Introduction: Sodium-ion batteries are being heavily explored as cheaper alternatives to expensive lithium-ion batteries. However, synthesizing electrolytes with sufficient room temperature ionic conductivities is a major bottleneck.

Methods and Results: In this work, gel-polymer electrolytes (GPEs) based on poly (ethylene oxide) (PEO) host polymer and NaClO₄ ionic salt have been synthesized by using ethylene carbonate (EC) and propylene carbonate (PC) plasticizer/solvent mixture. The optimized PEO:NaClO₄:EC:PC composition (7:13:40:40 wt. %) exhibits a room temperature ionic conductivity of 9.5 mS cm^-1 and the variation of ionic conductivity with inverse temperature showed Arrhenius behavior with almost constant activation energies for all the compositions. DC polarization data on optimized composition confirms that the electrolyte is dominantly an ionic conductor (t_i = 0.99) with negligible electronic conductivity (t_e = 0.01), which is highly desirable for an ideal electrolyte. Cyclic voltammetric studies reveal that the optimized composition is electrochemically stable for a voltage window of -2 to +2 V. Absence of any crystalline peaks in the X-ray diffractograms of the GPEs is a clear evidence for the amorphous nature of these GPEs. Broadening of the FTIR optical vibrational bands at 1000-1300 and 1700-1850 cm^-1 is ascribed to the lowering of crystallinity resulting from the formation of PEO/Na⁺ complex through Na⁺ ion solvation.

Conclusion: The GPE with optimized composition and highest ionic conductivity at room temperature is a suitable electrolyte for practical applications in sodium-ion rechargeable batteries.

Keywords: Gel-polymer electrolytes; Ionic conductivities; DC polarization; Electrochemical impedance spectroscopy; Sodium-ion batteries

This research was supported by the AHEAD Operation of the Ministry of Higher Education, Sri Lanka funded by the World Bank (DOR # 62) and the Pakistan-Sri Lanka collaborative research project funded by the NSF - Sri Lanka (NSF-PSF/ICRP/2017/EA&ICT/01).

* Corresponding author: K. Vignarooban (kvignarooban@gmail.com)