To improve the ionic conductivity of solid electrolytes, it is generally thought that anions with high polarizability should be used. However, the relationship between polarization and conductivity is not clear because conductivity largely depends on the crystal structure. In this study, we focus on amorphous materials with no long-range ordered structure. The conductivity and ductility properties of lithium boron oxide (Li3BO3), lithium boron nitride (Li3BN2), and lithium boron sulfide (Li3BS3) are compared. Li3BS3 glass is prepared from Li2S, B, and S using heat treatment and a mechanochemical process. It has high ductility and a higher ionic conductivity (3.6 × 10–4 S cm–1 at 25 °C) than that of Li3BO3 and Li3BN2 glass with a low activation energy of 32 kJ mol–1. Li3BS3 glass is therefore suitable as an ionic conductor with high conductivity. The electronegativity of anions and glass properties such as ionic conductivity and ductility are correlated, and it is proposed that this relationship can be used as a basis for investigating fast ionic conductors.

Development of oxide solid electrolytes for all-solid-state batteries is attracting increasing attention. In this study, amorphous Li2O-LiI materials are prepared via a mechanochemical process to achieve high lithium ionic conductivity and good compatibility to lithium metal. Amorphous 66.7Li2O•33.3LiI (mol%) electrolyte shows a high ionic conductivity of 3.1 × 10 -5 S cm -1 at 25 °C with a relative density of 96%. An all-solid-state Li symmetric cell (Li/66.7Li2O•33.3LiI/Li) operates without an increase in overvoltage. A simple combination of lithium oxide and lithium iodide exhibits high ionic conductivity, ductility, and stability to lithium metal.

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.

Li metal is a promising negative electrode material for the development of all-solid-state batteries with high energy densities. However, the short-circuiting of batteries owing to Li dendrite formation is challenging. To solve this issue, it is crucial to form a suitable interface between the Li metal and solid electrolyte. This study demonstrates that the incorporation of a Sn interlayer at the interface between the Li metal and Li 3 PS 4 (LPS) electrolyte improved the Li stripping/plating performance of all-solid-state Li symmetrical cells. The cycling performance was further enhanced by replacing pure Li metal with a Li−Mg alloy. Galvanostatic cycling tests were conducted on a symmetrical cell of Li−Mg/Sn/LPS/Sn/Li−Mg at a current density of 1.0 mA cm −2 and a temperature of 100 °C. The operation of the cell was stable for 5000 h without short-circuiting. X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed that Sn film incorporation into the Li or Li−Mg/LPS interface suppressed the reductive LPS decomposition and maintained a stable interface. These findings will facilitate the development of interfacial modifications between Li metal and sulfide solid electrolytes to enhance the cycling performance of all-solid-state Li metal batteries.

Sulfide-based all-solid-state Li/S batteries have attracted considerable attention as next-generation batteries with high energy density. However, their practical applications are limited by short-circuiting due to Li dendrite growth. One of the possible reasons for this phenomenon is the contact failure caused by void formation at the Li/solid electrolyte interface during Li stripping. Herein, we studied the operating conditions, such as stack pressure, operating temperature, and electrode composition, that could potentially suppress the formation of voids. Furthermore, we investigated the effects of these operating conditions on the Li stripping/plating performance of all-solid-state Li symmetric cells containing glass sulfide electrolytes with a reduction tolerance. As a result, symmetric cells with Li–Mg alloy electrodes instead of Li metal electrodes exhibited high cycling stability at current densities above 2.0 mA cm–2, a temperature of 60 °C, and stack pressures of 3–10 MPa. In addition, an all-solid-state Li/S cell with a Li–Mg alloy negative electrode operated stably for 50 cycles at a current density of 2.0 mA cm–2, stack pressure of 5 MPa, and temperature of 60 °C, while its measured capacity was close to a theoretical value. The obtained results provide guidelines for the construction of all-solid-state Li/S batteries that can reversibly operate at high current densities.