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Interphase formation at the interface between a solid electrolyte and negative electrode is one of the main factors limiting the practical use of all-solid-state sodium batteries. Sulfide-type solid electrolytes with group 15 elements (P and Sb) exhibit high ductility and ionic conductivity, comparable to those of organic liquid electrolytes. However, the electronically conductive interphase formed at the interface between Na 3 PS 4 and sodium metal increases the cell resistance and deteriorates its electrochemical properties. Contrarily, Na 3 BS 3 , containing boron as an electrochemically inert element, forms an electronically insulating thin passivate interphase, facilitating reversible sodium plating and stripping. Sodium-metal symmetric cells with Na 3 BS 3 exhibit steady operation over 1000 cycles. Thus, reduction-stable solid electrolytes can be developed by substitution with an electrochemically inert element versus sodium.

All-solid-state batteries with sulfur-based positive electrode active materials have been attracting global attention, owing to their safety and long cycle life. Li2S and S are promising positive electrode active materials for high energy density in these batteries because of high theoretical capacities. All-solid-state batteries with these active materials generally require the addition of solid electrolytes (SEs) and conductive carbons to the positive electrode layer to form ionic and electronic conducting pathways due to their insulating nature. In this study, we developed electrode–electrolyte bifunctional materials in the system Li2S–V2S3–LiI with high ionic and electronic conductivity. All-solid-state batteries with Li2S–V2S3–LiI in the positive electrode layer work without SEs and conductive carbons. In particular, an all-solid-state battery with 90(0.75Li2S·0.25V2S3)·10LiI (mol %) showed a high capacity of 370 mA h g–1 at 25 °C and retained 83% of the initial discharge capacity even after 100 cycles. 90(0.75Li2S·0.25V2S3)·10LiI were composed of LiVS2 and Li2S–LiI nanoparticles embedded in the amorphous matrix. Both LiVS2 and Li2S–LiI solid solution showed electrode activity, which contribute to the high reversible capacity. Our findings offer new solutions for increasing the energy density of all-solid-state batteries.

Hard carbon is a promising negative electrode material for sodium-ion batteries that operate at low potentials. However, reversible and high-capacity charging and discharging in all-solid-state sodium batteries with hard carbon electrodes using sulfide solid electrolytes have not been reported. This study reports that reductive decomposition of the sulfide solid electrolyte occurs at both the negative composite electrode and the interface between the negative electrode layer and the solid electrolyte layer. In the first cycle, the all-solid-state cell with a composite electrode containing a Na3PS4 solid electrolyte exhibited a large irreversible capacity of 561 mAh g −1 because of the reductive decomposition of Na3PS4 to Na2S and Na3P. The use of a Na3BS3 glass electrolyte with reduction stability can lead to the successful charging and discharging of all-solid-state cell that utilizes hard carbon. This glass electrolyte can serve as a solid electrolyte for the negative composite electrode and as a buffer layer between the negative electrode layer and the solid electrolyte layer. Hence, it can also help suppress the irreversible capacity of the cell to 122 mAh g −1 .

All-solid-state batteries (ASSBs) have attracted significant attention as alternatives to Li-ion batteries. In ASSBs, solid electrolytes (SEs) play a key role. While many halide Li-ion conductors have been reported, only a few Na-ion conductors have been reported. In this study, a new phase of Na 3 InCl 6 with a cryolite-type monoclinic structure was prepared using a mechanochemical method. The new phase showed higher conductivity than the previously reported trigonal Na 3 InCl 6 and underwent a phase transition to trigonal phase when heat-treated at 90 °C. A Zr-substituted system of Na 3−x In 1−x Zr x Cl 6 was mechanochemically prepared. The obtained solid solutions with monoclinic structures based on Na 3 InCl 6 were formed in the compositions of x = 0.1-0.9. The Rietveld refinement results showed a decrease in Na occupancy at the octahedral sites and slight change at the prismatic sites. Bond valence sum mapping results showed that Na ions diffused alternately through two types of sites, suggesting that the introduction of Na vacancies at either site had a positive effect on Na-ion conduction. The ionic conductivity increased to approximately 10 −5 S cm −1 with an increase in the number of Na vacancies when x was greater than 0.6. This report describes one of the few Na-ion conducting chlorides with high conductivity.