Crystal structure changes of thio-LISICON electrolytes in humid atmosphere

Abstract

Sulfide electrolytes are among the most notable materials because of their high ionic conductivity and ductility. Their primary drawback is that they are sensitive to humidity and must be handled in an inert atmosphere to prevent degradation. Understanding the degradation mechanism is necessary to establish methods for overcoming it; however, whether or not it depends on the composition of the electrolytes is not yet clear. In this study, we systematically investigated the crystal structure changes in a humid atmosphere for crystalline sulfide electrolytes (thio-LISICONs). Eight thio-LISICONs, Li_xMS₄ (x = 3–5; M = Al, Ga, Si, Ge, Sn, P, As, and Sb), were prepared using a mechanochemical process and subsequent heat treatment. The exposure behavior was observed using in situ humidity exposure X-ray diffraction (XRD). In a humid atmosphere with a dew point of −8 °C, the electrolytes, transformed into new hydrates, all except for Li₅AlS₄ and Li₅GaS₄, which did not change. Hydrated Li₄MS₄ (M = Si and Ge) has the same crystal structure as that of the known hydrate Li₄SnS₄·13H₂O. The crystal structures of hydrated Li₃MS₄ (M = P, As, and Sb) were identical to each other but different from those of Li₃AsS₄·8H₂O. This study revealed that the hydration behavior of the sulfide electrolytes depends on their central element groups.