Sintering temperature dependency on sodium‐ion conductivity for Na₂Zn₂TeO₆ solid electrolyte

Abstract

We investigated the sintering temperature dependency on the properties of Na2Zn2TeO6 (NZTO) solid electrolyte synthesized via a conventional solid‐state reaction method. Sintering temperature of calcined NZTO powder, which was obtained by the calcination of precursor at 850℃, was changed in the range from 650 to 850℃. X‐ray diffraction analysis showed that P2‐type layered NZTO phase was formed in all sintered samples without forming any secondary phases. The relative densities of sintered NZTO samples were approximately 83%−85% for the samples sintered at 700℃ or higher. The all sintered samples showed sodium‐ion conductivity above 10−4 S cm−1 at room temperature and the highest conductivity of 4.0 × 10−4 S cm−1 in the sample sintered at 750℃. The sintering temperature to obtain the highest room temperature conductivity is 100℃ lower than that used in previous works. Such low sintering temperature compared to other Na‐based oxide solid electrolytes could be useful for co‐sintering with electrode active materials for fabrication of all‐solid‐state sodium‐ion battery.

Characterization of Na2Zn2TeO6 ceramic solid electrolyte densified by hot pressing

“…The sample pressure‐molded at 60 MPa has fewer voids and a higher density than the one pressure‐molded at 20 MPa. In fact, the relative density of the former reached 91%, while that of the latter was only 83%, nearly the same for NZTO prepared by conventional FS 11,12 …”

“…Calcined NZTO powder was reground and hot‐pressed at 700°C for 1 h with a graphite mold in a vacuum chamber of the furnace designed for HP and spark plasma sintering (CSP‐I‐02121, SS Alloy Co. Ltd.). The densification temperature is 100−150°C lower than the temperature for conventional FS in our previous works 11,12 . During the densification process, the uniaxial pressure of 20 or 60 MPa was applied to a sample.…”

“…Na‐ion conducting oxide solid electrolyte Na 2 Zn 2 TeO 6 (NZTO) with a P2‐type layered structure is an attractive candidate for the application to all‐solid‐state SiBs, because of the high ionic conductivity well above 0.1 mS cm −1 at room temperature and excellent chemical and electrochemical stabilities against Na metal 6–12 . NZTO has a honeycomb‐structured compound with a space group of P 6 3 22 and in the layers, each TeO 6 octahedron is surrounded by six ZnO 6 octahedra via edge‐sharing.…”

“…As reported in the literature, sintering temperature for densification of NZTO is usually set to be 800−900°C 6–12 . Although this temperature may be still high for the formation of good solid–solid interface with most electrode active materials by a co‐sintering process, there is a possibility to suppress the formation of undesired secondary phases by co‐sintering with cathode active materials with similar crystal structure and constituent elements 30–35 .…”

“…Although this temperature may be still high for the formation of good solid–solid interface with most electrode active materials by a co‐sintering process, there is a possibility to suppress the formation of undesired secondary phases by co‐sintering with cathode active materials with similar crystal structure and constituent elements 30–35 . However, reported relative densities of NZTO by a conventional sintering are limited to approximately 85%, mainly due to the nonuniformity in NZTO grain size and morphology 6–12 . Recently, an aerosol deposition process without any thermal treatment was applied for fabricating NZTO thick film, but the density and conductivity were not as high as sintered samples 36 …”