Mg-doped NASICON-type electrolyte for rechargeable solid-state sodium-ion batteries

Designing high sodium-ion conducting solid electrolytes is essential for realizing solid-state sodium-ion batteries. The present study investigates the earth-abundant divalent Mg2+ doping in the NASICON-type compound with a general compositional formula Na(3.2+2x)Zr(2-x)MgxSi(2.2)P(0.8)O(12) (x ranging from 0 to 0.228). Polycrystalline specimens are prepared via the conventional solid-state reaction method. A dominant monoclinic NASICON and the impurity m-ZrO2 exist in all the tested compositions. Na3PO4 and rhombohedral NASICON are also detected in compositions with high Mg2+ content. The conductivity significantly improves to 3.2 mS.cm(-1) at 25 degrees C with maximum occurring at x = 0.128. The optimal microstructure, high excess Na content, expanded unit-cell volume, and conducting rhombohedral phase contribute to excellent conductivity in this composition. The electrochemical performance of various solid-state batteries with Na3.1V2P2.9Si0.1O12 electrode and optimized solid electrolyte is also evaluated. The symmetrical-cells having solid electrolyte/electrode interface modified by liquid electrolyte show a stable capacity of 70 mAh.g (-1) after 70 cycles at C/10. Moreover, the solid-state half-cell in which a composite cathode is employed with no liquid electrolyte at the interface delivered a considerably high capacity of 92 mAh.g(- 1) at a C/5 rate. An outstanding energy density of -300 Wh.kg(-1) at 25 degrees C demonstrates the exciting prospect of Na3.456Zr1.872Mg0.128Si2.2P0.8O12 electrolyte for rechargeable solid-state batteries.