Atomic‐Scale Visualization of Electrochemical Lithiation Processes in Monolayer MoS₂ by Cryogenic Electron Microscopy

Abstract

While lithium ion batteries with electrodes based on intercalation compounds have dominated the portable energy storage market for decades, the energy density of these materials is fundamentally limited. Today, rapidly growing demand for this type of energy storage is driving research into materials that utilize alternative reaction mechanisms to enable higher energy densities. Transition metal compounds are one such class of materials, with storage enabled by “conversion” reactions, where the material is converted to new compound upon lithiation. MoS2 is one example of this type of material that has generated a large amount of interest recently due to its high theoretical lithium storage capacity compared to graphite. Here, cryogenic scanning transmission electron microscopy techniques are used to reveal the atomic‐scale processes that occur during reaction of a model monolayer MoS2 system by enabling the unaltered atomic structure to be determined at various levels of lithiation. It is revealed that monolayer MoS2 can undergo a conversion reaction even with no substrate, and that the resulting particles are smaller than those that form in bulk MoS2, likely due to the more limited 2D diffusion. Additionally, while bilayer MoS2 undergoes intercalation with a corresponding phase transition before conversion, monolayer MoS2 does not.

Advances in Cryo-Electron Microscopy for Understanding Energy Materials

“…1). Bilayer MoS2 on the other hand undergoes an intercalation reaction before conversion to Mo and Li2S nanoparticles [9]. The cryo-STEM imaging stability demonstrated here, have allowed us to gain unique atomic-scale insights into the lithiation processes 2D layered materials down to the monolayer limit.…”

Advances in Liquid‐Phase and Intercalation Exfoliations of Transition Metal Dichalcogenides to Produce 2D Framework

“…[138,180,359,360] There are also difficulties in evaluating and controlling the degree of intercalation since the incomplete intercalation has been shown to produce low yields of monolayer products, while the intermediate Li x TMDCs with corresponding metal nanoparticles and chalcogens are decomposed over the insertion. [252,343,361,362] Electrochemical Intercalation: The electrochemical technique is also employed as a quick and controllable tool for lithium intercalation [344,[363][364][365][366][367][368][369] and considered as an effective technique for exfoliating and/or intercalates layered material to single or multilayered 2D nanosheets, summary of electrochemical exfoliation of various bulk TMDCs summarized in Table 5. [63,[370][371][372] Electrochemical reactions that have occurred on electrode with layered structure will yields as intercalation and/or exfoliation of electrode.…”

Molybdenum Disulfide Based Nanomaterials for Rechargeable Batteries

“…Apart from designing different nanostructured MoS 2 ‐based materials for battery applications, the fundamental understanding of reaction mechanisms, structural properties, and phase transitions is vital to the development of improved electrode materials. A series of in situ techniques, including XRD, X‐ray absorption spectroscopy (XAS), Raman, and TEM, have been adopted to provide insights into the electrochemical mechanisms related to MoS 2 ‐based electrodes, see Table . Ex situ characterization methods require the cells to be disassembled and probed at different discharge–charge states searching for clues about electrochemical mechanisms of electrode materials.…”

“…Upon further lithiation, Li x MoS 2 decomposes to form Li 2 S and Mo. Figure a shows the in situ XRD pattern obtained for bulk MoS 2 during the first discharge and charge cycle at a current density of 100 mA g −1 . It is found that the MoS 2 (003) peak downshifted during discharge, indicating the expansion in the c ‐axis after Li + intercalation with the formation of Li x MoS 2 (0< x <1).…”