Highly conductive Na5YSi4O12 ceramic solid electrolyte by controlling Na deficiency in the precursors

Na₅YSi₄O₁₂ (NYSO) is known as a fast sodium-ion conductive oxide with both high ionic conductivity above 1 mS cm⁻¹ at room temperature and excellent electrochemical stability against sodium metal anode, making it promising for solid-state sodium battery application. In this study, we investigated the influence of non-stoichiometry in the precursor composition on the crystal phase, microstructure and ionic conducting property of NYSO. It is found that NYSO prepared from the precursor with slight sodium-deficient composition showed the highest total (bulk + grain boundary) conductivity of 2.8 mS cm⁻¹ at room temperature, which is superior to NYSO with stoichiometric precursor composition (∼ 1 mS cm⁻¹). Although excess Na deficiency in the precursor results in the formation of Na₃YSi₃O₉ phase, it does not influence so much on the sinterability and grain-boundary resistance of NYSO. Mitigation of Na₉YSi₆O₁₈ phase formation during the fabrication is very important for the densification and reduction of grain-boundary resistance in NYSO ceramic electrolyte.