In-situ TEM Investigation Of Alkali Metal Intercalation Of Transition Metal Dichalcogenides

Belonging to the class of various two-dimensional materials,transition metal dichalcogenides（TMDCs）show a wide range of electronic,optical,mechanical,chemical and thermal properties and have great potential in areas ranging from nanoelectronics,optoelectronics,catalysis to energy storage.The van der Waals gap between the layers of TMDCs allows insertion of guest ions such as alkali metal.Intercalation of TMDCs is one important field in the chemistry of inclusion compounds.With guest materials intercalated in the host ones,the structure and electronic properties of the host materials can be drastically manipulated.Here we investigated the alkali metal intercalation process in TMDCs（MoS₂ and WS₂）via an in-situ transmission electron microscopy（in-situ TEM）.TEM provides ability to correlate structural data with chemical information;as such,in-situ TEM is unique in its ability to obtain the above corelation under various additional environment in real time.By in-situ TEM we investigated the lithiation and sodiation process in MoS2 and WS2 nanosheets.The main results are as follows.1.We constructed electrochemical half-cell inside TEM,which involved MoS₂naonsheet as one electrode,Li metal as the counter electrode and Li₂O on the lithium as the solid-state electrolyte.The dynamic lithiation process was recoreded by in-situ TEM and the structural transformation of Mo S₂ are revealed by the electron diffraction patterns.The pristine MoS₂ transformed into an intermediate phase characterized by its 2×2 superstructure,during which time phase transition from 2H-1T occurred.With excessive lithium intercalated,Li₂S and Mo were generated.2.We investigated the sodium intercalation process in MoS₂ nanosheets via in-situ TEM and obtained real-time images and diffraction patterns of the sodiated MoS₂,which helps to better understand the electrochemical process and phase transition mechanism at the atomic level.Based on the high resolution transmission electron microscopy and electron diffraction pattern,we demonstrate that MoS₂ undergoes an intermediate phase that should be denoted as 2H-Na_0.25MoS₂,showing particular electron diffraction patterns which can be defined as 2×√3 superstructure.For more sodium insertion,2×2 patterns emerged,indexed as 2H-Na_0.5MoS₂ and 1T-Na_0.5MoS₂respectively according to the intensity distribution of the electron diffraction spots.Phase transition from 2H-Na_0.5MoS₂ to 1T-Na_0.5MoS₂ occurs,resulting from the decreased stability of 2H-MoS₂ lattice and the enhanced intervalence charge transfer between Mo³⁺and Mo⁴⁺ion.3.By using in-situ TEM method,we investigated the lithium ions insertion process in WS₂ nanosheet and obtained real-time images and diffraction patterns of the lithiated WS2.The electron diffraction patterns demonstrated that pristine WS2 transformed into2H-Li_0.5WS₂ with 2×2 diffraction patterns,followed by a phase transiton from 2H-1T and transformed into 1T-Li_0.5WS₂.With more lithium intercalated,Li₂S and W were generated.Upon sodium insertion,WS₂ underwent an intermediate phase 2H-Na_0.25WS₂ with its 2×（?） superstructure,and Na_0.5WS₂ with its 2×2 superstructure,which is similar to the sodiation process of MoS₂.