In Situ Raman Mapping of Electrochemical Sodium Intercalation and Phase Stability in MoS ₂ for Sodium-Ion Batteries

High Resolution Image Download MS PowerPoint Slide Understanding phase transformations in layered materials such as transition metal dichalcogenides during ion intercalation is critical for developing advanced battery electrodes. Given that the electrochemical behavior of molybdenum disulfide (MoS 2 ) is fundamentally governed by its nanoscale layered structure, understanding its phase stability during sodium-ion (Na + ) intercalation is essential for optimizing MoS 2 as an anode material for next-generation sodium-ion batteries. In this study, we investigate Na + intercalation into MoS 2 and its effect on structural phase evolution by using in situ Raman spectroscopy and ex situ X-ray photoelectron spectroscopy (XPS). It is widely believed that electrochemical intercalation of sodium ions into MoS 2 leads to transformation from the semiconducting 2H phase to the metallic 1T phase. Surprisingly, we find that the electrochemical Na + intercalation method does not lead to phase transformation in MoS 2 . In situ Raman mapping over 900 μm 2 regions of electrochemically sodiated MoS 2 shows only a weak and spatially sparse J 1 mode (∼153 cm –1 ) that is characteristic of the metallic phase. Ex situ XPS measurements further confirm the absence of the 1T phase and reveal the onset of MoS 2 structure degradation below the 0.85 V intercalation voltage, with significant breakdown occurring near 0.65 V. This work presents large-area comprehensive in situ Raman mapping during Na + intercalation in MoS 2 particles at different voltages with ex situ XPS, which shows that contrary to published results, 2H to 1T phase transformation is not always achieved in electrochemically sodium-ion-intercalated MoS 2 .