Core‐Shell Structured Composite Solid Electrolyte Enables High‐Rate All‐Solid‐State Sodium Batteries

All‐solid‐state sodium batteries (ASSSBs) have emerged as the promising candidates for large‐scale energy storage; however, they still face challenges related to ionic transport across multi‐scale interfaces, which leads to suboptimal battery performances. In this study, we propose a core‐shell structured composite electrolyte with Na3PS4 sulfide solid electrolyte (SE) as the core and Na2.25Y0.25Zr0.75Cl6 halide SE as the shell. This design strategy boosts high SE’s oxidative stability (4.0 V), ionic conductivity (0.44 mS cm‐1) and mechanical strength (Young’s modulus of 9.19 GPa). These properties enable the composite SE to effectively mitigate the chemo‐mechanically‐induced multi‐interfacial contact losses at the cathode side. Additionally, the homogeneous full‐cell configuration, utilizing the core‐shell structured composite electrolyte as both catholyte and SE layer, demonstrates superior rate capability, achieving a discharge capacity of 76.4 mAh g⁻¹ at 2.0 C, significantly outperforming the conventional sandwich‐like designs. This work not only provides a novel design strategy for functional SE, but also offers valuable insights into the chemo‐mechanical failure mechanism at multiscale interfaces of ASSSBs.