Conductivity enhancement in K+-ion conducting dry Solid Polymer Electrolyte (SPE): [PEO: KNO3]: A consequence of KI dispersal and nano-ionic effect

Abstract

Flexible films of dry Solid Polymer Electrolytes (SPEs): [PEO: KNO3] in varying salt concentrations have been hot-press cast. Salt concentration dependent conductivity study revealed two SPE films: [95PEO: 5KNO3] and [70PEO: 30KNO3] exhibiting relatively higher room temperature conductivity (σrt) ∼ 2.76 × 10−7 S/cm and ∼4.31 × 10−7 S/cm respectively. In order to increase σrt further, two strategies have been adopted. Firstly, fractional amount of KI has been dispersed as IInd-phase active filler into above two SPE film compositions which acted as Ist-phase host and Composite Polymer Electrolyte (CPE) films were hot-press cast. Filler particle concentration dependent conductivity study identified CPE films: [(95PEO: 5KNO3) + 7KI] and [(70PEO: 30KNO3) + 10 KI] as optimum conducting films with σrt ∼ 6.15 × 10−6 S/cm and ∼3.98 × 10−6 S/cm respectively. σrt-enhancement of approximately an order of magnitude was achieved by this approach. In second approach, dry powder mixture of (KNO3 + KI), in ratio that of above two CPE films, were subjected to high energy ball-milling separately for different durations prior to casting the films again. The conductivity measurements as a function of milling time identified CPE films: [(95PEO: 5KNO3) + 7KI] and [(70PEO: 30KNO3) + 10 KI] in which two respective (KNO3 + KI) ratios milled for 4- and 6-h, exhibited almost similar value of σrt ∼ 2.09 × 10−5 S/cm. This approach increased σrt further by ∼3–6 fold. The reason attributed for this has been Nano–ionic effect introduced at the interphase boundaries between KNO3 and KI, as a consequence of milling. These films have been referred to as milled CPE films. Subsequently, all the optimum conducting SPE and CPE (unmilled/milled) films were subjected to various characterization studies in order to evaluate their utility in potential All–Solid–State batteries. Ion transport behaviour has been characterized in terms of ionic conductivity (σ), total ionic (tion) and cation (t+) transference numbers, evaluated using different ac/dc techniques. Temperature dependent conductivity measurements have also been done to compute activation energy (Ea) value by linear least square fitting of respective ‘log σ −1/T’ plots. Materials characterization vis-a-vis complexation of salt in polymeric host has been confirmed by SEM/XRD/FTIR/DSC analysis.