Liquid Metal Interlayer for Ultrastable Solid‐State Sodium Metal Battery

Solid-state sodium metal batteries have attracted wide attention due to their high energy density, remarkable safety, and abundant sodium resources. However, the growth of Na dendrites and poor interfacial contact between Na metal anode and Na₃Zr₂Si₂PO₁₂ (NZSP) solid-state electrolytes severely limit their practical application. Herein, a wettable liquid metal (GaIn) interlayer significantly reduces the interfacial resistance and avoids the formation of voids at the Na/NZSP interface. Moreover, the Ga₄Na and NaIn alloys at the interface caused by the spontaneous reaction of GaIn with Na metal enhance the bond of NZSP with Na anode, which provides a continuous Na⁺ diffusion pathway and homogeneous Na⁺ flux to suppress Na dendrite growth. The symmetric cell can cycle stably for over 6500 h at 0.05 mA cm^-2 and over 3000 h at 0.1 mA cm^-2, with a critical current density of 0.8 mA cm^-2 at 25 °C, and the interfacial resistance is significantly reduced to 21.6 Ω from 1095.1 Ω. The full cell coupled with NaNi_1/3Fe_1/3Mn_1/3O₂ also shows outstanding cycling performance, maintaining 85.1% capacity after 100 cycles at 0.5 C. This work demonstrates that the liquid metal interlayer has a large potential for the practical application of solid-state metal batteries.