Synthesis Of Graphene-like MoS₂/graphene Nanocomposites And Their Electrochemical Performances As Anodes In Rechargeable Lithium Ion Batteries

Graphene-like transition metal dichalcogenides MoS2and graphene possess many superior physical and chemical characteristics, due to their unique molecular structure. Owning to their matching crystal structure and micro-morphology, as well as their complementary electrical properties, the duo constitutes excellent hybrid materials thanks to their synergy effect. Working as Li-ion cathode materials on the nano-scale, this novel hybrid material does not only offer high reversible Li-ion storage capacity, but also facilitates excellent cycle stability and good high rate charge and discharge performance. In this thesis, we find a novel synthesis method assisted by cation surfactant to fabricate graphene-like MoS2/Graphene hybrid nanomaterial and proposed its growth mechanism. Li-ion battery has been fabricated using the as-made materials as cathode and its electrochemical Li-ion storage capacity has been studied.A facile and scalable process was developed for the synthesis of single-layer MoS2/graphene nanosheet (SL-MoS2/GNS) composites based on the concurrent reduction of (NH4)2MoS4and graphene oxide sheets by hydrazine in the presence of cetyltrimethylammonium bromide (CTAB); followed by annealing in N2atmosphere. The morphology and microstructure of the composites were examined by X-ray diffraction, field emission scanning electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy. The formation process for the SL-MoS2/GNS composites was also investigated. The SL-MoS2/GNS composites delivered a large reversible capacity and good cycle stability as a Li-ion battery anode. In particular, the composites easily surpassed MoS2in terms of rate performance and cycle stability at high current densities. Electrochemical impedance spectroscopy revealed that the GNS in the composite not only reduced the contact resistance in the electrode but also significantly facilitated the electron transfer in lithiation and delithiation reactions. The good electrochemical performance of the composites for reversible Li+storage could be attributed to itself unique stable structure and the synergism between the functions of SL-MoS2and GNS.A facile and scalable process is developed for the synthesis of graphene-like MoS2/GNS composites with different cationic surfactant, such as dodecyltrimethylammonium bromide (DTAB), and octyltrimethylammonium bromide (OTAB). These cationic surfactants contain alkyl chains of different length, and we investigated their effects on the microstructures and electrochemical performances of the composites. The MoS2/GNS composites are systematically characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM). In addition, we also studied the impact of steric effects on the fabrication of the composite by using tetrabutylammonium bromide addition. We found that the tetrabutylammonium bromide has little effect on the the forming of graphene-like MoS2, because the steric structure of the four n-butyl hindered the electrostatic interaction between the positively charged quaternary ammonium salt with GO. It is also found that the layer number of MoS2can be controlled when varying cationic surfactants. The MoS2sheets were single-layered in MoS2/GNS-D composites, and the MoS2sheets are4-6layers for the MoS2/GNS-O composites. For MoS2/GNS-T composites, the MoS2sheets are8-12layers. Among these three MoS2/GNS samples, MoS2/GNS-D and MoS2/GNS-O composites delivered higher reversible capacity of900-1100mAh g-1, with no fading of the capacity after110cycles and better high-rate capability (670mAh go-) at a high current density of1000mA g-1.A facile process was developed to synthesize MoS2/graphene nanosheet (GNS) composites by a solution-phase method. From the characterization of XRD, SEM and HRTEM, it was found that the graphene oxide sheets provided a novel substrate for the nucleation and subsequent growth of Mo2. The synergistic effect between MoS2and the graphene molecular layer greatly enhances the diffusivity of Li+ons in the process of lithiation and delithiation. These Mo2/GNS composites therefore exhibit extraordinary capacity, especially MoS2/GNS (1:1) composites up to1065mAh g-1, and excellent rate capability and cycling stability as an anode material for lithium ion batteries.