Highly Sodium-Ion Conductive Sulfide Electrolyte Film Prepared By Pulsed Laser Deposition

Abstract

All-solid-state sodium secondary batteries exhibit significant potential for use in next-generation batteries that offer advantages associated with safety and cost. The key material in this battery technology is a solid electrolyte that conducts sodium ions. The most typical solid electrolyte for all-solid-state sodium batteries is Na3PS4 [1]. All-solid-state batteries using Na3PS4 and several electrode active materials operate at room temperature. At their present stage of development, the operation of all-solid-state sodium batteries under high current densities is challenging. It was recently reported that Na3SbS4 solid electrolytes showed a high ionic conductivity that exceeded 10-3 S cm-1 [2-4], which was higher than that of Na3PS4 glass-ceramic [1]. The evaluation of bulk ionic conductivity is important for determining the potential of the electrolytes. However, most previous studies have reported on the ionic conductivities of compressed powder pellets.Numerous studies have employed pulsed layer deposition (PLD) for fabricating lithium-ion-conducting sulfide electrolyte films. However, PLD has been seldom used for developing sodium-ion-conducting solid electrolyte films thus far. Herein, a thin film of highly dense Na3SbS4 solid electrolyte was prepared by PLD to evaluate its potential sodium-ion conductivity. The as-deposited film was observed to be amorphous, whereas a cubic crystalline phase of Na3SbS4 was formed in the thin film after heat treatment at 190°C. Crystallization of the thin film after heat treatment increased ionic conductivity and decreased activation energy. The heat-treated film showed a conductivity of 1.7×10-2 S cm-1 at 25°C and an activation energy of 15 kJ mol-1 for conduction. Further, it was discovered that the Na3SbS4 solid electrolyte has a high conductivity of more than 10-2 S cm-1 at room temperature. The evaluation of the chemical composition of the film suggested that there are some vacancies in sodium site. Acknowledgements: This work was supported by the MEXT program "Elements Strategy Initiative to Form Core Research Center (ESICB)" and JSPS KAKENHI Grant Nos. 18H05255 and 19H05816.