of proton mobility from structural relaxation and/or viscosity in a series of protic ionic liquids. Mechanism of enhanced proton conductivity is discussed at the end.