Enhancing Cycling Stability In Symmetric Solid‐State Na‐Ion Batteries Via Composite Polymer Buffer Layer

Solid‐state sodium‐ion batteries (SSIBs) represent an advanced energy storage technology, offering superior safety, thermal stability, and robust long‐term cycling performance. However, their practical deployment is critically constrained by the low ionic conductivity of solid electrolytes (SEs) and pronounced interfacial instability—stemming from poor physical contact between the electrode and SE, as well as parasitic reactions involving metallic sodium and liquid electrolyte‐based interfacial modifiers. In this work, the development of a composite polymer buffer layer (CPBL) is reported as an interfacial modifier to establish stable and intimate contact at the electrode‐SE interface. Integrated into a symmetric full‐cell configuration using Fe‐doped Na 3 V 2 (PO 4 ) 3 electrodes and NASICON‐type ceramic electrolyte, the SSIB delivers a discharge capacity of ≈45 mAh g −1 (theoretical capacity ≈58.8 mAh g −1 ) at C/5, with 89% capacity retention over 200 cycles at 25 °C. Notably, the SSIB assembly is carried out in ambient conditions without the need for an inert atmosphere. The enhanced electrochemical performance is attributed to the synergistic effects of improved ionic conductivity and superior interfacial contact, which collectively facilitate efficient Na + transport across the electrode‐SE interface. These findings underscore the potential of CPBL to overcome interfacial challenges in SSIBs and advance the development of safe, high‐performance, and sustainable sodium‐based energy storage systems.