A Study On The Preparation And Energy-storage Performance Of Carbon-based Nanohybrids

In order to solve the increasingly serious environmental and energy problem, scientists pay more and more attention on the high efficient and environmental energy storage and conversion equipment, such as supercapacitor and Li-ion secondary battery. The high rate performance and long cycling life are the two important parameters for the application of supercapacitor. To improve the specific capacitance and rate performance of the electrode material is crucial to increase the energy density of the supercapacitor. Porous carbon materials are widely used in the field of energy storage due to its high specific surface area, good electrical conductivity and structure stability as well as extensive source. However, it has a weakness of low specific capacitance, hence it is commonly to induce media with pseudocapacitance to the carbon materials mainly with electrochemical double-layer capacitance. Making the carbon material doped with heteroatom or combined with metal oxide/sulfide is used widely to improve the rate and cycling performance of the electrode material. According to the characteristics of energy storage, this work attempts to design the structure of electrode materials and to synthesize them with simple and efficient way. The effects of micro-structure on energy-storage performance were clarified according to the analysis of their micro-structure and the test of electrochemical performance. Several materials have also been used as Li-ion anode materials to explore the common features of electrode materials used both in supercapacitor and Li-ion secondary battery. The main results are summarized as follows:?1? This work designs and develops an effective method to accomplish the 2D layered metal oxide and carbon layer to be stacked layer-by-layer. A sandwich MoO₃/C hybrid nanostructure was obtained via calcination of the dodecylamine-intercalated layered a-MoO₃, leading to the in situ production of the interlayered graphene layer. The sample with a high degree of graphitization of graphene layer and more interlayered void region exhibits the most outstanding energy storage performance. The obtained material is capable of delivering a high specifi c capacitance of 331 F·g^-1 at a current density of 1 A·g^-1 and retained 71% capacitance at 10 A·g^-1. In addition, nearly no discharge capacity decay between 1000 and 10000 continuous charge-discharge cycles is observed at a high current density of 10 A·g^-1, indicating an excellent specific capacitance retention ability. The exceptional rate capability endows the electrode with a high energy density of 41.2 Wh·kg^-1 and a high power density of 12.0 kW·kg^-1 simultaneously. The excellent performance is attributed to the sandwich hybrid nanostructure of MoO₃/C with broad ion diffusion pathway, low charge-transfer resistance, and robust structure at high current density for long-time cycling. The present work provides an insight into the fabrication of novel electrode materials with both enhanced rate capability and cyclability for potential use in supercapacitor and other energy storage devices.?2? Porous hierarchical architectures of few-layer MoS₂ nanosheets dispersed in carbon matrix are prepared by a microwave-hydrothermal method followed by annealing treatment via using glucose as C source and structure-directing agent and ?NH₄?2MoS₄ as both Mo and S sources. It is found that the morphology and size of the secondary building units ?SBUs?, the size and layer number of MoS₂ nanosheets as well as the distribution of MoS₂ nanosheets in carbon matrix, can be effectively controlled by simply adjusting the molar ratio of ?NH4?2MoS4 to glucose, leading to the materials with a low charge-transfer resistance, many electrochemical active sites and a robust structure for an outstanding energy storage performance including a high specific capacitance (589 F·g^-1 at 0.5 A·g^-1), a good rate capability (364 F·g^-1 at 20 A·g^-1), and an excellent cycling stability ?retention 104% after 2000 cycles? for application in supercapacitors. The exceptional rate capability endows the electrode with a high energy density of 72.7 Wh·kg^-1 and a high power density of 12.0 kW·kg^-1 simultaneously. This work presents a facile and scalable approach for synthesizing novel heterostructures of MoS₂-based electrode materials with an enhanced rate capability and cyclability for potential application in supercapacitor.?3? A selective extraction approach for hierarchically porous S-doped carbon was developed. In this method, the in-situ formed SnS is vaporized away in N2?g? at 800? and the excessive sulfur as S-doping agent. The doped S is uniformly distributed in the carbon matrix, combining with O or C atoms in the carbon material to form new functional groups such as aromatic sulfide, sulfoxide and sulfone. Owing to the obviously reduced particle size and greatly enhanced meso-/macropores, electronic conductivity and surface polarity as well as the active sites available for faradic reactions, the resultant S-doped carbon electrode shows a super specific capacitance of 443 F·g^-1 at 0.5 A·g^-1 and a capacitance of 284 F·g^-1 even at a high current density of 20 A·g^-1. The present work would supply a new guide for the preparation of S-doped functional materials with a high supercapacitor electrode performance.?4? Porous S-doped carbon was also used as anode material in Li-ion battery and its electrochemical performance was compared with that of the pure carbon material. The electrochemical tests including CV, charge-dischage, EIS and GITT suggest that the boundary and interior defects of the material were increased due to the S doping and porous structure, which can shorten the diffusion distance of Li+ion and accelerate the charge to transfer in the material, further to enhance the electrochemical performance of porous S-doped carbon. The S-doped carbon and carbon display the discharge capacity of 598 mAh·g^-1 and 325 mAh·g^-1 at a current desity of 100 mA·g^-1, respectively. Furthermore, the two electrode materials maintain the discharge capacity of 369 mAh·g^-1 and 211 mAh·g^-1 after 150 cycles at a current density of 200 mA·g^-1, respectively.?5? MoS₂/C hybrid nanostructure was obtained using uniform-heating microwave- hydrothermal method and single precursor that could easily be dispersed in the reaction system. It has been used as anode material of Li-ion battery. The small size of basic unit, the MoS₂ nanosheets with few stacking layers and an interval uniform distribution of few-layer MoS₂ nanosheets and carbon enhance the conductivity of the electrode material as well as shorten diffusion distance of Li ion. Hence, the specific capacity and rate performance of the electrode material was improved. The carbon matrix can prevent the volume effect and restacking of MoS₂ nanosheets, resulting in the structure stability of the electrode material during charge-discharge process. In addition, carbon can also be the adsorbing agent of the formed sulfide and prohibit its dissolving into the electrolytes, further ensure the MoS₂/C hybrids with a good cycling stability. It is confirmed that the composite with MoS₂ well dispersed in carbon matrix has the application potential as anode of Li-ion battery.