Revisiting the Structural Evolution of MoS2 During Alkali Metal (Li, Na, and K) Intercalation

Transition metal dichalcogenides, especially MoS2-based materials, have been actively investigated for application as high-performance electrode materials for metal-ion batteries. To further improve the electrochemical behavior of the electrode, it is important to have a strong fundamental understanding of the metal-ion intercalation mechanism. However, much uncertainty and disagreement remain on the structural evolution of MoS2 during the intercalation process due to its complexity. Therefore, in this study, results from multiple approaches including in situ and ex situ X-ray diffraction, in situ Raman spectroscopy, and density functional theory calculations are integrated to construct a comprehensive picture of the structural changes in MoS2 for the different alkali metal (Li, Na, and K) ion battery systems. In addition, the reversibility of such structural changes at different depths of intercalation is further examined. Several notable observations are made in the process. For example, rather than the K1.0MoS2 phase reported in previous studies, a stable K1.5MoS2 phase is identified instead. Also, we find that lithium ions cannot be completely pulled out even at a low concentration of intercalation, which explains the irreversible discharge/charge profile observed for the lithium-ion system. Furthermore, the different alkali metal systems show difference in stability of the Mo–S bond, influencing the cycle stability of the battery. This study demonstrates the value of combining multiple in situ techniques with density functional theory calculations for providing physical insight into the intercalation process and accelerating the rational design of electrode materials for metal-ion batteries.