High Na sup(+) conducting Na sub(3)Zr sub(2)Si sub(2)PO sub(12)/Na sub(2)Si sub(2)O sub(5) composites as solid electrolytes for Na sup(+) batteries

Abstract

Sodium superionic conductor Na sub(3)Zr sub(2)Si sub(2)PO sub(12) (NZSP) is a promising material as a solid electrolyte for sodium-ion batteries. The highest conductivity of approx. 1.0 mS/cm at room temperature (RT) was reported for the compound with a Na content of approximately 3.3 per formula unit (f. u.) and when the material is synthesized with a final sintering temperature greater than or equal to 1220 degrees C. Herein, we propose a new synthesis method to enhance the conductivity of the NZSP by liquid-phase sintering with the optimum amount of additive of amorphous-Na sub(2)Si sub(2)O sub(5). In this regard, a series of composite materials were prepared by mixing Na sub(3)Zr sub(2)Si sub(2)PO sub(12) with amorphous-Na sub(2)Si sub(2)O sub(5) (NZSP/NS-x wt.percent; with x = 0.0, 2.5, 5.0, 7.5, 10.0) and sintering at a lower temperature of 1150 degrees C. Enhanced conductivity of 1.7 mS/cm at RT has been achieved for the Na sub(3)Zr sub(2)Si sub(2)PO sub(12)/Na sub(2)Si sub(2)O sub(5)-5.0 wt.percent (NZSP/NS-5.0) composite. The effects of additives on the NZSP phase formation, microstructure, and ion conductivity have been investigated by XRD, MAS NMR, SEM, and impedance spectroscopy. Our study demonstrates that the higher conductivity of the NZSP/NS-5.0 composite is caused by the combined effect of increased Na content in the NZSP phase (by diffusion of Na sup(+) ions from the liquid phase of NS to bare NZSP phase), higher density, and microstructures with lesser pores.