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, Na 3 Zr 2 Si 2 PO 12 solid electrolyte is doped with Ca 2+ , 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 3.4 Zr 1.8 Ca 0.2 Si 2 PO 12 (Na 3.4 Zr 1.8 Ca 0.2 Si 2 PO 12 ‐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 3.4 Zr 1.8 Ca 0.2 Si 2 PO 12 . In addition, the assembled Na 3 V 2 (PO 4 ) 3 /Na 3.4 Zr 1.8 Ca 0.2 Si 2 PO 12 ‐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.