Calcium Doped NASICON Electrolyte with Graphite Coating for Stable All-solid-state Sodium Metal Batteries

All-solid-state sodium metal batteries face the challenges of low ionic conductivity of solid electrolytes and poor wettability towards metallic Na anode. Herein, Na₃Zr₂Si₂PO₁₂ solid electrolyte is doped with Ca²⁺, obtaining a high ionic conductivity of 2.09×10^-3 S cm^-1 with low electronic conductivity of 1.43×10^-8 S cm^-1 at room temperature, which could accelerate Na⁺ transportation and suppress sodium dendrite growth. Meanwhile, a graphite-based interface layer is coated on Na_3.4Zr_1.8Ca_0.2Si₂PO₁₂ (Na_3.4Zr_1.8Ca_0.2Si₂PO₁₂-G) in order to improve the solid-solid contact between solid electrolyte and Na anode, realizing a uniform current distribution and smooth Na metal plating/stripping, and thus achieving a triple higher critical current density of 3.5 mA cm^-2 compared with that of Na_3.4Zr_1.8Ca_0.2Si₂PO₁₂. In addition, the assembled Na₃V₂(PO₄)₃/Na_3.4Zr_1.8Ca_0.2Si₂PO₁₂-G/Na all-solid-state battery exhibits excellent electrochemical performances with a reversible capacity of 81.47 mAh g^-1 at 1 C and capacity retention of 97.75 % after 500 cycles.