A Hetero‐Layered, Mechanically Reinforced, Ultra‐Lightweight Composite Polymer Electrolyte for Wide‐Temperature‐Range, Solid‐State Sodium Batteries

Abstract The practical use of polyethylene oxide polymer electrolyte in the solid‐state sodium metallic batteries (SSMBs) suffers from the retard Na⁺ diffusion at the room temperature, mechanical fragility as well as the oxidation tendency at high voltages. Herein, a hetero‐layered composite polymeric electrolyte (CPE) is proposed to enable the simultaneous interfacial stability with the high voltage cathodes (till 4.2 V) and Na metallic anode. Being incorporated within the polymer matrix, the sand‐milled Na₃Zr₂Si₂PO₁₂ nanofillers and nanocellulose scaffold collectively endow the thin‐layer (25 µm), ultralightweight (1.65 mg cm⁻²) CPE formation with an order of magnitude enhancement of the mechanical strength (13.84 MPa) and ionic conductivity (1.62 × 10⁻⁴ S cm⁻¹) as compared to the pristine polymer electrolyte, more importantly, the improved dimension stability up to 180 °C. Upon the integration of the hetero‐layered CPE with the iron hexacyanoferrate FeHCF cathode (1 mAh cm⁻²) and the Na foil, the cell model can achieve the room‐temperature cycling stability (93.73% capacity retention for 200 cycles) as well as the high temperature tolerance till 80 °C, which inspires a quantum leap toward the surface‐wetting‐agent‐free, energy‐dense, wide‐temperature‐range SSMB prototyping.