Rhombohedral Li_1+xY_xZr_2-x(PO₄)₃ Solid Electrolyte Prepared by Hot-Pressing for All-Solid-State Li-Metal Batteries

NASICON-type solid electrolytes with excellent stability in moisture are promising in all-solid-state batteries and redox flow batteries. However, NASIOCN LiZr₂(PO₄)₃ (LZP), which is more stable with lithium metal than the commercial Li_1.3Al_0.3Ti_1.7(PO₄)₃, exhibits a low Li-ion conductivity of 10^-6 S cm^-1 because the fast conducting rhombohedral phase only exists above 50 °C. In this paper, the high-ionic conductive rhombohedral phase is stabilized by Y³⁺ doping at room temperature, and the hot-pressing technique is employed to further improve the density of the pellet. The dense Li_1.1Y_0.1Zr_1.9(PO₄)₃ pellet prepared by hot-pressing shows a high Li-ion conductivity of 9 × 10^-5 S cm^-1, which is two orders of magnitude higher than that of LiZr₂(PO₄)₃. The in-situ formed Li₃P layer on the surface of Li_1.1Y_0.1Zr_1.9(PO₄)₃ after contact with the lithium metal increases the wettability of the pellet by the metallic lithium anode. Moreover, the Li_1.1Y_0.1Zr_1.9(PO₄)₃ pellet shows a relatively small interfacial resistance in symmetric Li/Li and all-solid-state Li-metal cells, providing these cells a small overpotential and a long cycling life.