In Situ Transmission Electron Microscopy Observation of Electrochemical Behavior of CoS₂ in Lithium-Ion Battery

Metal sulfides are a type of potential anode materials for lithium-ion batteries (LIBs). However, their electrochemical behaviors and mechanism during the charge and discharge process remain unclear. In the present paper, we use CoS₂ as a model material to investigate their electrochemical process using in situ transmission electron microscopy (TEM). Two kinds of reaction behaviors are revealed. The pure CoS₂ particles show a side-to-side conversion process, in which large and anisotropic size expansion (47.1%) occurs that results in the formation of cracks and fractures in CoS₂ particles. In contrast, the CoS₂ particles anchored on reduced graphene oxide (rGO) sheets exhibit a core–shell conversion process involving small and homogeneous size expansion (28.6%) and few fractures, which attributes to the excellent Li⁺ conductivity of rGO sheets and accounts for the improved cyclability. Single-crystalline CoS₂ particle converts to Co nanocrystals of 1–2 nm embedded within Li₂S matrix after the first lithiation. The subsequent electrochemical reaction is a reversible phase conversion between Co/Li₂S and CoS₂ nanocrystals. Our direct observations provide important mechanistic insight for developing high-performance conversion electrodes for LIBs.