Interface Ionic/Electronic Redistribution Driven by Conversion‐Alloy Reaction for High‐Performance Solid‐State Sodium Batteries

Abstract NASICON‐type Na + conductors show a great potential to realize high performance and safety for solid‐state sodium metal batteries (SSSMBs) owing to their superior ionic conductivity, high chemical stability, and low cost. However, the interfacial incompatibility and sodium dendrite hazards still hinder its applications. Herein, a conversion‐alloy reaction‐induced interface ionic/electronic redistribution strategy, constructing a gradient sodiophilic and electron‐blocking interphase consisting of sodium‐tin (Na‐Sn) alloy and sodium fluoride (NaF) between NASICON ceramic electrolyte and Na anode is proposed. The Na x Sn y alloy‐rich layer near the side of the sodium electrode acts as a superior conductor to enhance the anodic sodium‐ion transport dynamics while the NaF‐rich layer near the side of the ceramic electrolyte serves as an electron insulator to confine the interfacial electron turning ability, achieving uniform and dendrite‐free Na deposition during the cycling. Profiting from the synergistic effect of the gradient interphase, the critical current density (CCD) of the assembled Na symmetric cell is significantly increased to 1.7 mA cm −2 and the cycling stability of that is as high as 1200 h at 0.5 mA cm −2 . Moreover, quasi‐solid‐state sodium batteries with both Na 3 V 2 (PO 4 ) 3 and NaNi 1/3 Fe 1/3 Mn 1/3 O 2 cathode display outstanding electrochemical performance.