Ionic liquid based sodium ion conducting gel polymer electrolytes

A novel thermally and electrochemically stable sodium ion conducting gel polymer electrolyte has been reported, which comprises a solution of NaCF3SO3 (sodium triflate or NaTf) in a room temperature ionic liquid 1-ethyl 3-methyl imidazolium trifluoro-methane sulfonate (EMI-triflate or EMITf) immobilized in poly (vinylidine fluoride-co-hexafluoropropylene) (PVdF-HFP). Different structural, thermal, electrical and electrochemical studies demonstrate promising characteristics of the polymer films, suitable as electrolytes in rechargeable sodium batteries. The possible conformational/structural changes in the polymer PVdF-HFP due to the entrapment of EMITf or EMITf/NaTf solution have been investigated by Fourier transform infra-red (FTIR), scanning electron microscopic (SEM) and X-ray diffraction (XRD) techniques. The electrolyte with the composition of EMITf:PVdF-HFP (4:1 w/w) + 0.5 M NaTf possesses excellent dimensional stability in the form of free standing thick film, which offers the ionic conductivity of 5.74 × 10−3 S cm−1 at room temperature (∼ 27 °C). The temperature dependence of electrical conductivity of polymer–ionic liquid blends and gel polymer electrolyte follows the Vogel–Tammen–Fulcher (VTF) behaviour. The sodium ion conduction in the gel polymer electrolyte film is confirmed from cyclic voltammetry and transport number measurement. The Na+ ion transport number has been determined to be ∼ 0.23, which suggests the significant contribution of anionic transport along with the possible conduction of component ions of ionic liquid.