Facile alloying strategy at the Na|NZSP interface realizes high-performance solid-state Na batteries

Achieving stable and low-resistance interfaces remains a critical challenge for sodium metal solid-state batteries. A facile sputtering technique is employed to deposit an 80 nm gold coating on Na₃Zr₂Si₂PO₁₂ (NZSP-Au) solid electrolyte, enabling the formation of a conformal Na[sbnd]Au alloy interphase through controlled in situ alloying with metallic sodium. This engineered interfacial layer reduces the interfacial resistance between NZSP and Na metal from 629 Ω·cm² in unmodified symmetric Na||Na cells to 33 Ω·cm². Under continuous plating/stripping at 0.2 mA·cm⁻², the NZSP-Au electrolyte maintains stable cycling for over 10,000 h with only 33.1 mV overpotential, whereas the unmodified NZSP shows a high initial overpotential of 348.2 mV and short-circuits after approximately 1780 h. When paired with a Na₃V₂(PO₄)₃ cathode, the Na₃V₂(PO₄)₃|NZSP-Au|Na full cell delivers an initial discharge capacity of 113.7 mAh g⁻¹ and retains 89.3 % of this capacity after 600 cycles at a 2C rate. These results underscore interfacial alloy engineering as a promising strategy to enhance the stability and safety of Na metal anodes for solid-state sodium batteries.