Stable Sodium Metal Anodes Enabled by an In-situ Generated Mixed-Ion/Electron-Conducting Interface

• A mixed-ion/electron-conducting interface layer was constructed by a conversion reaction. • The interface layer can improve the affinity with Na metal, leading to dendrite-free Na deposition. • The full cells based on the designed layer show a very stable cycling over 1000 cycles. Sodium metal has been regarded as one of excellent candidates of anode materials for the next-generation high-energy sodium-ion batteries owing to its low redox potential, low cost, and high theoretical capacity. However, the poor reversibility and the dendrite growth of Na anode on cycling have significantly hindered the practical application of sodium metal anodes (SMAs). Herein, we report that a mixed-ion/electron-conducting interface (MIECI) layer, in-situ generated through the conversion of CuP 2 into the mixed conductor of Cu and Na 3 P, enable dendrite-free Na plating/stripping in ether-based electrolyte. The MIECI layer can significantly improve the affinity with the deposited Na, homogenize the Na + flux, and reduce the local current density. The MIECI layer enables Coulombic efficiency (CE) of Na plating/stripping as high as 99.71% over 420 cycles at 2 mA h cm −2 and 2 mA cm −2 as well as the Na||Na cell to stably cycle for 1200 h (300 cycles) with a depth of discharge of 33.33 % at 2 mA h cm −2 . It is found that the current density plays a greater influence on the SMA stability. The in-situ construction of MIECI buffer layer opens up a simple and facile avenue to stabilize SMAs.