Separator-Integrated Design of High-Loading Na3Zr2Si2PO12 Composite Electrolytes for Sodium Metal Batteries

80 wt% Na 3 Zr 2 Si 2 PO 12 in ∼35 μm separator, significantly thinner than 50–200 μm membranes. • High ceramic loading achieves ∼16 GPa modulus while maintaining flexibility. • NASICON-induced α-to-β phase transition in PVDF-HFP enhances Na⁺ transport. • High ceramic loading enhances ionic conductivity and Na + transference number. • Over 2000 h stable Na|Na cycling and 96% retention at 5C over 350 cycles. Sodium metal batteries promise cost-effective energy storage, but face critical safety and stability challenges from flammable liquid electrolytes and uncontrolled dendrite growth. Quasi-solid-state electrolytes combining NASICON (Na 3 Zr 2 Si 2 PO 12 ) ceramic fillers with PVDF-HFP polymers offer a pragmatic solution, but existing freestanding membranes encounter a fundamental “thickness-performance-manufacturability” trilemma: thick films (50–200 μm) reduce energy density, high ceramic loadings cause brittleness, and specialized fabrication hinders scalability. Here, we demonstrate a separator-integrated design that resolves this trilemma by directly coating a thin (∼10 μm) high-loading (80 wt%) NASICON–PVDF-HFP–NaClO 4 composite layer onto a commercial polypropylene separator. This architecture shifts mechanical demands onto the PP substrate, allowing the active layer to be optimized for ionic transport without the brittleness that limits freestanding films. The resulting 80N membrane combines a total thickness of ∼35 μm, an ionic conductivity of 0.725 mS cm −1 , a Na + transference number of 0.54, stable Na|Na cycling over 2000 h, a critical current density(CCD) of 1.9 mA cm –2 , and full slurry-coating compatibility, with successful validation in ∼88 mAh g –1 proof-of-concept pouch cells (96.9% capacity retention after 100 cycles at 0.5 C). The work establishes the separator-integrated architecture as a practical route to high-ceramic-loading composite electrolytes for sodium metal batteries. Separator-integrated NASICON-PVDF-HFP composite electrolytes achieve 80 wt% ceramic loading—exceeding typical freestanding membrane limits (30-50 wt%)— through optimized slurry coating on porous PP substrates. High ceramic content establishes percolative Na + transport pathways, enabling stable Na plating/ stripping for over 2000 hours in symmetric cells. Flexible pouch cells validate practical viability for high-performance sodium metal batteries. Download: Download high-res image (217KB) Download: Download full-size image