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 (∼0.5 V) would cause the irreversible loss of the Raman peaks due to decomposition of the K+ intercalated compound (K x MoS2), 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. K+ 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 K x MoS2 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 (∼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 μm. 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.