Correlation between mechanical properties and ionic conductivity of sodium superionic conductors: a relative density-dependent 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 interrelation remains inadequately understood. Addressing this gap is vital for concurrent enhancements in both properties. This study summarizes recent literature on NASICON solid electrolytes Na1+xZr2SixP3-xO12 (NZSP, 0≤x≤3), highlighting the mechanical properties and ionic conductivity, and identifies a positive correlation between mechanical strength, in particular hardness, and ionic conductivity at ambient temperatures. Microstructural analysis reveals that a range of factors, including relative density, grain size, secondary phases, and crystallographic structures, significantly influence material properties. Notably, an increase in relative density uniquely contributes to simultaneous enhancements in both ionic conductivity and mechanical strength. Consequently, future research should prioritize enhancing the relative density of NASICON solid electrolytes, possibly employing advanced techniques, including sol-gel process, spark plasma sintering (SPS), and microwave-assisted sintering. The correlation between mechanical properties and ionic conductivity observed in NASICON solid electrolytes extends to other high-temperature sintered oxide electrolytes like Li7La3Zr2O12 (LLZO). This investigation not only suggests a potential linkage between these crucial properties but also guides subsequent strategies for refining solid electrolytes for advanced battery technologies.