Japan Fine Ceramics Center develops new method for real-time observation of intercalation reactions at the atomic level

Understanding intercalation reactions (reactions where atoms are inserted into materials), which play crucial roles in batteries and catalysts, holds the key to materials development. Since microstructural changes occurring during reactions directly affect device performance, elucidating these changes is extremely important for developing next-generation devices. While in situ observation using electron microscopy to observe reaction processes in real-time has been actively pursued, observations have often been limited to the nanometer scale, making it difficult to track more microscopic structural changes at the atomic scale.

Senior Researcher Kei Nakayama and Senior Researcher Shunsuke Kobayashi of the Japan Fine Ceramics Center successfully achieved atomic-scale real-time observation of lithium insertion reactions using annular dark-field (ADF) scanning transmission electron microscopy (STEM) instead of the conventionally used high-resolution transmission electron microscopy (HRTEM). The major advantage of ADF-STEM is that atomic positions can be easily identified just by looking at the images. The research was published in ACS Nano.

The material studied was molybdenum disulfide (MoS₂), which is also attracting attention as an electrode material for secondary batteries. Using a specially designed specimen holder and electron beam, the researchers induced reactions causing lithium insertion into MoS₂ within the microscope and employed an approach to capture video footage of the process. First, they confirmed that lithium insertion reactions occurred through low-magnification observation. Next, they performed high-magnification observation of similar reactions. As a result, they successfully captured the progression of crystal structure changes dynamically and directly at the atomic level 16.74 seconds after insertion. Furthermore, after 19.69 seconds, they revealed that local regions with different crystal orientations appeared successively after the initial crystal structure change had occurred.

Through examination from a crystallographic perspective, it was found that these changes were closely related to changes in internal stress that vary with increasing lithium insertion amounts, revealing the mechanism of microstructural changes accompanying the progression of lithium insertion reactions. These results specifically demonstrate that atomic-scale in situ observation techniques using ADF-STEM serve as a powerful tool for elucidating the dynamic processes of lithium insertion reactions.

By achieving atomic scale in situ observation of lithium insertion reactions using an imaging method different from conventional approaches, the group have opened new avenues for in situ observation methods of lithium insertion reactions. Moving forward, in addition to expanding to various battery materials, they plan to advance atomic scale in situ observation techniques further, with applications to lithium elimination reactions, observations under applied voltage, and direct visualization of lithium ions in their sights.

Journal Information
Publication: ACS Nano
Title: Atomic-Scale In Situ Scanning Transmission Electron Microscopy of MoS₂ during Lithiation
DOI: 10.1021/acsnano.5c05218