Ionic liquid-based sodium ion-conducting composite gel polymer electrolytes: effect of active and passive fillers

We report the studies on composite gel polymer electrolytes (GPEs) comprising 0.5 M solution of sodium trifluoromethane sulfonate (Na-triflate or NaTf) in ionic liquid 1-ethyl 3-methyl imidazolium trifluoromethane sulfonate (EMITf) entrapped in poly (vinylidinefluoride-co-hexafluoropropylene) (PVdF-HFP) dispersed with passive filler Al2O3 and active filler NaAlO2 particles. The free-standing films of the composite GPEs, prepared from solution-cast method, offer optimum ionic conductivity at room temperature (6.3–6.8 × 10−3 S cm−1 and 5.5–6.5 × 10−3 S cm−1 for Al2O3- and NaAlO2-dispersed GPEs, respectively), with sufficient electrochemical stability and excellent thermal stability up to 340 °C. As observed from XRD and SEM, the composites are of predominantly amorphous and porous character, which support the high ionic conduction. The sodium ion transport number has been found to be ∼0.27 for Al2O3-dispersed GPE and 0.42 for NaAlO2-dispersed GPE, which indicates the predominant role of passive and active fillers, Al2O3 and NaAlO2, respectively. The dispersion of NaAlO2 enhances the sodium ion conductivity in composite GPE substantially. The overall ionic conductivity is same as in the case of Al2O3 dispersion. The performance characteristics of GPE, particularly, dispersed with active filler NaAlO2 show its potential applicability as electrolyte/separator in sodium batteries.