Construction of an ultra-stable mixed conductive layer to stabilize the solid-state electrolyte/Na interface by in-situ interface chemistry.

• A thin SnF 2 layer is coated on the NZSP electrolyte surface via a powder polishing method for interfacial modification. • A mixed ionic/electronic conductive NaF@Na 15 Sn 4 layer is in situ generated at the Na|NZSP interface. • The NaF@Na 15 Sn 4 layer improves wettability, promotes interfacial Na+ transport and prevents the electron from attacking NZSP. • The NaF@Na 15 Sn 4 modified Na symmetric cell cycles stably more than 6000 h at 0.3 mA cm−2. The quest for high energy density, essentially safe, and cost-effective energy storing systems is powering the evolution of solid-state sodium metal batteries (SSSMBs). However, insufficient Na/solid-state electrolyte (SSE) interface contact leads to the formation of sodium dendrites, causing premature cell failure and safety issues that limit their practical application. Here, a novel ultra-stable mixed conductive layer (MCL) composed of Na 15 Sn 4 and NaF is introduced at the Na|Na 3 Zr 2 Si 2 PO 12 (NZSP) interface through in-situ interface chemistry. Such an MCL not only enables the transformation of the NZSP surface from sodiophobic to sodiophilic to achieve intimate contact with Na but also homogenizes the Na+ flux without dendritic sodium. Benefiting from the construction of the multifunctional interface, a high critical current density (CCD) of 1.3 mA cm−2 and an exceptionally long cycle life of 6000 h at 0.3 mA cm−2 are achieved for the Na|SnF 2 @NZSP|Na cell at room temperature (RT). Furthermore, the Na|SnF 2 @NZSP|Na 3 V 2 (PO 4) 3 full cell shows excellent cycling performance (91 % of maintained capacity for over 500 cycles at 1 C) and rate capability (101.4 mAh/g at 5 C) at RT. This work presents a validated method for the coordinated regulation of interfacial transfer kinetics and inhibition of sodium dendrite growth. [ABSTRACT FROM AUTHOR]