Correlation between mechanical properties and ionic conductivity of sodium superionic conductors: a relative density-dominant relationship

Sodium superionic conductors (NASICON) are pivotal for the functionality and safety of solid-state sodium batteries. Their mechanical properties and ionic conductivity are key performance metrics, yet their correlation remains inadequately understood. Addressing this gap is vital for concurrent enhancements in both properties. This study summarizes recent literature on the sintered polycrystalline NASICON solid electrolyte Na1+xZr2SixP3-xO12 (NZSP, 0≤x≤3), focusing on its mechanical properties and ionic conductivity, and identifies a positive correlation between these properties at ambient temperatures. Microstructural analysis reveals that a range of factors, including relative density, grain size, secondary phases, and crystal structures, significantly influence the properties of NZSP. Notably, an increase in relative density uniquely contributes to simultaneous enhancements in both hardness and ionic conductivity. Consequently, future research should prioritize enhancing the relative density of NZSP, potentially by employing advanced sintering techniques such as spark plasma sintering (SPS) and microwave-assisted sintering. The correlation between mechanical properties and ionic conductivity observed in NZSP is also evident in other oxide solid electrolytes, such as garnet Li7La3Zr2O12 (LLZO). This investigation not only suggests a potential linkage between these crucial properties but also guides subsequent strategies for refining polycrystalline oxide solid electrolytes for advanced battery technologies.