Ion transport in a polymer-plastic solid soft matter electrolyte in the light of solvent dynamics and ion association

Abstract

Ion transport in a recently demonstrated promising soft matter solid plastic−polymer electrolyte is discussed here in the context of solvent dynamics and ion association. The plastic−polymer composite electrolytes display liquid-like ionic conductivity in the solid state, compliable mechanical strength (∼1 MPa), and wide electrochemical voltage stability (≥5 V). Polyacrylonitrile (PAN) dispersed in lithium perchlorate (LiClO₄)−succinonitrile (SN) was chosen as the model system for the study (abbreviated LiClO₄−SN:PAN). Systematic observation of various mid-infrared isomer and ion association bands as a function of temperature and polymer concentration shows an effective increase in trans conformer concentration along with free Li⁺ ion concentration. This strongly supports the view that enhancement in LiClO₄−SN:PAN ionic conductivity over the neat plastic electrolyte (LiClO₄−SN) is due to both increase in charge mobility and concentration. The ionic conductivity and infrared spectroscopy studies are supported by Brillouin light scattering. For the LiClO₄−SN:PAN composites, a peak at 17 GHz was observed in addition to the normal trans−gauche isomerism (as in neat SN) at 12 GHz. The fast process is attributed to increased dynamics of those SN molecules whose energy barrier of transition from gauche to trans has reduced under influences induced by the changes in temperature and polymer concentration. The observations from ionic conductivity, spectroscopy, and light scattering studies were further supplemented by temperature dependent nuclear magnetic resonance ¹H and ⁷Li line width measurements.