A Multifunctional Na_xBi/NaCl Flexible Interface Layer for Solid‐State Na Metal Batteries

Abstract Na₃Zr₂Si₂PO₁₂ (NZSP) electrolyte with excellent air stability and acceptable ionic conductivity demonstrates a significant potential for application in solid‐state Na metal batteries. However, the poor interfacial contact between NZSP and Na anode has severely hindered its practical application. Herein, a BiCl₃/polytetrafluoroethylene flexible interface layer is constructed between NZSP and Na metal anode. BiCl₃ can react with Na metal to form a multifunctional Na_xBi/NaCl‐rich flexible interface layer during the cycling process, which can effectively enhance the interfacial contact, avoid the formation of voids and reduce the interfacial resistance. Na_xBi with low energy barrier can provide fast Na⁺ diffusion path, and NaCl with a wide band gap can inhibit electron injection and suppress Na dendrite growth. Owing to the multifunctional Na_xBi/NaCl flexible interface layer, the Na/BiCl₃@NZSP/Na symmetric cell exhibits a reduced interfacial resistance from 1252.1 to 67.6 Ω and achieves a high critical current density of 2 mA cm⁻² at 25 °C. At 0.1 mA cm⁻²/0.1 mAh cm⁻² and 0.3 mA cm⁻²/0.3 mAh cm⁻², the symmetric cell can stably cycle for 3000 and 1100 h. Additionally, the Na₃V₂(PO₄)₃/BiCl₃@NZSP/Na full cell can retain 96.7% of initial capacity (≈111 mAh g⁻¹) after 300 cycles at 0.5 C and excellent rate capability.