3D NASICON Ceramic Skeleton Enabled 18 µm‐Thick High Performance Bicontinuous‐Phase Ultrathin Composite Quasi‐Solid‐State Electrolyte

Ultrathin composite solid‐state electrolytes with ultrathin thicknesses and ultra‐low weight exhibit significant prospects for constructing high‐energy‐density solid‐state sodium metal batteries (SSSMBs). However, composite quasi‐solid‐state electrolytes (CSSEs) based on ceramic powder usually show discontinuous ionic transport channels and uneven agglomeration of the powder, which limits their practical performance. In this work, a 3D continuous self‐supporting Na 3.3 Mg 0.15 Zr 1.85 Si 2 PO 12 (NMZSP) ultrathin ceramic skeleton is sintered and further combined with an in situ UV curing process of trihydroxymethylpropyl triacrylate (TMPTA) to achieve an 18 µm‐thick ultrathin NMZSP ceramic skeleton composite quasi‐solid‐state electrolyte (UNSCE). The introduction of a ceramic skeleton effectively prevents ceramic powder agglomeration. Moreover, the synergistic interaction between the ceramic skeleton and the polymer matrix creates an abundant continuous two‐phase interface, which promotes the selective and rapid transportation of Na + . Therefore, UNSCE demonstrates a high Na + transference number (0.76). COMSOL simulations confirm that the 3D ceramic skeleton facilitates uniform current density, reducing dendrite formation risks. The Na | UNSCE | Na 3 V 2 (PO 4 ) 3 cell obtains a capacity retention of 91% after 500 cycles at 1C and a discharge capacity of 83.8 mAh g −1 at 10C. In summary, this work presents a scalable fabrication strategy for high‐performance ultrathin CSSEs based on a non‐inert ceramic skeleton, advancing practical deployment in SSSMBs.