Effects of microwave-assisted reflux in wet-chemical synthesis of high sodium-ion conductive Na5DySi4O12

Sodium rare-earth silicate with Na5DySi4O12 components is known as a highly conductive and stable sodium ion solid electrolyte though are difficulties in obtaining a single-phase, highly conductive Na5DySi4O12 pellet because it has low conductivity impurity phases. In this study, microwave heating for aqueous solution of starting materials was performed for the formation of homogeneous precursor of Na5DySi4O12. A single-phase Na5DySi4O12 was successfully obtained from the aqueous solution heated with 2.45GHz microwave at 80 °C after calcination at 700 °C and sintering at 1050 °C. The Na5DySi4O12 showed high conductivity of 2.9 × 10−4 at 30 °C and an activation energy of 0.25 eV. The precursor particles with microwave heating were fine particles with a narrower particle size distribution than those without microwaving. The results of thermal analysis showed different thermal behaviors with and without microwaves. The crystalline phase after calcination at 700 °C was different with and without microwave. After sintering at 1050 °C with microwaves was a single-phase Na5DySi4O12, but the pellets without the microwaves heating process did not have a crystalline phase of Na5DySi4O12. These results demonstrated that microwave heating of aqueous solution affects the formation of single-phase Na5DySi4O12. [Display omitted] •Almost single-phase Na5DySi4O12 could be synthesized by the wet method with microwave-assisted reflux for precursor solution.•Microwave-assisted reflux made particles smaller and uniform in precursor solution.•Synthesis route via Na3DySi3O9 as an intermediate to Na5DySi4O12 is proposed.