| Fossil energy crisis and environmental pollution propose great challenges to the development of sustainable society. To solve these problems, clean and renewable energy sources, such as solar and wind energy, are greatly needed to integrate into electric girds. Due to the inherent intermittency and fluctuation of the new revewable energy systems, large-scale electric energy storage (EES) is urgently demanded for efficient utilization of the new revewable energy systems. Among these new energy systems, electrochemical rechargeable batteries are most convenient and portable choices. Recently, Sodium-ion (Na-ion) batteries have been considered as an attractive alternative to their lithium counterparts for EES applications because of their natural abundance, low cost and environmental friendliness. The key technology to enable Na-ion battery applications depends on the development of suitable Na-storage materials, especially the novel anodes with high capacity and excellent cyclability. Herein, this thesis focused on the exploration and development carbon-based anode materials with high capacity and long cycle life, including various hard carbons, high-capacity alloy@carbon composites and metal oxides@carbon composites. In particular, we emphasized on the Na storage mechanism of hard carbons and the formation mechanism of the carbon-based microsphere structure, thus providing experimental experiences and theoretical knowledge for the development of carbon-based anode materials. The main research contents and innovative results are as follows:1. We prepared various hard carbon materials from different carbon sources and investigated their electrochemical Na-storage properties. Firstly, the widely distributed and low-cost cellulose was selected as hard carbon precursor. By optimization of pyrolysis conditions (the heating rate:1?/min; the pyrolysis temperature:1300?; the insulated time:2 h), the as-prepared hard carbon demonstrated best Na-storage performance with specific capacity of 320 mAh g-1 at a current density of 20 mA g-1, and 214 mAh g-1 at 200 mA g-1, and excellent cycling stability with an 83% capacity retention over 200 cycles. Secondly, we synthesized a series of hard carbons from different precursors by using the as-optimized pyrolysis condition, and investigated and compared their electrochemical performance from different structural hierarchy, such as molecular level (?A), micro-structure level (-nm), and macro-morphology level (-?m). Electrochemical results showed that the structural hierarchy influenced their Na-storage performance differently. To our knowledge, the precursors with macro molecular structures are beneficial for the improvement of electrochemical capacity, while microchannels and micropores have inferior impact on the specific capacity, which might be related to the missing of graphene layers. Besides, well-defined morphology or molecular arrangement can improve the Na-storage performance to a certain extent.2. We investigated the Na-storage mechanism of hard carbon by using cellulose-pyrolyzed carbons treated at different temperatures. XRD, TEM, BET, CV and CD (charge-discharge) experiments were carried out to characterize the reaction mechanism at the "sloping" region and the "plateau" region during the whole electrochemical Na-storage process. Results demonstrated that the slopping region at higher potential corresponds to the Na-ion adsorption on the active sites of the carbon surfaces, whereas the plateau region at lower potential corresponds to the Na-ion intercalaction into the graphene sheets. Moreover, we also studied the electrochemical Li ion insertion behaviors into cellulose-pyrolyzed carbons, and found that much smaller Li ions could not intercalate into graphene sheets to form stable graphite intercalation compound (GIC). These results clarify the previous misunderstandings on the Na-storage mechanism of hard carbons, and providing valuable experimental experiences to develop superior hard carbon materials.3. In pursuit of higher capacity anode materials, we successfully synthesized alloy-type Sb@C and Sn@C microspheres in non-aqueous systems, on basis of previously developed solvothermal method to synthesize hard carbon microspheres. Physical characterizations revealed that Sb nano-particles (-20 nm) were homogeneously encapsulated in carbon microspheres. The as-prepared Sb@C microspheres electrode demonstrated superior electrochemical performance with a high capacity of 640 mAh g-1 at 0.1 C (1C= 600 mA g-1), and an excellent capacity retention of 92.3% over 300 cycles at 3C rate. Such impressive Na-storage properties mostly likely resulted from the even dispersion of Sb nanocrystals in the carbon matrix, thus leading to the increase of the reversible capacity due to high electrochemical utilization of Sb. Furthermore, the unique microsphere structure could alleviate mechanical stress that brought from the volume expansion during Na alloy/dealloy process, and effectively suppress the electrode pulverization or aggregation, contributing to the structural stability during repeated Na insertion/extraction process. To understand the formation mechanism of the monodispersed Sb@C microspheres, we designed comparative experiments by using different raw materials. A facile self-catalyzing mechanism has been disclosed by us to explain the possible reaction process:furfural firstly polymerized under solvothermal conditions to produce H2O, which could induce the hydrolysis of Sb3+to generate H+. And then the generated H+ could further promote the polymerization of furfural to produce H2O in return. This continuous self-catalyzing reaction ensured the homogeneous dispersion of metal nanocrystals in hard carbon microsphere matrix. At the same time, Sn@C microspheres have been successfully prepared by applying the self-catalyzing mechanism in the SnCl4-furfurnal system, and demonstrated adequate electrochemical Na-storage performance. These results confirmed the effectiveness of the solvothermal self-catalyzing mechanism in synthesize alloy anode materials using hydrolysable metal salts as precursors, thus offering an appealing choice for developing high capacity and long cycle alloy anodes.4. We prepared metal oxides@carbon (TiO2@C and SnO2@C) microspheres composites by exploiting the solvothermal self-catalyzing method, and investigated thoroughly the formation mechanism of the hierarchical hollow TiO2@C microspheres, further confirming the generality of this self-catalyzing approach. The TiO2@C microspheres consist of hierarchical yolk-shell structures made up of carbon-coated TiO2 nano-particles. Electrochemical results showed that the TiO2@C electrode displayed superb Na-storage properties with high capacity of 210 mAh g-1 at 0.1C (1C=200 mA g-1), excellent rate performance of 36% capacity retention at 40C rate, and impressive cycle life of 71.6% capacity retention over 3,000 cycles at 1C rate. Such superior Na insertion performance probably resulted from the nano-sized TiO2 particles with elevated insertion reaction kinetics, even carbon coating with enhanced electric conductivity, and yolk-shell structure containing large amounts of meseporous voids with increased reaction interfaces and improved structural stability. At the same time, SnO2@C microspheres have been fabricated and showed good Na-storage performance, with specific capacity of 348 mAh g-1 at current density of 20 mA g-1, and satisfying capacity retention of 82% over 100 cycles at 100 mA g-1. These research results demonstrate that the self-catalyzing method can be used to fabricate metal oxides@carbon microsphers with superior electrochemical performance, such as TiO2@C and SnO2@C composites. Therefore, we propose a novel and facile method to develop MOx@C microspheres using hydrolysable metal salts as raw materials. Besides, the morphologies features of the MOx@C products differ with Mn+species, which might be related to the hydrolysis behavior of Mn+ions or system acidity. |
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