Mechanically Adaptive Interpenetrating Separator for Ultralong Cycling and High‐Rate Sodium Metal Batteries

ABSTRACT Sodium metal batteries (SMBs) show great promise due to their high theoretical capacity and the abundance of sodium. However, the high reactivity of the sodium metal anode induces heterogeneous and dynamic interfacial stresses, which disrupt the solid‐electrolyte interphase (SEI) and result in uncontrollable dendrite growth during cycling. Conventional separators lack mechanical adaptability to mitigate interfacial stress, exacerbating battery failure risks. To address this critical challenge, we design an adaptive separator that mitigates interfacial stress concentration and suppresses dendrite growth via synergistic mechanical adaptation. Specifically, rigid glass fibers (GF) possess a high modulus to serve as a mechanical scaffold, while the flexible supramolecular nanofibers (CSNF) act as viscoelastic components to dissipate interfacial stress. They collectively achieve interfacial stress distribution uniformity, which is beneficial for the battery cycling stability. Consequently, the Na||GF/CSNF10%||Na symmetric cell demonstrates a long cycle life for over 2600 h. The Na||GF/CSNF10%||Na 3 V 2 (PO 4 ) 3 cell retains 88.4% of the initial capacity (89.8 mAh g −1 ) after 5000 cycles at a high rate of 10 C. This work highlights the synergistic effect of viscoelastic and rigid components in the composite separator, achieving exceptional long‐term cycling stability and high‐rate performance in SMBs.