Grain Boundary Design of Solid Electrolyte Actualizing Stable All‐Solid‐State Sodium Batteries

Abstract Advanced inorganic solid electrolytes (SEs) are critical for all‐solid‐state alkaline metal batteries with high safety and high energy densities. A new interphase design to address the urgent interfacial stability issues against all‐solid‐state sodium metal batteries (ASSMBs) is proposed. The grain boundary phase of a Mg 2+ ‐doped Na 3 Zr 2 Si 2 PO 12 conductor (denoted as NZSP‐ x Mg) is manipulated to introduce a favorable Na 3−2 δ Mg δ PO 4 ‐dominant interphase which facilitates its intimate contact with Na metal and works as an electron barrier to suppress Na metal dendrite penetration into the electrolyte bulk. The optimal NZSP‐0.2Mg electrolyte endows a low interfacial resistance of 93 Ω cm 2 at room temperature, over 16 times smaller than that of Na 3 Zr 2 Si 2 PO 12 . The Na plating/stripping with small polarization is retained under 0.3 mA cm ‐2 for more than 290 days (7000 h), representing a record high cycling stability of SEs for ASSMBs. An all‐solid‐state NaCrO 2 //Na battery is accordingly assembled manifesting a high capacity of 110 mA h g ‐1 at 1 C for 1755 cycles with almost no capacity decay. Excellent rate capability at 5 C is realized with a high Coulombic efficiency of 99.8%, signifying promising application in solid‐state electrochemical energy storage systems.