Ferroelectric Nanofiber Reinforced Electrolyte Empowers Fully 3D‐Printed Solid‐State Sodium‐Ion Batteries

Abstract Composite solid electrolytes for sodium batteries face persistent challenges in their widespread adoption, including limited operation stability, compromised interfacial compatibility, and insufficient fabrication controllability. As relevant, fully 3D‐printed solid‐state sodium batteries are waited to be demonstrated. Herein, a ferroelectric BiFeO 3 (BFO) filler for the poly(ethylene oxide) (PEO) based electrolyte is introduced to harness spontaneous polarization that promotes the dissociation of Na salt (NaClO 4 ). The electrospun BFO, obtained in its nanofiber form, also facilitates cross‐linked piezoelectric networks to induce uniform Na deposition and restrain dendritic growth. As a result, the prepared PEO‐BFO‐NaClO 4 electrolyte enables a 93.7% retention after 300 cycles at 1 C for Na 3 V 2 (PO 4 ) 3 ||Na batteries. The dissociation mechanism of NaClO 4 and the deposition behavior of Na modulated by the ferroelectric effect are elucidated throughout theoretical simulations. The thus‐fabricated solid electrolyte is conducive to 3D printing customization, and the fully 3D printed solid‐state sodium batteries utilizing aqueous slurries showcase excellent electrolyte‐electrode interfacial compatibility. This encourages stable operation at room temperature when coupled with ferroelectric BFO to regulate uniform ion flux. This work demonstrates an appealing material paradigm with application‐relevant manufacturing strategy toward advanced and customizable solid‐state alkali batteries.