All-Solid-State Na/S Batteries with a Na₃PS₄ Electrolyte Operating at Room Temperature

Bulk-type all-solid-state Na/S cells, which are expected to have high capacity, be highly safe, and have low material cost, were fabricated using a Na₃PS₄ glass-ceramic as a solid electrolyte. The sulfur composite electrodes were prepared by mechanical milling of sulfur active material, a conductive additive (acetylene black), and a Na₃PS₄ glass-ceramic electrolyte. The all-solid-state Na/S cells used the reaction up to the final discharge product of sulfur active material, Na₂S, and achieved a high capacity of ∼1100 mAh (g of S)⁻¹ at room temperature. The rate of utilization of sulfur active material was ∼2 times higher than that of high-temperature-operating NAS batteries (commercially available NAS batteries, Na/sintered β″-alumina/S), where Na₂S_<i>x</i> melts with bridging sulfurs contribute to redox in the sulfur electrodes. The open circuit potential curve of the discharge process of the Na/S batteries operating at room temperature was similar to that of the NAS batteries operating at high temperatures; X-ray diffraction and X-ray photoelectron spectroscopy measurement indicated that amorphous Na₂S_<i>x</i> with a structure similar to the structure of these melts contributed to sulfur redox reaction in the all-solid-state Na/S cells. A galvanostatic intermittent titration technique and impedance measurement suggested that the overpotential during the discharge process in the all-solid-state Na/S cells was mainly derived from the sodium diffusion resistance in the solid sulfur active material. The finding would be an effective guide for achieving higher performance for all-solid-state Na/S cells

Lithium-Ion-Conducting Argyrodite-Type Li₆PS₅X (X = Cl, Br, I) Solid Electrolytes Prepared by a Liquid-Phase Technique Using Ethanol as a Solvent

Argyrodite-type crystals, Li₆PS₅X (X = Cl, Br, I), are promising solid electrolytes (SEs) for bulk-type all-solid-state lithium-ion batteries with excellent safety and high energy densities because of their high ionic conductivities and electrochemical stabilities. However, these advantageous features alone are not sufficient to achieve good cell performance. It is also critically important to have a simple and effective synthetic route to SEs and techniques for forming favorable solid–solid interfaces with large contact areas between the electrode and electrolyte particles. Here, we report an effective route for the preparation of argyrodite-type crystals using a liquid-phase technique via a homogeneous ethanol solution to improve cell performance using an SE-coating on the active material. The preparation conditions, such as appropriate halogen species and alcohol solvents, dissolution time, and drying temperature, are examined, finally resulting in Li₆PS₅Br with a lithium-ion conductivity of 1.9 × 10^–4 S cm^–1. Importantly, the obtained solution forms a favorable solid–solid electrode–electrolyte interface with a large contact area in the all-solid-state cells, resulting in a higher capacity than conventional techniques such as hand mixing using a mortar

High-Sodium-Concentration Sodium Oxythioborosilicate Glass Synthesized via Ambient Pressure Method with Sodium Polysulfides

The practical utilization of all-solid-state sodium batteries necessitates the development of a mass synthesis process for high-alkali-content sulfide glass electrolytes, which are characterized by both high ionic conductivity and remarkable formability. Typically, vacuum sealing and quenching are conventional techniques employed during the manufacturing process. In this paper, we present a novel approach, a pioneering method for the production of sulfide glass electrolytes with high alkali concentrations, achieved through ambient-pressure heat treatment and a gradual cooling process. We enhance the glass-forming ability of Na3BS3 by incorporating a small quantity of SiO2. The ionic conductivity of the resulting Na3BS3·0.225SiO2 (molar ratio) glass exhibited 1.5 × 10–5 S cm–1 at 25 °C, surpassing that of Na3BS3 glass. An all-solid-state cell utilizing Na3BS3·0.225SiO2 glass is successfully operated as a secondary battery at 60 °C. Our findings suggest that sodium oxythioborosilicate glass with electrochemical properties identical to those of Na3BS3 can be prepared without the need for quenching. These results propel the advancement of research in the domain of mass production processes tailored for high-alkali-content sulfide glass

Oxide-Based Composite Electrolytes Using Na₃Zr₂Si₂PO₁₂/Na₃PS₄ Interfacial Ion Transfer

All-solid-state sodium batteries using Na₃Zr₂Si₂PO₁₂ (NASICON) solid electrolytes are promising candidates for safe and low-cost advanced rechargeable battery systems. Although NASICON electrolytes have intrinsically high sodium-ion conductivities, their high sintering temperatures interfere with the immediate development of high-performance batteries. In this work, sintering-free NASICON-based composites with Na₃PS₄ (NPS) glass ceramics were prepared to combine the high grain-bulk conductivity of NASICON and the interfacial formation ability of NPS. Before the composite preparation, the NASICON/NPS interfacial resistance was investigated by modeling the interface between the NASICON sintered ceramic and the NPS glass thin film. The interfacial ion-transfer resistance was very small above room temperature; the area-specific resistances at 25 and 100 °C were 15.8 and 0.40 Ω cm², respectively. On the basis of this smooth ion transfer, NASICON-rich (70–90 wt %) NASICON–NPS composite powders were prepared by ball-milling fine powders of each component. The composite powders were well-densified by pressing at room temperature. Scanning electron microscopy observation showed highly dispersed sub-micrometer NASICON grains in a dense NPS matrix to form closed interfaces between the oxide and sulfide solid electrolytes. The composite green (unfired) compacts with 70 and 80 wt % NASICON exhibited high total conductivities at 100 °C of 1.1 × 10^–3 and 6.8 × 10^–4 S cm^–1, respectively. An all-solid-state Na₁₅Sn₄/TiS₂ cell was constructed using the 70 wt % NASICON composite electrolyte by the uniaxial pressing of the powder materials, and its discharge properties were evaluated at 100 °C. The cell showed the reversible capacities of about 120 mAh g^–1 under the current density of 640 μA cm^–2. The prepared oxide-based composite electrolytes were thus successfully applied in all-solid-state sodium rechargeable batteries without sintering

Mechanochemical Synthesis and Characterization of Metastable Hexagonal Li₄SnS₄ Solid Electrolyte

A new crystalline lithium-ion conducting material, Li₄SnS₄ with an <i>ortho</i>-composition, was prepared by a mechanochemical technique and subsequent heat treatment. Synchrotron X-ray powder diffraction was used to analyze the crystal structure, revealing a space group of <i>P</i>6₃/<i>mmc</i> and cell parameters of <i>a</i> = 4.01254(4) Å and <i>c</i> = 6.39076(8) Å. Analysis of a heat-treated hexagonal Li₄SnS₄ sample revealed that both lithium and tin occupied either of two adjacent tetrahedral sites, resulting in fractional occupation of the tetrahedral site (Li, 0.375; Sn, 0.125). The heat-treated hexagonal Li₄SnS₄ had an ionic conductivity of 1.1 × 10^–4 S cm^–1 at room temperature and a conduction activation energy of 32 kJ mol^–1. Moreover, the heat-treated Li₄SnS₄ exhibited a higher chemical stability in air than the Li₃PS₄ glass-ceramic