Effect of Excess Na Contents in Precursor on the Property of Na2Zn2TeO6 Ceramic Solid Electrolyte

Abstract

P2-type layered sodium (Na) ion conducting oxide Na2M2TeO6 (M = Mg, Zn, etc.) have attracted much attention for the application to all-solid-state Na-based batteries with high safety and lower cost, because of the high ionic conductivity above 10-4 S/cm at room temperature range and excellent chemical and electrochemical stability [1, 2]. Generally, Na in precursor materials is easy to volatilize in high temperature processing, so that the excess Na in precursors must be added to compensate the Na loss and suppress the composition shift in final products and undesired secondary phase formation. In this work, we investigated the effect of excess Na contents in precursor on the property of Na2Zn2TeO6 (NZTO) solid electrolyte synthesized via a conventional solid state reaction method.Na2CO3, ZnO and Te2O3 were used as precursor materials and the excess Na contents were adjusted to 3, 7 and 10% over the stoichiometric composition. They were pulverized and mixed for 3 hours by planetary ball milling and then calcined at 850ºC for 6 hours in air using a SiC crucible. Calcined powders were ground again and then pelletized by cold isostatic pressing at 300 MPa. Finally, the pellets with different excess Na contents were sintered at 850ºC for 12 hours in air using a SiC crucible.From XRD analysis, P2-type layered NZTO phase were mainly confirmed in all sintered samples, but only the sample with 3% excess Na in precursor contained small amount of secondary phases, which is attributed to the Na volatilization during high temperature processing. From SEM observation, it was confirmed that all sintered samples have porous structures. The relative densities were in the range from 80 to 83% and nearly independent of the excess Na contents.Au film was formed on both the end surfaces of each sintered NZTO sample as an ion-blocking electrode and electrochemical impedance was measured at 27-202ºC to investigate the ionic conducting property. As results, the ionic conductivity of NZTO samples strongly depends on the excess Na contents in precursor. The sample with 3% excess Na showed the lowest total (grain + grain boudary) conductivity of 0.5 x 10-4 S/cm at room temperature, while the conductivity for the samples with 7 and 10% excess Na was well above 10-4 S/cm. In our experiment, 7% excess Na in precursor is the best and the highest total conductivity of 3.8 x 10-4 S/cm at room temperature and low activation energy of 0.27 eV were obtained. Since the relative density of our NZTO samples are not so high and the contribution of grain-boudary resistance is large at present, the total conductivity could be improved further by densification via optimizing sintering processing.This work was partly supported by Iketani Science and Technology Foundation.[1] Y. Li, et al., Chem. Eur. J. 24, 1057-1061 (2018).[2] Y. Li, et al., ACS Appl. Mater. Interfaces 10, 15760−15766 (2018). Figure 1