Aliovalent dual element co-assisted strategy to enhance ionic conductivity and stability of NASICON-type solid electrolyte for all-solid-state sodium batteries

The urgent need for high-performance sodium batteries has drawn great attention to the development of novel solid-state electrolyte systems. However, it is still a great challenge to obtain excellent ionic conductivity and interfacial stability performance for most solid-state electrolytes. In this study, a new NASICON-type solid-state electrolyte Na3.1+xZr2-xScxSi2.1P0.9O12 (donated as NZSSP, x = 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3) was successfully prepared by solid-phase method. Due to the aliovalent substitution of Sc3+/Si4+, the phase transition or unwanted rhombohedral phase is effectively suppressed, the Na+ transport channel bottlenecks are greatly enlarged, and the concentration of Na+ is largely increased, giving rise to the significant improvement of the ionic conductivity and stability of NZSSP solid-state electrolyte. The Na3.35Zr1.75Sc0.25Si2.1P0.9O12 solid-state electrolyte has an ultra-high ionic conductivity of 4.92 mS cm- 1 at room temperature. Meanwhile, it shows an excellent sodium plating/stripping stability by the optimization of the Na+ transport channel. The critical current density of the Na//Na3.35Zr1.75Sc0.25Si2.1P0.9O12//Na symmetric battery is achieved up to 1.0 mA cm-2, and it can stably cycle for nearly 3000 h at 0.1 mA cm-2 or 1000 h at 0.5 mA cm-2 at room temperature. A detailed analysis of these aliovalent dual element substitution samples has been performed, and the mechanism of the conductivity enhancement is discussed systematically. This study can provide a feasible method to improve the ionic conductivity and electrochemical stability of NZSP materials and paves a way for the application of allsolid-state sodium batteries.