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

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 Mg²⁺-doped Na₃Zr₂Si₂PO₁₂ conductor (denoted as NZSP-xMg) is manipulated to introduce a favorable Na_3−2δMg_δPO₄-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 cm² at room temperature, over 16 times smaller than that of Na₃Zr₂Si₂PO₁₂. 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 NaCrO₂//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.