Sodium-based all-solid-state batteries (ASSBs) are economically feasible alternatives to lithium-based batteries that show high potential to provide energy storage systems that are safe and possess high energy density in an ever-increasing energy demand around the globe. Among the many sodium-ion conducting solid electrolytes (that are integral to developing an ASSB), monoclinic-structured Na₃Zr₂Si₂PO₁₂ (part of the NASICON-family of compounds) are promising materials that exhibit high ionic conductivity in the range of 0.67 mS/cm. However, to be as effective as liquid electrolytes, the innate low grain boundary conductivity in Na₃Zr₂Si₂PO₁₂ needs to be resolved. In this work, by adopting a strategic co-doping of the Zr-site by divalent Zn²⁺ and Mg²⁺-ions, the bulk and grain boundary conductivities were improved concomitantly in Na₃Zr₂Si₂PO₁₂. With a general stoichiometry of Na_3+2(x+y)Zr_2-xZn_xMg_ySi₂PO₁₂, an enhanced total ionic conductivity (bulk + grain boundary) of 2.8 mS/cm at room temperature was achieved with an optimum composition of x = 0.2 and y = 0.125. A considerable increase in the grain boundary conductivity from 0.3 mS/cm for undoped Na₃Zr₂Si₂PO₁₂ to 4.4 mS/cm for Zn/Mg-co-doped Na₃Zr₂Si₂PO₁₂ contributed to such improvement in its total ionic conductivity. In addition, increase in bulk conductivity was down to the structural transformation from monoclinic (C 2/c) of Na₃Zr₂Si₂PO₁₂ to rhombohedral (R-3c) crystal structure as observed from the significant peak shifts in the X-ray diffraction patterns of the Zn/Mg-co-doped Na₃Zr₂Si₂PO₁₂ samples. To understand the effect of co-doping Zn²⁺- and Mg²⁺-ions were investigated by comparing its structural and impedance characteristics to that of the undoped-, Zn²⁺- and Mg²⁺-doped Na₃Zr₂Si₂PO₁₂ (single dopant), as shown in Fig. 1 (a) and (b).

The inclusion of dual dopants in the Na₃Zr₂Si₂PO₁₂ structure had a significant effect on the migration barrier (around 0.22 eV) for ionic conduction relative to 0.33 eV obtained from undoped Na₃Zr₂Si₂PO₁₂. Additionally, the migration barriers for bulk and the grain boundary conduction were resolved by studying the impedance properties at lower temperatures (21 ^oC to –40 ^oC). Although, there was considerable increase in the ionic conductivity after doping with divalent ions, the electronic conductivity of the system remained unaffected (in the range of 10^–8 S/cm) indicating that the system can withstand relatively higher critical current densities. To understand the Na stripping and plating characteristics, symmetric cells were assembled with the dual-doped Na₃Zr₂Si₂PO₁₂ as a solid electrolyte sandwiched between sodium metal. Exhibiting an area specific resistance at the sodium-electrolyte interface as low as 34 Ω cm², stable Na cycling was observed for 25 cycles at different current densities ranging from 0.05 mA/cm² to 0.3 mA/cm² recorded at room temperature (as shown in Fig. 1 (c)­). Dendrite growth, resulting in a cell-shortage, was observed at current density of 0.35 mA/cm² for the Zn/Mg-doped Na₃Zr₂Si₂PO₁₂. The results presented in this work shows the potential of dual doping Na₃Zr₂Si₂PO₁₂ crystal structure to attain high sodium ion conducting solid electrolyte for ASSBs.

Figure 1 (a) X-ray diffraction profiles and (b) ionic conductivities for undoped-, Zn²⁺-, Mg²⁺- and dual Zn²⁺/Mg²⁺-doped Na₃Zr₂Si₂PO₁₂ and (c) Na cycling characteristics of dual Zn²⁺/Mg²⁺-doped Na₃Zr₂Si₂PO₁₂ as the solid-state electrolyte.