Direct Studies on the Lithium-Storage Mechanism of Molybdenum Disulfide

Transition metal sulfides are regarded as a type of high-performance anode materials for lithium ion batteries (LIBs). However, their electrochemical process and lithium-storage mechanism are complicated and remain controversial. This work is intended to give the direct observation on the electrochemical behavior and find out the lithium-storage mechanism of molybdenum disulfide (MoS₂) using in situ transmission electron microscopy (TEM). We find that single-crystalline MoS₂ nanosheets convert to Mo nanograins (~2 nm) embedded in Li₂S matrix after the first full lithiation. After the delithiation, the Mo nanograins and Li₂S transform to a large number of lamellar MoS₂ nanocrystals. The discharge-charge cycling of MoS₂ in LIBs is found to be a fully reversible conversion between MoS₂ and Mo/Li₂S rather than the electrochemical conversion between S and Li₂S proposed by many researchers. The in situ real-time characterization results give direct evidence and profound insights into the lithium-storage mechanism of MoS₂ as anode in LIBs.