Insight into synergetic effect of bulk doping and boundary engineering on conductivity of NASICON electrolytes for solid-state Na batteries

Na superionic conductor-type Na1+xZr2SixP3-xO12 (0 ≤ x ≤ 3, NZSPO) is considered as one of the most promising solid electrolytes for solid-state sodium batteries, while its relatively low ionic conductivity of 10−4 S cm−1 requires improvement for application. In this study, a synergetic strategy is applied to improve the ionic conductivity of NZSPO, i.e., by combining bulk doping and grain-boundary engineering. Hf4+ is selected as the substitution for Zr4+ to stabilize the highly conducting rhombohedral phase. La2O3 is added as a sintering aid to promote the ion transport along the grain boundaries with Na3La(PO4)2 formed therein. The optimized Na3Zr1.8Hf0.2Si2PO12-Na3La(PO4)2 (NZHSPO-NLPO) ceramic electrolytes show a high relative density of 98.8% and the superior ionic conductivity of 1.66 × 10−3 S cm−1 at 30 °C. The Na symmetric batteries display the stable plating/stripping cycling over 500 h at 0.1 mA cm−2 and 0.05 mAh cm−2. With ionic liquid as a wetting agent at cathode sides and PEO as an intermediate layer at anode sides, the NZHSPO-NLPO-based Na batteries with Na3V2(PO4)3 cathodes exhibit the discharge capacity of 109.9 mAh g−1 and capacity retention of 92.7% at 0.1 and 30 °C for 50 cycles. It is proved that the simultaneous optimization of bulk and grain boundaries is powerful for increasing the ionic conductivity of ceramic solid electrolytes.