Ionic conduction mechanism of PEO-type polymer electrolytes investigated by the carrier diffusion phenomenon using PGSE-NMR

The diffusive behavior of the cation and anion species of the PEO-type polymer electrolyte has been investigated using pulsed gradient spin-echo NMR to elucidate the carrier conduction mechanism of the polymer electrolyte. Compared with the random diffusive behavior generally observed in electrolyte solutions, the carriers in the PEO-type polymer electrolyte showed a characteristic migration feature of a restricted condition especially after the application of stress on the membrane. This is attributed to carrier hopping along the polymer chains through Coulombic interaction between the carriers and the ethylene oxide sites on the chains. The restricted feature was in agreement with the simple boundary restriction model. The diffusion time dependence of the echo intensity change also supported the belief that the carrier migration in the polymer electrolyte followed the simple boundary model. Considering the actual situation of the polymer electrolyte, the polymer chains spread in all directions to create a random network structure, which consequently permits three-dimensional migration as random diffusion under the condition of long-time limit. The diffusion manner of the cation species along and across the stretched direction was different at 80°C. This was due to the difference in the diffusion coefficient between the two directions from the fitted results according to the simple boundary model. This confirmed that the alignment of the sites in the polymer electrolyte by strain would be effective for creating a highly conductive pathway for lithium ion transport even if application of stress is disadvantageous for segmental mobility.