Innovative rechargeable batteries that can effectively store renewable energy, such as solar and wind power, urgently need to be developed to reduce greenhouse gas emissions. All-solid-state batteries with inorganic solid electrolytes and electrodes are promising power sources for a wide range of applications because of their safety, long-cycle lives and versatile geometries. Rechargeable sodium batteries are more suitable than lithium-ion batteries, because they use abundant and ubiquitous sodium sources. solid electrolytes are critical for realizing allsolid-state sodium batteries. Here we show that stabilization of a high-temperature phase by crystallization from the glassy state dramatically enhances the na + ion conductivity. An ambient temperature conductivity of over 10 − 4 s cm − 1 was obtained in a glass-ceramic electrolyte, in which a cubic na 3 Ps 4 crystal with superionic conductivity was first realized. All-solid-state sodium batteries, with a powder-compressed na 3 Ps 4 electrolyte, functioned as a rechargeable battery at room temperature.

Sulfide solid electrolytes with cubic Na 3 PS 4 phase has relatively high sodium ion conductivity of over 10-4 S cm-1 at room temperature, and all-solid-state sodium batteries Na-Sn/TiS 2 with the electrolyte operated as a secondary battery at room temperature. To improve battery performance, conductivity enhancement of sulfide electrolytes is important. In this study, we have succeeded in enhancing conductivity by optimizing preparation conditions of Na 3 PS 4 glass-ceramic electrolytes. By use of crystalline Na 2 S with high purity of 99.1%, cubic Na 3 PS 4 crystals were directly precipitated by ball milling process at the composition of 75Na 2 S•25P 2 S 5 (mol%). The glass-ceramic electrolyte prepared by the milling for 1.5 h and consecutive heat treatment at 270 o C for 1 h showed the highest conductivity of 4.6 x 10-4 S cm-1 , which is twice as high as the conductivity of the cubic Na 3 PS 4 glass-ceramic prepared in a previous report. All-solid-state Na-Sn/NaCrO 2 cells with the newly prepared electrolyte exhibited charge-discharge cycles at room temperature and kept about 60 mAh per gram of NaCrO 2 for 15 cycles.

Glass–ceramic electrolyte in the system Na3PS4–Na4SiS4 exhibits a high Na+ ion conductivity of 7.4 × 10−4 S cm−1 at 25 °C.

Rietveld analysis of cubic Na3PS4 in the 94 Na3PS4⋅6 Na4SiS4 (mol %) glass ceramic indicates that two Na+ sites (Na1 and Na2) can be found in cubic Na3PS4 and that the phosphorus atoms in cubic Na3PS4 are partially replaced by silicon in the 94 Na3PS4⋅6 Na4SiS4 glass ceramic. The electron‐density distribution of the cubic Na3PS4 structure, obtained by the maximum‐entropy method/Rietveld method, revealed that cubic Na3PS4 contains three‐dimensional Na+ conduction pathways along the Na1 and Na2 sites. From the Rietveld analysis, the cubic Na3PS4 in the 94 Na3PS4⋅6 Na4SiS4 glass ceramic has a larger site‐occupancy of Na2 than that in the Na3PS4 glass ceramic, which may be responsible for the improvement in Na+ conductivity.

The xNa 2 S ･ (100−x)P 2 S 5 (mol%; x=67, 70, 75 and 80) glasses were prepared by mechanochemical processing. Composition dependence of local structure, thermal behavior and electrical conductivity of the prepared glasses were examined. NMR and Raman spectroscopic studies revealed that the xNa 2 S･(100−x)P 2 S 5 glasses were composed of the thiophosphate units corresponding to their nominal compositions. The room temperature conductivities of the glasses increased with an increase in the Na 2 S content and attained a maximum at the x=80 composition; the highest conductivity is 1 × 10 −5 S cm −1. The xNa 2 S･(100−x)P 2 S 5 glass-ceramics were obtained by heating the glasses beyond their crystallization temperatures. The relationship between conductivity and crystalline phase was studied for the glass-ceramics. The glass-ceramics at the compositions of x=70 and 75 exhibited higher conductivities than those of corresponding glasses due to the precipitation of the superionic cubic Na 3 PS 4 crystal. In particular, the x=75 glass-ceramic showed the highest conductivity of 2 x 10-4 S cm-1 at 25 o C and the lowest activation energy of 27 kJ mol-1. In addition, the x=75 glass-ceramic was electrochemically stable against sodium deposition and dissolution. The 75Na 2 S•25P 2 S 5 glass-ceramic electrolyte with high conductivity and high electrochemical stability is suitable for all-solid-state sodium rechargeable batteries.

Na3Zr2Si2PO12 (NASICON) is a promising material as a solid electrolyte for all‐solid‐state sodium batteries. Nevertheless, one challenge for the application of NASICON in batteries is their high sintering temperature above 1200°C, which can lead to volatilization of light elements and undesirable side reactions with electrode materials at such high temperatures. In this study, liquid‐phase sintering of NASICON with a Na3BO3 (NBO) additive was performed for the first time to lower the NASICON sintering temperature. A dense NASICON‐based ceramic was successfully obtained by sintering at 900°C with 4.8 wt% NBO. This liquid‐phase sintered NASICON ceramic exhibited high total conductivity of ~1 × 10−3 S cm−1 at room temperature and low conduction activation energy of 28 kJ mol−1. Since the room‐temperature conductivity is identical to that of conventional high‐temperature‐sintered NASICON, NBO was demonstrated as a good liquid‐phase sintering additive for NASICON solid electrolyte. In the NASICON with 4.8 wt% NBO ceramic, most of the NASICON grains directly bonded with each other and some submicron sodium borates segregated in particulate form without full penetration to NASICON grain boundaries. This characteristic composite microstructure contributed to the high conductivity of the liquid‐phase sintered NASICON.

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