A functional NaₓSn/NaBr interlayer for solid-state sodium metal batteries

Solid-state sodium metal batteries are hindered by critical challenges at the interface between sodium metal anode and solid-state electrolyte, including poor interfacial contact, dendrite growth and sluggish ion transport. Herein, a solution of SnBr 2 was drop-cast onto the sodium metal surface, inducing an in-situ alloying reaction to construct a functional interlayer composed of Na x Sn alloy and NaBr. The Na x Sn provides abundant sodiophilic sites, which enhance interfacial contact with solid-state electrolyte Na 3 Zr 2 Si 2 PO 12 (NZSP), facilitate rapid Na + transport across the interface, and improve interfacial diffusion kinetics. Moreover, the electron-insulating nature of NaBr promotes uniform sodium deposition and suppresses dendrite growth. Consequently, the critical current density is significantly improved to 3.1 mA cm −2 from 0.6 mA cm −2 and the area-specific resistance is decreased from 1759.1 Ω·cm −2 to 42.4 Ω·cm −2 . The solid-state symmetric cell (SnBr 2 @Na/NZSP/SnBr 2 @Na) demonstrates stable cycling for 1500 h at 0.5 mA cm −2 /0.5 mAh cm −2 . When paired with a Na 3 V 2 (PO 4 ) 3 cathode, the solid-state full cell exhibits exceptional cycling stability, retaining a capacity of 104.7 mAh g −1 after 700 cycles at 0.5C with a capacity retention of 95.5%. This strategy of constructing a functional interphase significantly enhances the performance of solid-state sodium metal batteries, presenting a promising path toward practical application.