Unveiling the Charge Storage Behavior in Flower-Like MnMoO₄/Mn-MoS₂ Nanosheet Anode Material by Operando Spectroscopy

Transition metal disulfides (TMDs) have recently attracted considerable attention on serving as the promising electrode materials for energy storage devices like Li-ion batteries (LIB) due to their rich reaction sites and unique structural features. Molybdenum disulfide (MoS₂) has a high theoretical capacitance (669 mAh g^-1) and a low volume expansion rate, which is a promising material for LIB. However, the low cycling stability (&lt;30 cycles) as well as high electric resistance have limited the practical application. Therefore, the combination of TMD with metal oxide such as MnMoO₄ is a possible solution to enhance the cyclability and conductivity of electrode materials for LIB applications. Herein, a binary transition-metal sulfide/oxide, MnMoO₄/Mn-MoS₂ nanosheet (MMSO) grown on nickel foam was developed as the anode material for LIB application. The MMSO electrode was prepared by a two-step hydrothermal method, and the well-distributed sheet structure of MnMoO₄ can freely deposit onto the Mn-MoS₂ surface to form the flower-like heterostructures, which can separately stand onto Ni form substrate to significantly enhance the electrochemical performance of the active material by increasing the number of active sites as well as by shortening the diffusion path for Li⁺ ions. The electrode exhibits both large mass and areal capacities (1885 mA h g^-1 and 3.56 mA h cm^-2) at a current density 0.5 mA cm^-2and an outstanding rate capability (2.5 mA h cm^-2) at a current density 3 mA cm^-2. Moreover, the energy storage mechanism of Mn-MoS₂ nanosheet was unveiled systematically by <i>operando</i> synchrotron X-ray absorption near edge structure (XANES) and <i>in operando</i> synchrotron X-ray diffraction (XRD) by continuously analyzing the crystal phase and valence state of the material during the charging and discharging process. Based on these observations, the anode material exhibits the conversion-type reaction by reducing Mn-MoS₂ to molybdenum metal (Mo) and Li₂S during lithiation process, and some of Li₂S would be oxidized to elemental sulfur (S₈⁰or S₄^2-) at the delithation process. This reaction leads to the formation of elemental sulfur at about &lt; 1.0 V, and the released sulfur would be further utilized in subsequent reactions to enhance the electrochemical performance greatly. Results in this study have clearly clarified the charging-discharging behaviors of TMD-based materials as the anode of LIB, which can provide a gateway to develop a platform for the utilization of 2-D sulfide materials for energy storage applications.