Elastomer-Assisted PEO Electrolytes: ENR50 Suppresses Crystallinity and Boosts Na+ Transport Enabling High-Voltage Solid-State Sodium-Ion Batteries

The quest for sustainable, greener, low-cost energy storage has propelled solid-state sodium-ion batteries as a vital alternative to lithium-based systems. Here, we report a solid polymer electrolyte (SPE) comprising a poly(ethylene oxide) (PEO) and 50% epoxidized natural rubber (ENR50) blend doped with sodium trifluoromethanesulfonate (NaCF 3 SO 3 ). Incorporation of the bio-based ENR50 elastomer improves flexibility and promotes ion transport by suppressing PEO crystallinity and enhancing amorphous conduction pathways. An optimal salt loading of 25 wt.% NaCF 3 SO 3 delivered the highest room-temperature ionic conductivity of 1.00 × 10 -5 S cm -1 . Temperature-dependent conductivity followed Arrhenius behaviour with two distinct regions at 30 – 60 °C (E a1 = 1.27 eV) and 60 – 100 °C (E a2 = 0.19 eV; σ 2 = 8.57 × 10 -2 S cm -1 ), suggesting a temperature-driven structural transition. Differential scanning calorimetry confirmed a semi-crystalline-to-amorphous transition with a melting temperature event at ∽ 60 ± 10 °C, consistent with the observed increase in ionic mobility. Compared with lithium analogue (PEO–ENR50–LiCF 3 SO 3 ), the sodium-based system exhibited improved electrochemical performance, achieving a higher sodium-ion transference number (t Na + = 0.23) than the lithium-ion transference number (t Li + = 0.06). Both systems showed dominant ionic conduction with total ion transport numbers approaching 0.99. ATR-FTIR confirmed polymer-salt coordination at the PEO ether group (∽1099 cm -1 ), while thermogravimetric analysis demonstrated improved thermal stability upon salt incorporation. Linear sweep voltammetry revealed a wide electrochemical stability window up to 5.0 V. Overall, these results demonstrate the synergistic role of ENR50 in combining mechanical compliance with improved ionic transport, highlighting its potential as a high-voltage SPE for next generation sodium-ion batteries.