A Promising Gel Polymer Electrolyte Composition Comprising a Green-Glyme Di(propylene glycol) Dimethyl Ether for Application in Sodium-Ion Batteries

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