Quasi-solid electrolyte based on cost-effective sodium salt for high-performance sodium-ion supercapacitors

Abstract

Developing hydrogel-based quasi-solid electrolytes (QSEs) that simultaneously achieve an extended electrochemical window, improved interfacial stability, and an optimal balance between ionic conductivity and mechanical strength remains highly challenging. In this work, low-cost sodium nitrate (10 m NaNO₃)-based QSEs were synthesised via free radical polymerisation using tetra(ethylene glycol) diacrylate (TEGDA) as the monomer, 2-hydroxy-2-methylpropiophenone (HMPP) as the photoinitiator, and varying polyethylene glycol (PEG, 45–70 wt%) contents as the crosslinker. The QSEs were systematically characterised for their electrochemical performance, ionic conductivity, and mechanical properties at room temperature. Linear sweep voltammetry showed a widened electrochemical window of 2.4 V. As the PEG content increased from 45 to 70 wt%, ionic conductivity declined from 9.18 to 1.55 mS cm⁻¹, while the QSE with 60 wt% PEG exhibited the highest Young's modulus (2.95 MPa) and hardness (0.16 MPa), indicating the optimal balance between ionic conductivity and mechanical properties. The symmetric supercapacitor constructed using activated carbon electrodes and QSE containing 60 wt% PEG exhibited a specific capacitance of 168.3 F g⁻¹ when operated at 0.2 A g⁻¹ current density. The supercapacitor delivered a power density of 459.44 W kg⁻¹ alongside a peak energy density of 134.64 Wh kg⁻¹. When the power density was elevated to 4876.80 W kg⁻¹, the corresponding energy density was maintained at 20.32 Wh kg⁻¹. These results demonstrate the strong promise of NaNO₃-based QSEs as cost-effective, flexible, and high-performance electrolytes for next-generation sodium-ion supercapacitors.