Addressing Electro‐Chemo‐Mechanical Coupling Failure at the Sodium|Na ₃ PS ₄ Interface towards Durable All‐Solid‐State Sodium Metal Batteries

ABSTRACT All‐solid‐state sodium batteries utilizing sulfide electrolytes and sodium (Na) anodes are promising for energy storage due to their safety and cost‐effectiveness. However, their practical implementation is significantly hindered by the interfacial degradation at Na 3 PS 4 |Na interface, while the resolution strategies remain scarce due to the unclear underlying mechanisms. Herein, operando synchrotron X‐ray computed tomography is utilized to reveal an electro‐chemo‐mechanical coupling failure mechanism. Our results demonstrate the formation of a 65‐µm‐thick side‐reaction layer at the interface within two hours of contact. This electron‐conducting but ion‐blocking layer causes non‐uniform Na electrodeposition, triggering continuous mechanical cracking inward Na 3 PS 4 that leads to a short‐circuit. To address this issue, a thermodynamically stable thin Na 3 Sb‐NaF functional layer is engineered on Na to protect Na 3 PS 4 from being chemically reduced. Its high Na‐ion diffusivity ensures uniform ion transport for homogeneous Na electrodeposition, enabling a mechanically integrated interface. As a result, symmetric cells sustain over 1000 h of cycling at 0.1 mA cm– 2 (0.1 mAh cm −2 ), while full cells using NaNi 1/3 Fe 1/3 Mn 1/3 O 2 cathodes achieve 88.3% capacity retention after 300 cycles at 0.2 C, room temperature. Crucially, anode‐free all‐solid‐state batteries exhibit stable cycling for over 60 cycles. This work advances the development of high‐energy, durable all‐solid‐state sodium metal batteries.