Enhanced ionic conductivity of Na-excess Na3Zr2Si2PO12 solid electrolyte by tuning its elemental composition and sintering temperature

Na3Zr2Si2PO12 (NZSP)-based materials are promising solid electrolytes for all-solid-state-Na-ion batteries (ASSNIBs) because of their wide electrochemical stabilities and good ionic conductivities at room temperature. The major challenges in developing NZSP-based materials include the occurrence of unexpected secondary phases that often limits their ionic conductivity values. In addition, the development of these materials often needs higher sintering temperatures (> 1100 °C) that may lead to the minor loss of Na during the preparation. The Na content in NZSP is often increased to minimize the loss of Na during preparation and furthermore to enhance its concentrations in NZSP. The transport behaviour of NZSP material is often controlled by the stoichiometries of different elements present. In the current work, Na-excess NZSP materials are prepared by solid-state reaction using different types of Na-precursors such as Na2CO3, Na3PO4·12H2O, Na2SiO3, and NaF as an excess Na-source keeping its Na-stoichiometry same. However, the selection of the precursors changes stoichiometries of the other elements like Si, P, and O present in NZSP. The formations of the Na-excess NZSP materials exhibiting monoclinic NZSP structure are confirmed by X-ray diffraction. The conductivity results measured by impedance spectroscopy are explained with respect to their unit cell volume, density, and bottleneck area available for the Na-ion diffusion. Na-excess NZSP material using Na2CO3 (Na3.45Zr2Si2PO12.22, NC) as an excess Na-source, sintered at 1200 °C, exhibited highest ionic conductivity. The ionic transport number measurement by the dc polarization technique confirms the ionic nature of the principal charge carriers. Furthermore, the linear sweep voltammetry measurement shows its electrochemical stability window is up to 5.2 V implying the suitability of the Na3.45Zr2Si2PO12.22 material as a potential solid electrolyte for ASSNIBs.