Manipulating Plasticity and Conductivity of Composite Polymer Electrolytes with a ZrO2 Ceramic Filler for Improving the Performance of Sodium Batteries

Although sodium-ion batteries (SIBs) are touted as cheaper than lithium-ion batteries, their commercial adoption is hindered by interfacial instability, dendrite formation, and safety concerns arising from flammable liquid electrolytes. To counter these issues, in this study, a composite polymer electrolyte (CPE) was developed comprising poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)), poly(ethylene oxide) (PEO), sodium hexafluorophosphate (NaPF6) salt, succinonitrile, and zirconium dioxide (ZrO2) via a simple solution casting method. The optimized CPE with 2.5 wt % ZrO2 (CPE-2.5) enhanced the amorphous nature of the matrix, porosity, voltage stability of the electrolyte, ionic conductivity (manipulating the Na+ ion transference number), and elasticity–plasticity balance. ZrO2 acted like a bridge between NaPF6 salt and the PEO matrix, establishing a Na+-conducting porous path when filled with a liquid electrolyte. The CPE-2.5 exhibited a high ionic conductivity of 1.26 ± 0.04 × 10–3 S cm–1, excellent electrochemical stability up to 4.96 V vs Na/Na+, and strong mechanical stability (tensile strength of ∼10.55 MPa). Direct evidence for the interaction of Lewis acid Zr4+ centers with PF6– was provided by differential scanning calorimetry. This interaction enhanced the matrix conductivity, preventing ion-pairing. The CPE-2.5 exhibited optimal porosity (22%) to retain the electrolyte and balanced plasticity and elasticity to accommodate volume change during 2000 h plating/stripping of Na and low polarization (∼100 mV). The Na||Na3V2(PO4)3 half-cell delivers a capacity of 107.5 mAh g–1 with 95% retention over 200 cycles. This work highlights ZrO2-induced manipulation of porosity, conductivity, and plasticity as a strategy for durable solid-state SIBs.