Development of Von-Alpen-Type NASICON Electrolytes with High Ionic Conductivities and Relative Densities

The NASICON compound, represented by the chemical formula Na_1+xZr₂Si_xP_3-xO₁₂ (0 ≤ x ≤ 3), exhibits a Na⁺ ion conductivity of approximately 10^-4 S cm^-1 and a wide electrochemical window. It is chemically stable not only in air but also in seawater. However, the ion conductivity of NASICON compounds is relatively lower compared to typical liquid electrolytes (10^-2 S cm^-1), leading to decreased performance. In this study, a method is proposed to enhance the ion conductivity and density of vA-NASICON (von-Alpen-type NASICON) ceramic electrolytes by doping them with heterogeneous element Mg²⁺ ions. Additionally, a synthesis method utilizing a sintering aid for vA-NASICON is suggested to achieve higher ion conductivity and density at lower temperatures and shorter times.<br/><br/>Considering factors such as raw material cost, ion radius, and oxidation state, Mg²⁺ ions were chosen as heterogeneous element dopants compared to the Zr⁴⁺ ions present in NASICON compounds. The effects of Mg dopant incorporation on the modified crystal structure, surface changes, and formation of secondary phases were analyzed, and their influence on the ion conductivity of vA-NASICON was studied. The dopant precursor, MgO, can enhance the surface diffusion coefficient, increase the densification rate, and promote crystal growth. However, excessive introduction of Mg²⁺ leads to the formation of undesirable secondary phases, such as Na_xMg_yPO₄, which results in a decrease in the ion conductivity of Mg-doped vA-NASICON. By analyzing the ion conductivity, the optimal Mg doping level was determined. The vA-NASICON synthesized with the optimal Mg doping exhibited a higher ion conductivity of 3.64x10^-3 S cm^-1 compared to undoped vA-NASICON (2.03x10^-3 S cm^-1).<br/><br/>The introduction of Glass-frit in vA-NASICON can increase the sinterability of the ceramic matrix and lower the densification sintering temperature. vA-NASICON was prepared by solid-state reaction, and systematic investigations were conducted on crystal structure, microstructure, and electrochemical properties. Glass-frit added vA-NASICON sintered at 1200 dgree for 1 hour showed a high ionic conductivity of 2.1x10^-3 S cm^-1 and a relative density of 97.2%, surpassing vA-NASICON (2.54x10^-3 S cm-1, relative density 65.7%) without addition. Glass-frit is that by forming an amorphous phase at the grain boundary, it can promote grain boundary contact and promote densification of ceramic electrolyte. The results of this study are expected to be effectively utilized in the development of oxide-based solid electrolytes with high ionic conductivity and density.