Crystalline phases, morphology and conductivity of PEO:LiTFSI electrolytes in the eutectic region

Polymer electrolytes formed from high molecular weight poly(ethylene oxide) with dissolved LiN(CF3SO2)2 salt were obtained by casting from solution. The compositions ranging from 16:1 to 8:1 EO:Li molar ratio were studied by the X-ray diffraction and by impedance spectroscopy simultaneously with polarizing microscope observation. Crystallization processes and the resulting morphology of electrolytes were investigated in the temperature range from 85 to −20 °C in various cooling and heating cycles. For all compositions, coexistence of crystalline phases of pure PEO and PEO6:LiTFSI complex was found. The results indicate that phase segregation plays a key role in the crystallization process. The amorphous phase acts like a reservoir of salt, which can be injected or drained by crystallizing spherulites of a defined composition. This interaction between pure PEO and PEO6:LiTFSI complex explains why effective crystallization process leading to a large decrease of conductivity can take place even in compositions close to the eutectic system. In such a system the concentration of salt in remaining amorphous phase can be very different from the initial composition of electrolyte. The ionic conductivity strongly depends on morphology of the electrolyte, which indicates a possibility of achieving high conductivity in the presence of the crystalline phases.