Tuning structures and morphology of MoS2/carbon nanocomposites for their applications as lithium-ion battery anode

Abstract

In recent years, lithium ion batteries (LIBs) have attracted more and more attention due to the high energy density, low gravimetric density, long cycle life and flexible design. The fast-growing market for portable electronic devices and the development of hybrid electric vehicles create a strong driving force for the development of advanced anode materials, because the commercial anode material, graphite, has a theoretical specific capacity of only 372 mAh/g, which is too low to meet the rising demands of these new applications. As an analogue of graphite, molybdenum disulfide (MoS2) shows great promise as an alternative in virtue of its predominant merits in capacity and relatively small volume change in cycling. MoS2, however, suffers from sluggish kinetics due to its semiconductor nature, especially along the direction perpendicular to the basal plane, leading to unsatisfied rate capability. In addition, the shuttle effect caused by the soluble intermediates, polysulfides (PS), results in a loss of active material and continuous capacity decay. Aimed to overcome the inherent weaknesses of MoS2-based anodes (sluggish kinetics and capacity fading), in my PhD work, ultrathin MoS2 nanosheets and their hybrids are prepared through novel morphology and structure design. The effects of layer number, interlayer distance, conductive additive (carbon) and crystal interfaces on the electrochemical properties of MoS2 are investigated. First, the effect of layer number is studied by aqueous exfoliation of MoS2 with the aid of alkali lignin (AL), a low-cost, environmentally benign and bio-renewable feedstock. It is found that, AL has strong stabilization effect on MoS2 nanosheets, as well as other two-dimensional (2D) materials. The stabilization mechanism and disperse conditions of AL are explored. Compared to bulk MoS2, the exfoliated MoS2 nanosheets exhibit improved electrochemical performance both as cathode and anode. However, their cycling stability is still not satisfied as the dissolution of PS is inevitable. In order to confine the active materials, disordered graphene-like MoS2 nanosheets are embedded in amorphous carbon nanofibers through hydrothermal treatment and electrospinning. The obtained binder-free, self-standing nanofibrous mats can be directly used as anodes after simply punching into suitable size and shape. The hydrothermally synthesized MoS2 exhibits expanded interlayer spacing, which accommodates extra Li+ and improves the Li+ storage capacity. The carbon mats alleviates the loss of PS, leading to excellent cycling stability. What’s more, thanks to the electrospinning technique and 2D layered structure of MoS2, high flexibility of the hybrid mats are achieved, enabling realization of diverse flexible devices. Nevertheless, the rate performance of the hybrid mats is still limited by the thick carbon shell and MoS2 nanosheets. To further increase the electronic and ionic diffusion, singlelayer MoS2 (SL-MoS2) with specially designed architecture is of great potential. Based on the coordination chemistry, a universal method for the fabrication of defect-rich, ultrafine crystals in carbon matrix is developed. Unique SL-MoS2/C sandwich structure is facilely prepared by simply mixing dopamine (DOPA) and Na2MoO4 aqueous solutions, followed by hydrothermal treatment and annealing. The coordination complex formed between DOPA and MoVI is found crucial for the formation of the unique structure. The structural features of the nanocomposites, large interlayer spacing, sandwich structure and crumpled naosheet morphology, render the SL-MoS2/C an excellent anode material, showing a reversible capacity of 500 mAh/g at 5 A/g. The engineering of SL-MoS2 with 2D carbon sheet is fascinating and may generate new class nanocomposite with idea atomic interface for high capacity and rate capability. In this regard, an electrostatic attraction-induced self-assembly strategy is developed for the fabrication of nitrogen-doped graphene (NDG)/MoS2 nanocomposite with well-defined alternating structure (van der Waals heterostructure). The maximized atomic contact triggers synergistic effect between the two components, endowing the NDG/MoS2 nanocomposite with low charge-transfer resistance, high sulfur reservation and structural robustness. A reversible capacity of 820 mAh/g can be achieved at 1 A/g after high-rate charge-discharge. This self-assembly approach may also be adopted for surface modification of MoS2 or fabrication of other NDG intercalated 2D materials. In summary, the electrochemical properties of MoS2 could be greatly improved by reducing the layer number, increasing interlayer distance, adding conductive and protective additive and creating interfaces. The knowledge obtained from this work provides useful insights for further studies on MoS2 or other metal sulfides as LIB anodes.