In situ Scanning Electron Microscopy Observation of MoS₂ Nanosheets during Lithiation in Lithium Ion Batteries

Using MoS2 nanosheets or composites of MoS2 nanosheets for the anode materials is beneficial to the energy storage performance of lithium ion batteries (LIBs). In order to understand the stability of MoS2 nanosheets as the anode material in LIBs (Li–MoS2 batteries), we study the structural evolution of MoS2 nanosheets during the lithiation process by means of in situ scanning electron microscopy (SEM) and ex situ microstructural analyses. By adjusting chemical vapor deposition growth parameters, monolayer MoS2 atomic sheets and continuous multilayer MoS2 nanosheets are respectively prepared for the experiments. In situ SEM analysis of the monolayer MoS2 atomic sheets shows that phase transformations occur at some voltages during the linear sweep voltammetry measurement. These reactions are further validated in Li–MoS2 batteries by cyclic voltammetry. Microstructural analyses of the MoS2 nanosheet anode confirm that the morphology of MoS2 anode significantly changes during the initial lithiation process from the open circuit voltage to 1.1 V. In addition to the reversible intercalation of Li+ ions, another irreversible reaction between MoS2 and Li+ ions also occurs in the lithiation process. This irreversible phase transformation plays an important role in battery performance when the MoS2 anode material is scaled down.