An In Situ Formed Composite Surface Layer to Stabilize Na₃Zr₂Si₂PO₁₂/Na Interfacial Performance

Na₃Zr₂Si₂PO₁₂ (NZSP) is regarded as a promising solid-state electrolyte (SSE) for solid-state sodium-metal batteries (SSSMBs) due to its high ionic conductivity and stable mechanical properties. However, the poor solid-solid contact between sodium (Na) and NZSP results in a high interfacial resistance and a reduced output capacity of the SSSMBs, thus restricting its practical application. To address this issue, a SnF₂ coating layer is introduced onto the surface of the NZSP with a simple drop-casting method. Then, through the in situ reaction of SnF₂ and molten Na metal, a composite conductive interfacial layer can be formed with its composition analyzed by CT and XPS. This composite layer facilitates intimate contact and decreases the interfacial resistance to 2.5 Ω cm². The critical current density is enhanced about 26 times, rising to 1.3 mA cm^-2. The symmetric cell displays a stable cycle life of 2500 h at 0.2 mA cm^-2 due to the advantageous interface. Moreover, the Na/SnF₂@NZSP/NaNi_1/3Fe_1/3Mn_1/3O₂ full cell exhibits an excellent cycle performance.