Zirconium-free sodium ion conducting NASICON-type structure NaMAl(PO₄)₃ (M = Nb, Ta)

Sodium superionic conductors (NASICONs) have shown promising sodium-ion conduction for application in all-solid-state sodium metal batteries. However, many factors hinder their large-scale application, such as high total resistance caused by secondary phases and the formation of zirconia impurities due to sodium oxide volatilisation at elevated temperatures. This work circumvents one of the major impurities by completely removing zirconium from the structure, thereby establishing the concept of zirconium-free NASICON-type materials. Oxides with nominal chemical formulae NaMAl(PO₄)₃ (M = Nb, Ta) were synthesised by solid-state reaction and found to crystallise in a rhombohedral NASICON structure. However, increasing the Na content using Al doping at the Nb and/or Ta sites was unsuccessful, resulting in glass-ceramic formation. Hence, the nominal composition formula Na_1+2xM_1-xAl_1+x(PO₄)₃ is primarily used to describe the targeted composition rather than to represent actual NASICON-type materials. Among the samples investigated, the multi-phase compound Na_1.5Ta_0.75Al_1.25(PO₄)₃ showed an ionic conductivity of 1 × 10^-4 S cm^-1 at 200 °C. X-ray photoelectron spectroscopy analysis showed that the Ta-based NaTaAl(PO₄)₃ was found to be chemically stable against reaction with molten Na, while the Nb analogue exhibited some degree of Nb^V reduction. This work presents a novel family of Zr-free NASICON structures designed to advance the development of all-solid-state sodium-metal batteries.