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

Abstract

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 Na3.2+2xZr2-xMgxSi2.2P0·8O12 (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 °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 °C demonstrates the exciting prospect of Na3·456Zr1·872Mg0·128Si2·2P0·8O12 electrolyte for rechargeable solid-state batteries.