High Na+ conducting Na3Zr2Si2PO12/Na2Si2O5 composites as solid electrolytes for Na+ batteries

Sodium superionic conductor Na₃Zr₂Si₂PO₁₂ (NZSP) is a promising material as a solid electrolyte for sodium-ion batteries. The highest conductivity of ～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 ≥1220°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-Na₂Si₂O₅. In this regard, a series of composite materials were prepared by mixing Na₃Zr₂Si₂PO₁₂ with amorphous-Na₂Si₂O₅ (NZSP/NS-x wt.%; with x = 0.0, 2.5, 5.0, 7.5, 10.0) and sintering at a lower temperature of 1150°C. Enhanced conductivity of 1.7 mS/cm at RT has been achieved for the Na₃Zr₂Si₂PO₁₂/Na₂Si₂O₅-5.0 wt.% (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⁺ ions from the liquid phase of NS to bare NZSP phase), higher density, and microstructures with lesser pores.