Synthesis and Na+ Ion Conductivity of Stoichiometric Na3Zr2Si2PO12 by Liquid-Phase Sintering with NaPO3 Glass

Abstract

Sodium super ionic conductor (NASICON)-type Na3Zr2Si2PO12 (NZSP) with the advantages of the high ionic conductivity, stability and safety is one of the most famous solid-state electrolytes. NZSP, however, requires the high sintering temperature about 1200 °C and long sintering time in the conventional solid-state reaction (SSR) method. In this study, the liquid-phase sintering (LPS) method was applied to synthesize NZSP with the use of NaPO3 glass with a low glass transition temperature of 292 °C. The formation of NZSP was confirmed by X-ray diffraction analyses in the samples obtained by the LPS method for the mixture of Na2ZrSi2O7, ZrO2, and NaPO3 glass. The sample sintered at 1000 °C for 10 h exhibited a higher Na+ ion conductivity of 1.81 mS/cm at 100 °C and a lower activation energy of 0.18 eV compared with the samples prepared by the SSR method. It is proposed that a new LPE method is effective for the synthesis of NZSP and the NaPO3 glass has a great contribution to the Na+ diffusion at the grain boundaries.

NASICONs‐type solid‐state electrolytes: The history, physicochemical properties, and challenges

“…[75] For a typical six basal planes in the rhombohedral structure, when there were one phosphorous and two silicon, the Na + conductivity could be promoted due to low energy barriers, which resulted from stronger coulombic repulsion of Na + /P 5+ than Na + /Si 4+ . In addition, the sodium was easy to volatile after calcined at high temperature, so excess Na 2 CO 3 , Na 2 O, [76] or other sodium compounds [77][78][79] were added to Na 3 Zr 2 Si 2 PO 12 to maintain the stoichiometric ratio. The incorporation of excess Na could increase the bottleneck size of the conducted channels and enhance the grain conductivity, and small extra quantities of sodium were beneficial for decreasing NASICON crystallization temperature and reduction of the appearance of ZrO 2 crystalline secondary phases.…”

“…[119] In addition, NaPO 3 glass exhibited low glass transition temperature, therefore NaPO 3 was often used as additives to synthesize the Na 3 Zr 2 Si 2 PO 12 , and achieved low sintering temperature and a decrease of grain barrier. [79] In summary, three common methods were utilized to improve the bulk of the electrolyte: novel sintering technologies with low sintering temperature and remaining time, chemical doping, and chemical additives. These methods mainly focused on increasing of Na + concentration and modifying the grain boundary to decrease the grain boundary resistance.…”

“…[ 119 ] In addition, NaPO 3 glass exhibited low glass transition temperature, therefore NaPO 3 was often used as additives to synthesize the Na 3 Zr 2 Si 2 PO 12 , and achieved low sintering temperature and a decrease of grain barrier. [ 79 ]…”

Rare Earth Oxide-Assisted Sintered NASICON Electrolyte Composed of a Phosphate Grain Boundary Phase with Low Electronic Conductivity

“…Similarly, Ji et al . proposed using NaPO 3 glass as raw materials to synthesize NASICON so as to promote the ion conductivity to 1.81 × 10 –3 S cm –1 . Meanwhile, it is worth noting that the latest research showed that La 2 O 3 can also have a similar effect as flux by adjusting the grain boundary chemical composition to improve the relative density .…”

“…proposed using NaPO 3 glass as raw materials to synthesize NASICON so as to promote the ion conductivity to 1.81 × 10 −3 S cm −1 . 19 Meanwhile, it is worth noting that the latest research showed that La 2 O 3 can also have a similar effect as flux by adjusting the grain boundary chemical composition to improve the relative density. 20 Sun et al fabricated La ions containing Na 3 Zr 2 Si 2 PO 12 electrolytes and proved that the introduced La 3+ ions exist in the form of phosphate impurities Na 3 La(PO 4 ) 2 , which can promote ceramic densification and increase the Si/P ratio in the NASICON phase.…”