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.