High-retention sodium supercapacitors with sodium hexametaphosphate-controlled water-processable/non-flammable sodium-ion solid-state electrolytes

Achieving high-performance sodium-based solid-state electrolytes (SSEs) through environmentally friendly processes is crucial to establishing a solid foundation for safe and inexpensive energy storage devices. Here we demonstrate nonflammable sodium cation-transporting SSEs prepared from aqueous solutions of branched poly(ethylene imine) (bPEI), sodium hydroxide (NaOH), and sodium hexametaphosphate (SHMP). The bPEI:NaOH:SHMP (PNaS) SSEs exhibited an outstanding ion conductivity of ~1 mS/cm at SHMP = 20 mol%, which is 5 times higher than 0.18 mS/cm for the bPEI:NaOH (PNa) SSEs, due to the SHMP-induced morphology optimization for efficient Na + transport. The optimum PNaS SSEs could deliver the output voltage of 4.4 V by galvanostatic charging at 0.5 mA/g, exhibiting long-term retention characteristics (>1000 s). The PNaS supercapacitors exhibited stable operation with 99.68% capacitance retained during 2000 charging/discharging cycles, while the PNaS films were considerably stable without burning upon the flammability test.