A niobium-substituted sodium superionic conductor with conductivity higher than 5.5 mS cm⁻¹ prepared by solution-assisted solid-state reaction method

Although research on all-solid-state sodium batteries (ASSSBs) have been conducted for many years, the solid-state electrolyte (SSE) material is still far from practical application at room temperature. One major reason is that no suitable electrolyte material with high ionic conductivity has been found yet. In this study, Nb⁵⁺ is introduced into NASICON-type solid electrolyte, where Nb⁵⁺ substituted Na_3.4Zr₂Si_2.4P_0.6O₁₂ (NZSP) is prepared by a solution-assisted solid-state reaction method. The best ionic conductivity is as high as 5.51 mS cm⁻¹ which is a significant improvement. High-frequency electrochemical impedance spectroscopy shows that the increase in total conductivity is mainly due to the decrease of grain boundary impedance as well as bulk impedance. The decrease in grain boundary impedance is probably owing to the increase in density of the electrolyte material after incorporation of the Nb content, which has a beneficial impact of the sintering of NZSP. The decrease in bulk impedance rather results from the ratio of sodium ion concentration and sodium vacancy concentration of 3.3:0.7 in the crystal structure, thus facilitating the sodium ion transport. Symmetric cells with sodium metal as electrodes (Na|Nb⁵⁺ substituted NZSP|Na) are subsequently assembled and cycled stably for 60 cycles at a current density of 0.05 mA/cm².