Thickness-dependent phase transition kinetics in lithium-intercalated MoS2

The phase transitions of two-dimensional (2D) materials are key to the operation of many devices with applications including energy storage and low power electronics. Nanoscale confinement in the form of reduced thickness can modulate the phase transitions of 2D materials both in their thermodynamics and kinetics. Here, using in situ Raman spectroscopy we demonstrate that reducing the thickness of MoS2 below five layers slows the kinetics of the phase transition from 2H- to 1T '-MoS2 induced by the electrochemical intercalation of lithium. We observe that the growth rate of 1T ' domains is suppressed in thin MoS2 supported by SiO2, and attribute this growth suppression to increased interfacial effects as the thickness is reduced below 5 nm. The suppressed kinetics can be reversed by placing MoS2 on a 2D hexagonal boron nitride (hBN) support, which readily facilitates the release of strain induced by the phase transition. Additionally, we show that the irreversible conversion of intercalated 1T '-MoS2 into Li2S and Mo is also thickness-dependent and the stability of 1T '-MoS2 is significantly increased below five layers, requiring a much higher applied electrochemical potential to break down 1T '-MoS2 into Li2S and Mo nanoclusters.