Li-ion batteries (LIBs) have become one of the most important renewable energy storage technologies and have been used in a variety of applications, such as electric vehicles and electronic devices. Due to the limited capacity and energy density of current LIBs, there has been a continual effort to explore high-capacity electrode materials. Molybdenum disulfide (MoS2) has been discovered as a promising anode material for LIBs because of its highly reversible capacity and excellent rate capability. Our recent study has shown that MoS2 nanoparticle electrodes achieved about 1300 mAh/g capacity, which is nearly twice as its theoretical capacity (669 mAh/g). To find the mechanism behind this phenomenon, MoS2 nanoparticle cells were made for operando synchrotron X-ray Diffraction (XRD) analysis. Two cycles were tested between 0 V and 3 V under 0.3 C rate during operando XRD scan. The voltage profiles show that the first discharge process is different from the second discharge process, which means that some of the reactions in the first discharge are not reversible. After the first discharge the voltage profile shows that the reaction is reversible. The XRD result shows that MoS2 only existed in the beginning stage of the discharging process in the first cycle and disappeared after discharging to 0.5 V. After the MoS2 phase disappeared, a new LiXMoS2 phase appeared. The operando XRD result shows that the LiXMoS2 peak shifted to lower 2θ angles during lithium insertion (discharge) and shifted back to higher 2θ angles during lithium removal (charge). Based on the analysis of operando XRD results and other characterizations, the extra capacity could be due to some Li-rich LiyMoS2 phase formation during the cycling process.

Investigation of lithium storage mechanisms in MoS2 anode material using operando synchrotron XRD

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