Synthesis Of Graphene-like Transition Metal Disulfides/Graphene Nanocomposites And Their Electrochemical Performances As Anodes In Rechargeable Lithium Ion Batteries

Li-ion batteries have been widely used in portable electronics such as mobile communication devices due to their high energy density, friendly-environment and long cycle life. Recently, the discovery of graphene has been greatly attracted many attentions due to its unique two dimensional structure with high electron conductivity which can be facilitating the insertion and extraction of lithium ions as Li-ion battery electrode. Current reports on graphene-based materials are mostly focused on metal or metal oxides supporting on the surface of graphene. The cycle stability of these composites, however, is still not satisfactory and～15-30%of capacity loss is typical after30-50cycles. The unsatisfactory cycle stability was probably caused by structural and morphological mismatches between graphene and metal or metal oxides. Here, we first reported the synthesis and preparation of graphene-like transition metal disulfides (GL-TMDs, e.g., MoS2and SnS2) supporting on the surface of graphene. The growth and forming mechanism of these GL-TMDs/graphene composites were investigated. We also studied the effects of reaction conditions on their electrochemical performances.A facile process to synthesize graphene-like MoS2/amorphous carbon (a-C) composites was developed. MoS2/C composites were firstly prepared by hydrothermal method employing sodium molybdate, sulfocarbamide and glucose as starting materials. The graphene-like MoS2/a-C composites were obtained after annealing at800℃in H2/N2. The samples were characterized by XRD, SEM, EDX and HRTEM. It was confirmed that in the composites MoS2has a structure of single-layer, which is named graphene-like nanostructure. The graphene-like MoS2nanosheets were uniformly dispersed in amorphous carbon. The interlaminar distance of the adjacent graphene-like MoS2nanosheets in the composites measured was～1.0nm. The mechanism of the formation of the graphene-like MoS2/a-C composites was investigated. The graphene-like MoS2/a-C composites exhibited high capacity and excellent cyclic stability used as anode materials for Li-ion batteries. The composite prepared by adding1.0g of glucose in hydrothermal solution exhibited the highest reversible capacity (962mAh/g) and excellent cyclic stability. After100cycles, it still retained912mAh/g. The significant improvements in the electrochemical properties of the graphene-like MoS2/a-C composites could be attributed to the graphene-like structure of the MoS2nanosheets and the synergistic effects of graphene-like MoS2and amorphous carbon.According to the disadvantages of graphene-like MoS2/a-C composites, here we reported a facile process to synthesize the novel nanocomposites comprised of single-layer MoS2, graphene and amorphous carbon (SL-MoS2/G@a-C) by a hydrothermal route employing sodium molybdate, sulfocarbamide, as-prepared graphene oxide and glucose as stating materials and then annealing in H2/N2atmosphere at800℃. The samples were systematically investigated by X-ray diffraction, field emission scanning electron microscopy, and high-resolution transmission electron microscopy. It was demonstrated that the single-layer MoS2and graphene in the composites highly uniformly dispersed in the amorphous carbon. The mechanism for the formation of SL-MoS2/G@a-C nanocomposites was investigated. It was found that that the SL-MoS2/G@a-C nanocomposites exhibited very high reversible capacity with excellent cyclic stability and high-rate capability as anode materials of Li-ion batteries. Among three SL-MoS2/G@a-C samples, the SL-MoS2/G@a-C (1:1) nanocomposite delivered the largest reversible capacity (1116mAh/g) with hardly fading of the capacity after250cycles, and still remained a high specific capacity of850mAh/g and good cyclic stability at a high current density of1000mA/g.A facile process was developed to synthesize MoS2/graphene nanosheet (GNS) composites by a one-step in-situ 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 MoS2. The synergistic effect between MoS2and the graphene molecular layer greatly enhances the diffusivity of Li+ions in the process of lithiation and delithiation. These MoS2/GNS composites therefore exhibit extraordinary capacity, i.e., up to1300mAh/g, and excellent rate capability and cycling stability as an anode material for lithium ion batteries.A facile process was developed to synthesize layered MoS2/graphene (MoS2/G) composites by an L-cysteine-assisted solution-phased method, in which sodium molybdate, as-prepared graphene oxide (GO), and L-cysteine were used as starting materials. As-prepared MoS2/G composites were then fabricated into layered MoS2/G composites after annealing in H2/N2atmosphere at800℃for2h. The samples were systematically investigated by X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, and high-resolution transmission electron microscopy. Electrochemical performances were evaluated in two-electrode cells versus metallic lithium. It is demonstrated that the obtained MoS2/G composites show three-dimensional architecture and excellent electrochemical performances as anode materials for Li-ion battery. The MoS2/G composite with a Mo:C molar ratio of1:2exhibits the highest specific capacity of～1100mAh/g at a current of100mA/g, as well as excellent cycling stability and high-rate capability. The superior electrochemical performances of MoS2/G composite as Li-ion battery anodes are attributed to their robust composite structure and the synergistic effects between layered MoS2and graphene.We develop a facile process for preparing few-layer SnS2/graphene (FL-SnS2/G) hybrid by solution-phase method employing L-cysteine as a complexing, sulfide source and reducing agent. The FL-SnS2/G hybrid is characterized by XRD, SEM and HRTEM. It is demonstrated that the few-layer SnS2with defects or disorder structure supports on graphene surface. Electrochemical tests show the FL-SnS2/G hybrid exhibits an extraordinary capacity of up to920mAh/g with excellent cycling stability and high-rate capability. The significant improvement in the electrochemical performances is attributed to the robust composite structure and some synergistic interactions between few-layer SnS2and graphene. Electrochemical impedance spectra confirm that the incorporation of graphene considerably improved the electric conductivity and electron rapid transfer of the FL-SnS2/G hybrid. Therefore, this new kind of FL-SnS2/G hybrid can be used as a promising anode material for lithium ion batteries. SnS2/SnO2composites were prepared in a microwave-assisted reaction of a mixture solution of SnCl4and L-cysteine and were characterized by XRD, TEM, SEM and EDX. The influence of the mole ratio of SnCl4, to L-cysteine (L-cys) on the sample was investigated. It was found that using a microwave method, SnS2/SnO2composites were formed, and SnS2/SnO2nanoparticles were obtained when the mole ratio of SnCl4to L-cysteine was1:2. With higher contents of L-cys, when the mole ratio of SnCl4to L-cys was1:4, the products were nanosheets instead of nanoparticles. Electrochemical tests demonstrated that the SnS2/SnO2composites with layer structure exhibited high reversible capacities and good cycling performances when used as anode materials of Li-ion batteries. When the mole ratio of SnCl4to L-cys was1:6, the initial reversible capacity of products was593mAh/g, and the retention capacity that was maintained was over88%. Besides, the retention capacity of products was still excellent at high current charge/discharge.