Diglyme-Incorporated Gelled Polymer: An Efficient Quasi-Solid-State Electrolyte for Sodium-Ion Batteries

Glyme-based electrolytes have been gaining increasing attention in sodium-ion batteries (NIBs) for their attractive electrochemistry involving good complexation ability with Na-ions, high electrochemical stability, good transport characteristics, low volatility, and high safety. Here, we report a flexible and freestanding sodium-ion conducting gel polymer electrolyte containing an equimolar ratio of Na-salt NaTFSI and diglyme (G2), referred to as a solvate ionic liquid (SIL), immobilized inside host polymer poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP). Fourier-transform infrared (FTIR) and Raman analyses are employed to investigate the conformational evolutions in PVdF-HFP on incorporating G2 and NaTFSI, whereas structural changes are observed using X-ray diffraction. The electrochemical investigations on the electrolyte revealing a wide electrochemical stability window (∼5.2 V) versus Na/Na+, high room-temperature ionic conductivity (∼1.12 × 10–3 S cm–1), and a large Na+ transference number (tNa+ ∼ 0.58) along with thermal stability up to ∼100 °C render the electrolyte suitable for Na-storage applications. The Na deposition/extraction tests over a long period indicate excellent cyclic stability of the cells, revealing low and stable polarization with values limited to ±19 mV at a current density of 0.01 mA cm–2. Further, the prepared solvate ionic liquid-incorporated gel polymer electrolyte (SIL-GPE) is tested in Na-ion battery cells comprising Na0.7CoO2 cathode using galvanostatic charge–discharge cycling, with a first discharge capacity of ∼91.76 mA h g–1 at a 0.05C rate. Studies indicate a class of gel polymer electrolytes based on solvate ionic liquids, suitable for Na-storage applications.