Synthesis of Two-dimensional Carbon-based Nanocomposites and Their Applications in Rechargeable Batteries

Abstract

Over the past decade, graphene, the thinnest and most representative two-dimensional (2D) carbon material, has aroused hugh research interest because of its remarkable optical, electronic and mechanical properties. Meanwhile, as other representative 2D material, transition matal dichalcogenides derived from layered bulk crystals have also been intensively studied in recent years due to their unique properties and a broad range of applications such as energy storage devices, catalysis, electronic, optoelectronics and so on. Especially, in energy storage system, these 2D nanomaterials have been considered promising building blocks for construction of advanced electrodes with high energy density and stability. Nanocomposite material is a class of materials that are consisted of two or several components with at least one of them having a dimension in the nanoscale. The nanocomposite material possesses the advantages of its individual components, and at the same time may show new functions and properties for practical application, especially energy storage devices. Hybridizing nanomaterials with carbon is regarded as an effective way for enhancing the electrochemical performances due to the high conductivity, mechanical stability and surface area. However, currently, complicated and harsh synthetic processes are required to synthesize carbon-based nanocomposite, such as hydrothermal, solvothermal, and chemical vapor deposition (CVD) techniques

these are the main obstacles to mass producing these materials because these methods require expensive facilities, are time consuming, and are limited to small-sized reaction vessels. Especially, synthesizing 2D carbon-based nanocomposites remains challenging. In my dissertation, I aim to describe 2D carbon-based nanocomposites, particularly centering on their preparation strategies and applications in rechargeable batteries (i. e. Li and Na ion batteries and lithium-sulfur batteries). Firstly, solventless and scalable strategy is developed for the synthesis of few-layer MoS2 incorporated into hierarchical porous carbon (MHPC) nanosheet composites as anode materials for both Li- and Na-ion battery. An inexpensive oleylamine is introduced to not only serve as a surfactant and hinder the stacking of MoS2 nanosheets but also to provide a conductive carbon, allowing large scale production. In addition, a SiO2 template is adopted to direct the growth of both carbon and MoS2 nanosheets, resulting in the formation of hierarchical porous structures with interconnected networks. Due to these unique features, the as-obtained MHPC shows substantial reversible capacity and very long cycling performance when used as an anode material for LIBs and SIBs, even at high current density. Indeed, this material delivers reversible capacities of 732 and 280 mA h g-1 after 300 cycles at 1 A g-1 in LIBs and SIBs, respectively. In addition, its Coulombic efficiency reached ~98 % after the 3rd cycle and exceeded 99 % after 100 cycles, indicating that efficient Li+ insertion and extraction occur in the MHPC composites. The results suggest that these MHPC composites also have tremendous potential for applications in other fields. Secondly, monodisperse carbon nanocapsule ensemble-on-graphene nanosheet composites (MCNC/G) were prepared by a facile strategy, which involves mixing of iron-oleate and graphene, heat treatment, and finally, acid etching of iron oxide nanoparticles. The composites comprised highly uniform, hollow structured carbon nanocapsules with a diameter of about 20 nm that were densely deposited on the surface of the graphene nanosheets (the specific surface area = 172.4 m2 g-1 and pore volume = 0.96 cm3 g-1). In lithium-sulfur (Li-S) battery test, the MCNC/G-sulfur (MCNC/G-S) composite delivered a high specific capacity of 524.7 mA h g-1 after 100 cycles at 0.5 C-rate. In contrast, the capacity of graphene-sulfur (G-S) dropped significantly under the same conditions, even though the initial specific capacity (1337 mA h g-1) was higher than that of MCNC/G-S (1262 mA h g-1). Finally, we report the successfully synthesis of honeycomb-like 2D mesoporous carbon nanosheet (OMCNS) by an etching of self-assembled iron oxide/carbon hybrid nanosheets as an advanced sulfur host for Li-S batteries. The obtained 2D nanosheets have close-packed uniform cubic mesopores of ~ 20 nm side length, resembling honeycomb structure (the specific surface area = 386.7 m2 g-1 and pore volume = 1.05 cm3 g-1). We loaded OMCNS with sulfur element simple melting infusion process (70 wt%) and evaluate the performance of the resulting OMCNS-sulfur composites as cathode material. the OMCNS-S electrode exhibits a reasonable cycling performance compared with G-S electrode, and it could maintain the specific capacity of 1237.7 mA h g-1 at 1st cycle (0.1 C). After the galvanostatic rates were changed to 0.5 C, a specific capacity of 694.1 mA h g-1 is obtained with the capacity retention of 548.6 mA h g-1 at 0.5 C after 300 cycles. On the contrary, the G-S composite shows the fast capacity fading owing compared to OMCNS-S composite. Such these 2D carbon-based nanocomposites enables the achievement of electrode materials with high capacity and long cyclability for high-performance rechargeable batteries. The results suggest that these nanocomposites also have tremendous potential for applications in other fields.