Highly Conductive Poly(ethylene oxide)-Based Composite Polymer Electrolyte for Sodium Battery Applications

Polymer electrolytes based on poly(ethylene oxide) (PEO) have been extensively investigated for battery applications due to their excellent flexibility, chemical stability, and ease of processing. One major issue with the PEO-based electrolytes is their insufficient ionic conductivity. In this work, we incorporated 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (PY 14 FSI) into P(EO) 20 NaFSI, which significantly increases the amorphous phase, promotes interactions between NaFSI and ether oxygen in PEO, and enables higher ionic conduction. For example, P(EO) 20 NaFSI-1.6PY 14 FSI shows ionic conductivities of 1.15 × 10 –4 and 1.64 × 10 –3 S cm –1 at room temperature and 60 °C, respectively, which are significantly higher than those of pure P(EO) 20 NaFSI. Symmetric Na/Na cells with the P(EO) 20 NaFSI-1.6PY 14 FSI composite electrolyte show a critical current density of 0.4 mA cm –2 and excellent cyclability at 60 °C (e.g., minimal performance degradation during ∼700 cycles under a current density of 0.1 mA cm –2 ). All-solid-state Na/P(EO) 20 NaFSI-1.6PY 14 FSI/Na 3 V 2 (PO 4 ) 3 cells achieve a capacity of ∼100 mAh g –1 at a 0.1C rate. The cells retain ∼85% of the discharge capacity and a high Coulombic efficiency of >99% over 300 cycles at a 1C rate and 60 °C, confirming the good electrochemical performance of the electrolyte. This work represents a simple method to fabricate polymer electrolytes with high ionic conduction and excellent interfacial stability with electrodes for solid-state sodium metal battery applications.