Abstract
Sulfide-based solid electrolytes are desirable for use in all-solid-state batteries owing to their high ionic conductivity and plasticity. However, they generally degrade upon exposure to water and can generate toxic hydrogen sulfide even in dry-room atmospheres. To prevent their degradation, surface stabilization is required and further research into the degradation mechanism is necessary. In the present study, the stability of Li3PS4–LiI glass ceramic (LPSI) has been examined under low-humidity conditions. In contrast to an argyrodite-type solid electrolyte, exposure of LPSI to dry air with a dew point of −20 °C resulted in low H2S-gas generation and reduced ionic conductivity of LPSI. Since the conductivity mostly recovered after vacuum heating at 100 °C, the H2S generation is not considered to be the major reason for the reduction in conductivity. On the contrary, it is suggested that water molecules are present on the LPSI powder particles after dry-air exposure, resulting in the formation of a degraded LPSI layer and low ionic conductivity, and that most of the water molecules are removed during vacuum heating, resulting in the recovery of conductivity. Furthermore, optimal vacuum-heating conditions were obtained from X-ray diffraction and temperature-programmed desorption-mass spectrometry measurements, indicating an optimal temperature and heating time of 100 °C and 2 h, respectively. Impedance measurements were used to probe the degradation of the surface layer. The condition of the surface layer was affected by the pellet-forming pressure, and it was easier to detect the degradation of the surface layer when the pellets were formed at low pressures. This paper contributes to the formulation of guidelines for the development of water-resistant solid electrolytes.
