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Shalini Tripathi2 Matthew Janish2 William Moyer Mook1 Katherine Jungjohann1 Avinash Dongare2 Arthur Dobley3 C Carter2 1 4

2, Materials Science & Engineering Center, University of Connecticut, Storrs, Connecticut, United States
1, Center for Integrated Nanotechnologies (CINT), Sandia National Laboratories, Storrs, Connecticut, United States
3, Yardney Division, EaglePicher Technologies LLC, East Greenwich, Rhode Island, United States
4, Chemical & Biomolecular Engineering, University of Connecticut, Storrs, Connecticut, United States

Understanding the structure and phase changes associated with two-dimensional (2D) layered transition metal dichalcogenides (TMDs) is critical in optimizing performance in lithium-ion batteries. The large interlayer spacing in MoS2 (∼0.65nm) accommodates species such as alkali metal ions (Li+/Na+/K+) during intercalation. Intercalation is reported to change the electronic structure of the host molecule, resulting in variations in their electrical and optical properties. In this work, we examine the solid-state reactions between Li and MoS2. Li+ ions can be inserted into vdW gap; the reaction is still unclear. Plan-view imaging has been extensively used, however, it is essential to visualize the process with the electron beam being parallel to the basal planes of the layer material to understand the reaction process. Lattice-fringe images have been discussed for several systems but relying on microtoming or simply using curved thin layers, the orientation of the specimen was less than ideally uncontrolled. Here, TEM specimens are made using FIB, and oriented for detailed study of the intercalation process. This study of TMDs uses a Tecnai F30 and a Cs/image-corrected Titan equipped with a direct electron detector camera, K2. This camera has two major advantages: the electron dose can be minimized and quick changes during reactions are recorded; both instruments have EELS and XEDS capabilities. DFT calculations are used to probe the structure and bonding changes during these reactions. Volumetric expansion, energy barriers, phase transformations and the role of doping, defects and interfaces can be modeled. The dynamics of the structural response are modeled using ab initio MD simulations. Electrochemical aspects can be monitored in situ in real-time and at atomic scale to provide understanding of lithium-ion storage mechanisms in these solid-state reactions and thus to test the modeling-based results.
In plan-view specimen, variations normal to the basal plane are not seen. Defects associated with the reactions were monitored real-time. As the reaction between MoS2 and Li proceeds, white-line defects were observed under high-resolution imaging by TEM. Lower-magnification images show that the defects are not equally spaced and do not correspond to ‘stage’ development. These defects can cross several basal planes in the MoS2 (either forwards or backwards) but maintain essentially the same width after the step; they are not completely constrained to the vdW gap.
This work is funded by NSF grant No. 1820565. MTJ is at LANL. TEM is at CINT, an Office of Science User Facility operated for the U.S. DOE. Sandia NL is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s NNSA under contract DE-NA-0003525. The views expressed in the abstract do not necessarily represent the views of the U.S. DOE or the U.S. Government.

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