Ex Situ and Operando XRD and XAS Analysis of MoS2: A Lithiation Study of Bulk and Nanosheet Materials

Molybdenum­(IV) sulfide (MoS2) has generated significant interest as an electroactive material for Li-ion batteries because of its high theoretical capacity, good rate capability, and minimal volume changes during cycling. An important challenge toward implementing this material is understanding the many polymorphs of MoS2 that can be (de)­stabilized by electrochemical lithiation and nanosizing. To this end, bulk MoS2 and nanosheet-type MoS2 were characterized both as solids (X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma-optical emission spectroscopy (ICP-OES)) and during electrochemical cycling within operando X-ray analysis compatible lithium cells (operando XRD and ex situ XAS). In situ XRD shows that the bulk 2H-MoS2 phase is converted to 1T-Li x MoS2 upon discharge and that this change is only partially reversible upon charge. Furthermore, operando XRD identifies the nanosheet MoS2 as the metastable 1T′ phase and shows that this phase is conserved upon discharge. Ex situ XAS provides additional structural insights into the local structure of MoS2, confirming that the 1T′ phase is the correct assignment of the nanosheet MoS2 and revealing an irreversible local distortion that occurs during cycling. This local distortion is likely a factor in the increased capacity fade observed in the nanosheet cells. This work provides important insights into the structure of MoS2 and how that structure is affected by nanosizing and cycling, which can inform other studies of nanosheet layered materials.