Bulk and grain-boundary ionic conductivity in sodium zirconophosphosilicate Na₃Zr₂(SiO₄)₂PO₄ (NASICON)

Sodium zirconophosphosilicates (Na_1+xZr₂(P_1-xSi_xO₄)₃ (0<x<3)) currently experience a kind of renaissance as promising ceramic electrolytes for safe all-solid-state Na batteries. Such energy storage systems are an emerging option for next-generation technologies with attractive cost due to the use of abundant elements as sodium. To identify the right candidates their ion transport properties need to be precisely studied. In many cases less is known about the contributions of blocking grain boundaries to the overall charge carrier transport. Here, we took advantage of broadband impedance and conductivity spectroscopy carried out at sufficiently low temperature to make visible these two contributions for polycrystalline Na₃Zr₂(SiO₄)₂PO₄. It turned out that ion transport across the grain boundaries of a sintered pellet do not greatly hinder long-range ion dynamics. While bulk ion dynamics in Na₃Zr₂(SiO₄)₂PO₄ is characterized by 1.0 mS cm⁻¹, the grain boundary ionic conductivity is only slightly lower viz. 0.7 mS cm⁻¹. The latter value is of large practical interest as it allows the realization of all-solid-state Na batteries without strong interfering resistances from grain boundaries.