Lithium Storage Mechanism And Modification Studies Of MoS₂

Currently, graphite is the state-of-the-art anode material in lithium-ion batteriesdue to good safety, stable charge-discharge properties and low-cost. But its limitedtheoretical specific capacity (372mAh/g) provides a motivation to develop alternativeanode materials. MoS2, with a high theoretical capacity of672mAh/g, has beendemonstrated as innovative high-energy anode materials for lithium-ion batteries.However, its cyclic stability is unsatisfactory and requires improvement. This article isintended to explore the reason for the rapid degradation in capacity of the MoS2according to its electrode/electrolyte interfaces reaction mechanisms, and do thefurther research to find the way to improve the cyclic stability of MoS2based on theresult. The main contents of this paper are as follows:The first lithiation and delithiation processes of commercial MoS2electrode asanode materials for lithium ion batteries were studied by electrochemical impedancespectroscopy (EIS). It is found that the typical EIS is composed of four parts, namely,high-frequency semicircle, middle-frequency semicircle, low-frequency short slopingline and low-frequency arc in the Nyquist diagram, and they can be attributed to solidelectrolyte interphase (SEI) film and ionic resistance in pores, charge transfer step,solid state diffusion process, and phase transformation, respectively. An equivalentcircuit that includes elements related to SEI film and charge transfer process, inaddition to phase transformation, is proposed to simulate the experimental EIS data.The change of kinetic parameters for lithiation and delithiation of MoS2electrode as afunction of potential in the first charge-discharge cycle is analyzed. The resultsrevealed that, in the conversion process, the SEI film formed on the MoS2electrodewas relative thick and electrochemically decomposable, and severe phasetransformation occurred. They might be the reason for the rapid degradation incapacity of the MoS2electrode when cycled between3.00and0.01V is discussed indetail.A facile hydrothermal process was developed to synthesize MoS2withhierarchical structure, in which sodium molybdate, thiourea, PEG as precursors. Theexperiments studied the influence of reaction temperature and amount of PEG on themorphology. The products were characterized by X-ray diffraction (XRD) andscanning electron microscopy (SEM). XRD patterns showed that the samples werepure above180℃. SEM images indicated that PEG was necessary to form the uniform MoS2microspheres owned nanosheet structure. The samples were composedof microspheres and nanorod/nanobelt without PEG, but the products were uniformMoS2microspheres after adding PEG. Furthermore, the electrochemical performancesof different samples were characterized over the range of3.00to0.01V. The resultsrevealed that the MoS2with uniform microspheres morphology, which wassynthesized by adding0.6g PEG, exhibited the better discharge capacity and cyclicstability. However, its electrochemical performance was still unsatisfactory andrequired further improvement.The methods to improve the electrochemical performances were investigatedbased on the above research results. Adding the effect film forming electrolyteadditive, increasing the binder ratio and coating carbon methods were taken,respectively. The results manifested that the carbon-coated was the best method toimprove the electrochemical performances of MoS2, what is more, the material alsopresented an excellent rate performance.