Sol–gel synthesis of Mg2+ stabilized Na-β″/β-Al2O3 solid electrolyte for sodium anode battery

• Mg2+ stabilized Na-β″/β-Al2O3 powder was synthesized by sol–gel method.
• PVP was used as the complexing agent of the sol–gel process.
• An intermediate phase promoted the formation of Na-β″/β-Al2O3 phase.
• The Na-β″/β-Al2O3 phase formed at the temperature as low as 1000 °C.
• The ionic conductivity of the ceramic prepared sample reached 0.24 S cm-1 at 350 °C.

Mg2+ stabilized Na-β″/β-Al2O3 powder was successfully synthesized by a sol–gel method using PVP-10000 as complexing agent and metal nitrates as corresponding metal sources. The thermal decomposition of the xerogel and the evolution of crystallization behavior of Na-β″/β-Al2O3 were studied. The obtained powders have been unambiguously characterized using X-ray diffraction (XRD), N2 adsorption, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The Mg2+ stabilized Na-β″/β-Al2O3 crystallized gradually from amorphous phase to metastable intermediate phases and further formed the major phase at 1000 °C. The results showed that the large surface area of the amorphous intermediate phase with mesoporous structure followed by the generation of intermediate phases named as m-Al2O3 and MgAl2O4 with spinel structure to a great extent promoted the formation of Na-β″/β-Al2O3 phase. The Na-β″/β-Al2O3 nano-powders exhibited a plate-like two dimensional structure with clear crystallographic plane for Na+ diffusion. The transformation from m-Al2O3 only required the shift of Na+ from alternate layers of oxygen and the accommodation of every fifth layer since the structure of m-Al2O3 is only slightly different from that of Na-β″/β-Al2O3. The relative density of the sintered ceramic was approximately 98.8% and the ionic conductivity can reach 0.24 S cm−1 at 350 °C.