Phase‐Transition Interlayer Enables High‐Performance Solid‐State Sodium Batteries with Sulfide Solid Electrolyte

All‐solid‐state sodium (Na) batteries (ASSSBs) using sulfide‐based solid electrolytes (SEs) have attracted intensive attention due to their superior safety, high energy density, and low cost. However, the interfacial issue is one of the biggest challenges to achieve high‐performance sulfide‐based ASSSBs due to the serious reactions between active Na metal and sulfide SEs at the interface. To address the interfacial challenges, a simple and efficient approach by introducing a phase transition polymer electrolyte as an interlayer to stabilize the interface is proposed. Na 3 SbS 4 as a model sulfide SE is used to demonstrate the interlayer strategy to stabilize the interface by preventing the detrimental reactions and inhibiting Na dendrites. As a result, stable Na plating/stripping is observed in Na symmetric cells under the current density of 0.1 mA cm −2 . Moreover, ASSSBs with Na metal and TiS 2 electrode deliver long‐term stability over 300 cycles remaining a specific capacity above 100 mAh g −1 , and FeS 2 ||Na cells exhibit an impressive specific capacity of up to 200 mAh g −1 after the 20th cycles. This study demonstrates an efficient strategy to address interfacial challenges between sulfide SEs and Na metal, which contributes to the development of ASSSBs in next‐generation energy storage systems.