Ex situ investigation of exothermal behavior and structural changes of the Li3PS4- LiNi1/3Mn1/3Co1/3O2 electrode composites

Abstract

Sulfide-based all-solid-state lithium batteries are expected to be a next-generation power source because of their high energy density, good charge–discharge characteristics, and incombustibility. A positive electrode composite comprising LiNi1/3Mn1/3Co1/3O2 (NMC) and Li3PS4 (LPS) glass electrolyte shows excellent charge–discharge cycle characteristics. To understand exothermal reactions during the heating process of this battery material, structural and morphological changes were investigated mainly by ex situ X-ray diffraction measurements as well as ex situ transmission and scanning electron microscopy observations. We found that a number of transition-metal sulfides, such as MnS and CoNi2S4, and Li3PO4 nanocrystallites were formed in a sample heated at a temperature above the exothermic peak temperature. In addition, no substantial difference was observed in the behavior of these structural and morphological changes due to charge and discharge cycles. The formation of transition-metal sulfides and Li3PO4 crystalline phases suggests that, when exposed to real cell operating environments, LPS and NMC undergo a chemical reaction via heat treatment, which directly leads to the exothermal reactions.