Economical, Ecofriendly and Easy to Handle Polymer-In-Salt-Electrolytes

Neelam Srivastava, Dipti Yadav, Kanak Aggarwal

Department of Physics (MMV Section), Banaras Hindu University, Varanasi-221005, India

neel@bhu.ac.in

Polymer-In-Salt-Electrolytes (PISEs) [1] have attracted scientific community since 1990s when the Angell’s [2] group has indicated that if the salt concentration is increased beyond the threshold value so that the ion-clusters are connected throughout the matrix, then the ion dynamics changes. At this concentration ion transport is decoupled from polymer segmental motion and now ions diffuse through ion-cluster and hence it is much faster. It is also reported that at high salt concentration the cationic transference number also enhances. Hence depending upon the salt concentration the polymer electrolytes are classified into two classes i) Salt-In-Polymer-Electrolytes (SIPEs) - salt concentration is lower than the polymer and ii) Polymer-In-Salt-Electrolytes (PISEs)- salt concentration is higher than the polymer. SIPEs, which are presently used at commercial level, needs to be improved on two main fronts i) slow ion movement and ii) poor cationic transference numbers and hence PISEs are highly desired materials. Unfortunately, suitable polymer host which can hold a large amount of salt and keep it in dissociated form is still to be recognized, to take PISEs to commercial level. When it comes to commercial application, the cost, ease of synthesis/ handling and ecofriendly nature are also equally important.

Literature reports that for PISEs synthesis it is important to have the salt/ salt-mixture in molten state so that when polymer is added the rubbery state may remain intact. Hence as per reported protocols one has to identify suitable salt-mixture and then suitable host. Still reported polymer-salt combinations suffers recrystallization of salt, brittleness of morphology and also aging affects resulting in poor electrochemical and/or mechanical properties making it unsuitable for commercial application. It is found that PISEs reported needs further thermal treatments/additions of additives to control the recrystallization and aging effects.

Our group [3-4] has developed a simple solution casting protocol for synthesis of an economical, ecofriendly and easy to handle PISEs from crosslinked starches, where there is no need of getting the molten state salt/salt-mixture. The thought process behind this protocol and selection of starch/ crosslinked starch as host polymer is that the salt breaks the starch into smaller molecules resulting in generation new –OH and –H to interact with salt, i.e. increasing salt concentration itself creates a favorable atmosphere for its acceptance. Starch is hydrophilic in nature and presence of large amount of salt adds up to it, and such materials have moisture content varying from ~5% to 25%, depending to salt and starch combination, which is a favorable property leading the synthesized PISEs to behave as Water-In-Polymer-Salt-Electrolytes (WiPSEs) [5]. By exposing the freshly synthesized samples to high humidity these materials were stabilized with respect to ambient humidity changes. These materials lead to ESR <10Ω (reaching to as low as <1Ω) and wide electrochemical stability window (ESW>2.5V) and ion relaxation time below µSec. The supercapacitor fabricated, using synthesized PISEs with commonly available supercapacitor electrodes, have the behavior at par to other electrolytes reported in literature. With lab synthesized activated carbons synthesized in laboratory, the capacity up to 150F/g has been obtained with columbic efficiency >98%. Since the synthesis protocol and chemical used are economical, the starch based PISEs are economical and also environment benign (because starch is renewable polymer). The material is flexible and can be molded in desired shape and size and hence is a potential candidate to reach at commercial level.