Zn/Mg Dual Dopant Strategy to Enhance Sodium Ion Conductivity in Na3Zr2Si2PO12
Authors: Jaschin Prem Wicram , Wachsman Eric 
Published: November 2022

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 Na3+2(x+y)Zr2-xZnxMgySi2PO12, 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.

Figure 1