In Situ Raman Mapping of Electrochemical Sodium Intercalation and Phase Stability in MoS2 for Sodium-Ion Batteries

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 (MoS2) is fundamentally governed by its nanoscale layered structure, understanding its phase stability during sodium-ion (Na+) intercalation is essential for optimizing MoS2 as an anode material for next-generation sodium-ion batteries. In this study, we investigate Na+ intercalation into MoS2 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 MoS2 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 MoS2. In situ Raman mapping over 900 μm2 regions of electrochemically sodiated MoS2 shows only a weak and spatially sparse J1 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 MoS2 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 MoS2 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 MoS2.