Lithium metal is considered the ultimate negative electrode material for constructing high-energy-density batteries. However, the combination of liquid electrolytes and the Li metal negative electrode has not been commercially successful because the growth of Li dendrites causes the batteries to short-circuit. Solid electrolytes with robustness have the potential to physically inhibit Li dendrite penetration. Therefore, all-solid-state batteries are promising for practical use due to the feasibility of the Li metal negative electrode. Sulfide electrolytes are one of the candidates for solid electrolyte materials for all-solid-state Li metal batteries because of their high ionic conductivity and formability. However, even in sulfide all-solid-state Li metal batteries, the batteries short-circuit during charging and discharging. We have reported that one of the triggers of the short circuits is the formation of a reaction layer at the interface between the sulfide electrolyte and the Li metal^[1]. The reaction layer is formed by the reductive decomposition of the sulfide electrolyte. Therefore, it is necessary to develop the sulfide electrolyte with high reduction tolerance. For example, it revealed that the doping of LiI to Li₃PS₄ enhanced reduction tolerance^[2]. The addition of LiF, which has a wider electrochemical window, is expected to further improve the stability against Li metal. However, in the case of LiF being added to the Li₂S-P₂S₅ system, the ionic conductivity decreases as the amount of LiF added increases^[3]. Since it is known that a higher conductivity is desirable for preventing short circuits, strategies to improve the conductivity are required^[4]. In recent years, we have revealed that the highly ion conductive phase, α-Li₃PS₄, is stabilized at room temperature by rapid heating of Li₃PS₄ glass to crystallize, followed by quenching^[5]. Consequently, it should be possible to stabilize α-Li₃PS₄ at room temperature and achieve both high conductivity and high reduction tolerance in LiF-doped Li₃PS₄.

In this study, Li₃PS₄-LiF glass-ceramics were prepared as solid electrolytes for all-solid-state Li metal batteries. To develop high ionic conductivity as well as high reduction tolerance, we aimed to stabilize α-Li₃PS₄ at room temperature using a process with rapid heating and rapid cooling. The conductivity and stability against Li metal of the fabricated solid electrolytes were evaluated.

Li₃PS₄-LiF glasses were prepared by a mechanochemical process. In the XRD patterns of Li₃PS₄-LiF glasses, halo patterns were mainly observed, although some peaks attributed to LiF remained. Li₃PS₄-LiF glass-ceramics were prepared by heat treatment of the glass electrolytes. The heat treatment was conducted with a rapid heating rate of 400 ºC min⁻¹ and rapid cooling by quenching. As a result, the α-Li₃PS₄ analog phase was successfully stabilized at room temperature in LiF-doped Li₃PS₄. Although the conductivity of Li₃PS₄·xLiF (x in molar ratio) glass decreased with the addition of LiF, the conductivity increased by rapid heating and quenching, and exceeded 10⁻³ S cm⁻¹ even at x = 1, which is 10 times higher than that of glass electrolyte. The electrochemical stabilities of Li₃PS₄·xLiF glass-ceramics against Li metal were evaluated using all-solid-state Li symmetric cells. The addition of LiF suppressed the reductive decomposition of the solid electrolyte and improved Li stripping/plating performance. Overall, the α-Li₃PS₄ analog was successfully stabilized at room temperature by the rapid heating and quenching process, even with LiF-doped Li₃PS₄. Li₃PS₄·LiF glass-ceramics were a desirable solid electrolyte for all-solid-state Li metal batteries due to its high reduction tolerance and high conductivity.