A Strategic Co-doping Approach Using Sc3+ and Ce4+ toward Enhanced Conductivity in NASICON-Type Na3Zr2Si2PO12

NASICON-framework ceramic electrolytes are crucial for realizing the promise of all-solid-state sodium-ion batteries. The present work investigates the strategic co-doping approach used to enhance the sodium-ion conductivity in NASICON-type Na3Zr2Si2PO12. Sc3+ and Ce4+ are added as co-dopants for Zr4+ such that Sc3+ content is fixed at 16.5 mol % while the Ce4+ amount is varied from 0 to 5 mol %. All the samples are fabricated using the conventional solid-state reaction with the pressureless sintering performed at 1250 °C for 5 h. Although bare Na3Zr2Si2PO12 is synthesized using the monoclinic-ZrO2 precursor, the cubic-ZrO2 precursors are utilized to prepare the other remaining compositions. The microstructure of all the samples contains cuboidal-shaped grains, with the grain size varying from 1.2 to 0.9 μm. The bare Na3Zr2Si2PO12 possesses monoclinic-ZrO2 as an impurity phase that is found absent in other samples, emphasizing the importance of using cubic-ZrO2 precursor to eliminate the formation of a poor conducting phase. Owing to the low solubility of Ce4+ in the NASICON phase, several secondary phases are formed by adding more than 1 mol % Ce4+ in Na3.33Sc0.33Zr1.67Si2PO12. Substituting 16.5 mol % Sc3+ for Zr4+ in Na3Zr2Si2PO12 improves the ionic conductivity from 0.61 to 0.96 mS·cm–1 at room temperature, which is attributed to the presence of excess Na-ions to maintain the charge neutrality in the doped composition. However, replacing 1 mol % Ce4+ for Zr4+ remarkably raises the conductivity of Na3.33Sc0.33Zr1.67Si2PO12 from 0.96 to 2.44 mS·cm–1 at 25 °C. The optimized composition of Na3.33Ce0.02Sc0.33Zr1.65Si2PO12 exhibits more than four times higher sodium-ion conductivity than the bare Na3Zr2Si2PO12 at 25 °C. The detailed electrochemical and structural characterizations of these materials and the possible reasons for the observed superionic conduction in Na3.33Ce0.02Sc0.33Zr1.65Si2PO12 are discussed.