In situ TEM observations of the lithiation of molybdenum disulfide

Abstract

The lithiation of molybdenum disulfide (MoS 2 ) has been directly studied in situ in the TEM by observing specimens with the viewing direction parallel to the basal planes. The MoS 2 lamella was characterized by bright-field imaging during the lithiation, and both selected-area diffraction and high-resolution imaging before and after. An overall expansion of $5% along the c-direction was observed with concurrent local contraction. The contraction can be related to the expulsion of Mo as Li reduces it to form Li 2 S.Research into new materials for energy storage is at an exciting stage with both new materials and well-known materials showing new physical phenomena [1][2][3][4][5]. Layered materials such as the transition metal dichalcogenides (TMDCs) are attracting renewed interest for battery applications [6] as a special example of insertion electrode materials that can take advantage of the intercalation mechanism [7,8]. Work on the intercalation of alkali metals in TMDCs began in the 1970s [9-13], during which time many fundamental observations were made. For example, it was shown that the sodiated phase of NbSe 2 is orthorhombic with Na atoms occupying the octahedral sites in the van der Waals gap [9], and that the general stoichiometry of a fully-lithiated MX 2 compound (where M is a transition metal, usually in group 4B or 5B, and X is S, Se, or Te) is LiMX 2 except in the case of VSe 2 , which formed Li 2 VSe 2 [10].Although bulk molybdenum disulfide (MoS 2 ) can exist with different structures the 2H (hexagonal) phase is the most stable and is the predominant phase that occurs in nature [14]. The orthorhombic 1T structure is commonly observed after exfoliation, which is achieved by lithiating the MoS 2 and reacting the lithiated material with water [15][16][17]. Transition metal clustering in this phase can lead to 2 Â 1 (1T 0 ), 2 Â 2 (1T 00 ) and other supercell variations [18]. However, if these materials are to be used commercially in batteries it is likely that they will be in the form of sheets that are rather thicker than those formed by the exfoliation process. The present study addresses the lithiation of such practical materials.While considerable understanding of these materials can be achieved by examining specimens after lithiation or after delithiation (i.e. ex situ), observing the same specimens before, during and after lithiation has obvious advantages. In situ TEM is the only technique that allows specific sites on the specimen to be monitored in this way with atomic resolution. The usual geometry used for such studies is to examine the layer materials with the beam normal to the layers since this orientation greatly simplifies specimen preparation (see, e.g., [9]). In the present report, a novel experimental geometry was used such that the electron beam was parallel to the (0 0 0 1) planes. This geometry allowed direct observation of the Li intercalation process, in situ in the TEM. Lamellae of single crystal MoS 2 have been examined in this way; this approach can easily be extended to o...

Vertically Stacked 2H‐1T Dual‐Phase MoS2 Microstructures during Lithium Intercalation: A First Principles Study

“…This study investigates these mechanisms of lithium intercalation, the role of distribution of intercalated layers, and the formation of dual‐phase 2H‐1T microstructures observed experimentally. As implied by previous experimental studies, and specifically shown by Janish et al, 36 the lithiated 2H phase of MoS 2 can serve as a precursor to obtain the 1T phase during lithiation. In order to gain a mechanistic understanding of lithiation and the phase transformation behavior in thin films of MoS 2 , the energetics of intercalation and the diffusion pathways are first identified in the bulk 2H phase for variations in the concentration of Li, followed by the investigation of the role of Li‐ion concentration and distribution on the 2H‐to‐1T phase transformation behavior.…”

“…These studies also suggest Li intercalation on monolayer MoS 2 above a concentration of 40% of the available adsorption sites renders a phase transformation from the initial 2H to 1T phase, 31,32 and has also been observed experimentally during Li intercalation 33‐35 . Recent in‐situ lithiation experiments 36 on thin film MoS 2 using high‐resolution transmission electron microscopy (HRTEM) suggest the formation of several thin bands (typically 6‐7 layers) of planar defects. The bands are observed to grow across the sample, parallel to the basal planes, and all of them have approximately the same thickness.…”

Recent Advances in Doping of Molybdenum Disulfide: Industrial Applications and Future Prospects

“…Possible alkali‐metal‐based doping elements on MoS 2 including K and Li are shown in Group IA of Figure .…”

Application of Operando X-ray Diffraction and Raman Spectroscopies in Elucidating the Behavior of Cathode in Lithium-Ion Batteries

“…Raman spectroscopy is also used to explore the variation in the local structure and oxidation state, as well as the thermal stability of electrode surfaces or electrode-electrolyte interfaces during charge-discharge and heat treatment (Dokko et al, 2002;Hardwick et al, 2008;Membreno et al, 2013;Stancovski and Badilescu, 2013;Wu et al, 2013); FTIR is another effective tool for studying the surface and evolution of electrodeelectrolyte interfaces under cell operating conditions (Aurbach and Chusid, 1997;Chusid et al, 2001;Cheng et al, 2007;Ye et al, 2016). In addition to these spectroscopic techniques, operando SEM (Miller et al, 2013;Hovington et al, 2014;Marceau et al, 2016) and TEM (Liu, S. et al, 2014;Janish and Carter, 2015;Wang, 2015;Chen et al, 2016a;Xu et al, 2016) show the evolution of direct images of cycling electrode at the micro-and nano-scales. By combining the images with energy dispersive X-ray spectroscopy, electron energy loss spectroscopy and electron diffraction, the information on particle morphology, crystal structure, phase transformation, multi-phase interface behavior, element distribution and element positions are obtained as a function of the electrochemical process.…”