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

Na 5 YSi 4 O 12 (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 Na 3 YSi 3 O 9 phase, it does not influence so much on the sinterability and grain-boundary resistance of NYSO. Mitigation of Na 9 YSi 6 O 18 phase formation during the fabrication is very important for the densification and reduction of grain-boundary resistance in NYSO ceramic electrolyte. • Characterizing Na 5 YSi 4 O 12 (N5) ceramic electrolyte with different Na composition in the precursors. • Slight Na deficient composition is effective to reduce residual Na 9 YSi 6 O 18 (N9) phase. • Maximum total conductivity of 2.8 mS cm −1 at 27 °C for Na: Y: Si = 4.9: 1: 4 in the precursors. • Avoiding residual N9 phase is crucial for densification and reduction in grain-boundary resistance.