Influence of green-synthesized NiFe2O4 nanoparticles: surface treatment, doping Ratio, and calcination temperature on ion transport and dielectric properties of PEO–NaNO3–glycerol electrolytes

This work investigates the effect of green-synthesized NiFe₂O₄ nanoparticles (NPs) on the ion transport and dielectric properties of polyethylene oxide (PEO)–NaNO₃–glycerol nanocomposite polymer electrolytes (NCPEs). NiFe₂O₄ NPs were synthesized via a microwave-assisted green route and incorporated at 0–5 wt.% with and without surface treatment, followed by calcination at 400 and 800 °C. FTIR analysis revealed strong polymer–filler interactions, evidenced by broadened O–H stretching and intensified Fe–O/Ni–O vibrations, which contributed to reduced crystallinity and enhanced ionic mobility. Samples with 0.5 wt.% NPs showed the highest dielectric response, whereas higher loadings suppressed polarization. The modulus formalism and Cole–Cole plots confirmed non-Debye relaxation and faster ion dynamics in optimized samples. For the best-performing composition (IB27: 0.5 wt.% surface-treated NiFe₂O₄ calcined at 800 °C), transport parameters were determined as tan δmax = 4.24, relaxation time τ = 1.049 µs, diffusion coefficient D = 1.13 × 10^⁻3 cm²/s, charge carrier density N = 5.9 × 1013 cm^⁻3, and mobility µ = 4.24 cm² V^⁻1 s^⁻1. To evaluate the combined effects of nanoparticle concentration, calcination temperature, and surface treatment, Response Surface Methodology (RSM) was employed, and contour plots are presented in the Results section. These statistical models confirmed that low filler content (0.5 wt.%), high calcination temperature (800 °C), and surface modification synergistically enhance conductivity. The optimized NCPE achieved a maximum DC ionic conductivity of 111 µS/cm, demonstrating the effectiveness of nanoparticle engineering and statistical optimization in tailoring polymer electrolytes for advanced solid-state sodium-ion batteries and related energy storage devices.