A Promising Gel Polymer\nElectrolyte Composition Comprising\na Green-Glyme Di(propylene glycol) Dimethyl Ether for Application\nin Sodium-Ion Batteries

Electrochemically efficient electrolytes with suitable\nsolvents\nare an essential requirement for sodium-ion batteries (NIBs). Glymes\n(i.e., glycol ethers) added with Na salts, also known as “solvate\nionic liquids (SILs)”, are highly appealing electrolytes for\nNIBs in terms of low flammability, less toxicity, high electrochemical\nstability, and environmental compatibility. Herein, we report flexible\ngel polymer electrolytes (GPEs), composed of SILs containing a “green-glyme”\ndi(propylene glycol) dimethyl ether (DPGDME) and sodium bis(trifluoromethanesulfonyl)imide\n(NaTFSI) of varying molar ratios, immobilized in a polymer poly(vinylidene\nfluoride-<i>co</i>-hexafluoropropylene) (PVdF-HFP). Thick\nfilms of SIL-GPEs are optimized for application in quasi-solid-state\nNIBs via structural, thermal, and electrochemical measurements. These\nfilms offer optimum room temperature ionic conductivity (∼1.1\nmS cm<sup>–1</sup>), a high Na-ion transport number (∼0.67),\na wide oxidative stability voltage limit (∼5.2 V vs Na/Na<sup>+</sup>), and thermal stability (up to ∼100 °C). The\nGPE film demonstrates excellent reversibility when interfaced with\na sodium metal anode observed from the stripping/plating tests on\nthe cell Na|SIL-GPE|Na. The SIL-GPE film is found to be suitable as\nascertained by galvanostatic charge–discharge (GCD) tests on\nthe cell Na|SIL-GPE|Na<sub>0.44</sub>MnO<sub>2</sub> offering a capacity\nof ∼105.8 mAh g<sup>–1</sup> at 0.05C. The cell offers\na steady performance for ∼50 GCD cycles, recorded at 0.33C.\nThe results confirm the potential use of DPGDME as a green solvent\nin a flexible GPE film for quasi-solid-state NIBs.