Exothermal behavior and microstructure of a LiNi1/3Mn1/3Co1/3O2 electrode layer using a Li4SnS4 solid electrolyte

Abstract

A positive electrode containing Li3PS4 (LPS) glasses and LiNi1/3Mn1/3Co1/3O2 (NMC) is a promising candidate for sulfide-based all-solid-state lithium batteries owing to its excellent charge–discharge cycle characteristics. However, sulfide-based solid electrolytes exhibit low chemical stability in air. This disadvantage affects process cost and thermal stability of all-solid-state cells. To resolve these issues, in this study, we focus on solid electrolytes, Li4SnS4 (LSS), that do not generate H2S gas in air. The thermal behavior and microstructure of LSS–NMC positive electrode composites before and after the initial charge–discharge cycle are investigated. The initially charged LSS–NMC composites exhibit several exothermal reactions above 250 °C. However, pristine and initially discharged samples do not show any considerable exothermal reactions. For LPS–NMC composites, by contrast, exothermal reactions are detected regardless of the charging and discharging state. To clarify the exothermic factors of initially charged LSS–NMC composites, we performed ex situ transmission electron microscopy observation and X-ray diffraction measurements. It is determined that SnS2, transition metal sulfides, and metal oxides are formed above 300 °C, which is attributable to LSS and NMC decomposition reactions. On the basis of the relation between thermal behavior and corresponding structural changes, exothermic factors and thermal stability of LSS–NMC composites are discussed in comparison with LPS–NMC composites.