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 (MoS) using in situ transmission electron microscopy (TEM). We find that single-crystalline MoS nanosheets convert to Mo nanograins (~2 nm) embedded in LiS matrix after the first full lithiation. After the delithiation, the Mo nanograins and LiS transform to a large number of lamellar MoS nanocrystals. The discharge-charge cycling of MoS in LIBs is found to be a fully reversible conversion between MoS and Mo/LiS rather than the electrochemical conversion between S and LiS proposed by many researchers. The in situ real-time characterization results give direct evidence and profound insights into the lithium-storage mechanism of MoS as anode in LIBs.