Flexoelectricity in polymer electrolyte membranes: ‘Polarity switching’ in solid polymeric ion conductors*

The present article describes a novel phenomenon of polarity flipping during mechano-electrical energy conversion subjected to bending deformation of flexible polymer electrolyte membranes (PEM), which were originally developed for polymeric ion conductors in solid Li-ion batteries. This bending induced electrical polarization is analogous to flexoelectricity found in insulators and dielectric materials. However, the basic principle of flexoelectricity in PEMs operates based on electrical energy generation driven by ion polarization/depolarization across the PEM subjected to a pressure (or stress) gradient during bending. The observed flexoelectric coefficient (~323 μC/m), i.e., a measure of the converted mechano-electrical energy, is the highest among all flexoelectric materials hitherto reported. Of particular importance is that the present work is the first to demonstrate the occurrence of polarity switching, i.e., reversal of the polarization direction with increasing succinonitrile (SCN) concentration, attributable to changing ionic size of solvated Li-SCN complexes.