Exfoliation and cracking in MoS₂ following in-situ lithiation

The applicability of layered transition-metal dichalcogenides (TMDs) for electrode application in lithium-ion batteries (LIBs) necessitates a fundamental understanding of the ion intercalation/de-intercalation mechanisms to identify the factors that affect the cycling stability and/or degradation behavior of the layers. This paper presents the results of an in-situ study of reactions between Li/Li₂O and MoS₂ inside the transmission electron microscope (TEM) with the basal plane in the edge-on orientation; this orientation makes it possible to examine directly the microstructural and microchemical changes that occur within the van der Waals (vdW) gaps. The experiments enable the characterization of the nanoscale defects that form in real-time as the Li moves into the gaps. Defects are observed to form along the c-planes of MoS₂ during in-situ reaction with Li in the TEM. Energy-filtered TEM (EFTEM) analysis shows that these lines are rich in Li and poorer in Mo and S. Atomic-resolution TEM studies in an aberration-corrected TEM show the different stages of the microstructural evolution along these white-line defects. The formation of these defects during intercalation results in the degradation of the layers as cracks are observed to form at the interfaces between the defects and the matrix when external tensile stresses are applied. This cracking subsequently leads to exfoliation and the structural degradation of the intercalated material. These direct observations complemented by atomistic modeling raise several critical questions concerning the ion intercalation and de-intercalation mechanisms that need to be addressed to understand the factors that affect the safety and reliability of layered TMDs in LIBs. This paper is submitted as part of a special issue honoring the contributions of Kamanio Chattopadhyay to Materials Science and celebrating his 75th birthday. Prof. Chattopadhyay’s distinguished career included research on metastable materials, which may have much in common with the materials being studied here, and also served for several years as an Editor of the Journal of Materials Science.