Study of Salt Precipitation in Polymer Electrolytes Based on Poly(ethylene oxide) and EMImTf Ionic Liquid

In solidlike polymer electrolytes, salt precipitation (SP) followed by the occurrence of neutral ion pairs in polymer electrolytes is known to deteriorate ionic conductivity, but has not been the subject of intense study so far. Here, the mechanism of SP is studied for a polymer electrolyte consisting of poly(ethylene oxide) (PEO) and ionic liquid salt of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMImTf), which is one of the promising prototype systems because of its technological relevance. It is found that Ångstrom-scale open spaces in the amorphous matrix sensitively change in response to the release of organic cation from PEO polymer chains as well as SP in the Arrhenius manner. The cation release, triggering off ion-pair formation, takes place at the temperature by far lower than that of SP with an activation energy of ∼0.2 eV, whereas SP involves an energy cost of ∼0.4 eV. The results suggest that SP in the PEO-EMImTf complex is mechanistically explained from the viewpoint of the abovementioned two processes, that is, cation release from the present PEO-EMImTf complex and ion pairing.