10. Flexible-High-Conducting Polymer-In-Salt-Electrolyte (PISE) Membranes: A Reality Due to Crosslinked-Starch Polymer Host

Polymer-electrolytes, used in commercial energy devices, need to have small liquid components to achieve the desired electrochemical properties. Besides this, these polymer electrolytes have a low cationic transference number and slow ion movement. To get rid of these drawbacks, polymer-in-salt-electrolytes (PISEs) were hypothesized in the 1990s. In PISEs, ion transport is decoupled from polymer segment movement and it occurs through ion cluster, resulting in much faster ion transport in comparison to SIPEs (salt-in-polymer-electrolytes) and cationic transference number is also supposed to approach 1. Unfortunately, a polymer host which can accommodate a large amount of salt above the threshold value required for continuous ion cluster formation, and retain mechanical properties, is still to be identified. Till now, the approach has been to get a mixture of salts in the molten state and then add a small amount of polymer to get a solid morphology. Even after trying a variety of permutation combinations of salt, polymers, and additives, the targeted conductivity (10^–4 S/cm) along with good mechanical properties is rarely reported. Owing to the state-of-art of electronic device technology which has reached to flexible device stage, the present-day energy devices (and hence the electrolytes) need to be flexible. Recently, a facile protocol, which does not use any sophisticated instruments and/or complicated chemical procedures, for the synthesis of PISEs using starch (a renewable polymer) as host polymer, has been reported. Conductivity up to 0.1 S/cm has been achieved in the flexible (bendable, stretchable, and twistable) morphology which can be easily cut into different shapes and sizes. Electrochemical-Stability-Window (ESW) is also quite good (>2.5 V). These electrolytes are quite stable with respect to ambient change. Presently, a new concept of Water-In-Polymer-Salt-Electrolyte (WIPSE) is being investigated. Because of the water-absorbing nature of starches, starch-based PISEs seem to inherently have this benefit also, and probably it is the reason for the exceptionally high conductivity observed in these materials. To the best of the author’s knowledge such high conducting, flexible, and economical PISE membranes were not reported in literature except for crosslinked-starch polymer host-based membranes.