In Situ Study of K+ Electrochemical Intercalating into MoS2 Flakes

By applying a single-flake microelectrode technique, a potassium ion (K+) intercalating into a MoS2 flake under potential control was observed using optical microscopy and in situ Raman spectroscopy. The K+ intercalation process showed high reversibility while cycling between open circuit potential (OCP) and 0.8 V, confirmed by the recovery of the Raman peaks. Further discharging to low potential (similar to 0.5 V) would cause the irreversible loss of the Raman peaks due to decomposition of the K+ intercalated compound (KxMoS2), which was confirmed by X-ray photoelectron spectroscopy analysis. On the basis of the diffusion behavior of K+ within the MoS2 layer observed visually by optical microscopy, we believed that K+ was inserted into MoS2 via a layer-by-layer fashion on a micrometer scale. intercalation behavior in MoS2 flakes was further studied by using a galvanostatic intermittent titration technique, in which the abrupt decrease of diffusion coefficient (D-K(+)) suggested the unfavorable energy change within KxMoS2 structure from 0.9 to 0.8 V. The in situ Raman spectra of MoS2 single flakes with a thickness of 2 nm (3 layers) and 47 nm (similar to 72 layers) during potassiation were compared with those of commercial microcrystalline MoS2 flakes that have a typical thickness of 50-80 nm and a size of 2 mu m. Our results reveal important kinetic information of electrochemical K+ insertion into MoS2 and provide useful insights for the investigation of high-rate electrode materials for metal ion batteries.