Research On Preparation And Electrochemical Performance Of Three Dimensional Carbon Materials For Energy Storage Devices

Recently, with the increased demands and attentions on energy and environmental issues, researchers have been working on new clean and sustainable energy sources to replace traditional fossil fuels. Among them, electrochemical energy storage systems are becoming attractive. Furthermore, Lithium-sulfur battery is considered as one of the most attractive devices of the next generation electrochemical energy storage systems. In addition, they believe that the positive electrode material sulfur "captured" into the carbon material with a certain nanostructure to inhibit the shuttle effect is an effective way to solve the commercial issues of lithium-sulfur battery. On the other hand, as the preferred commercial anode material for lithium-ion battery, carbon-based materials are increasingly unable to meet the commercial needs, due to the low capacity and poor rate capability. In this thesis, we synthesized several carbon materials with three-dimensional structures and applied these materials into lithium-sulfur batteries and lithium-ion batteries to improve the battery capacity and cycle performance. This thesis mainly completed the following works:Firstly, we used bark of plane tree as renewable carbon source to obtain an integrated three-dimensional structure, and the carbon material also has a large surface area of 528 m2/g. We made the cathodes without any additive but have a high loaded sulfur. As a result, the integrated carbon-sulfur cathode delivered an initial discharge capacity of 1159 mAh/g at 0.2 A/g for lithium-sulfur battery. Even after 60 cycles, a high specific capacity of 608 mAh/g with a high Coulombic efficiency (>98%) was retained, much better than the capacity of 410 mAh/g for the cathode with macropore-destroyed structure.Secondly, we reported a one-step approach for synthesizing in situ N-doped hollow carbon nanospheres (N-PHC). This method used melamine as the nitrogen and carbon source and Co(CH3COO)2·4H2O as the catalyst precursor. And the hollow carbon spheres have both micro and mesoporous structure. We combined the sulfur with N-PHC through heat infiltration and assembled lithium-sulfur batteries, then characterization and testing. The results showed that the structure of the carbon material can well adsorbed sulfur and the intermediate products that effectively improve the capacity and cycle performance of lithium-sulfur batteries. The N-PHC/S cathodes delivere a highly initial discharge capacity of 1164 mAh/g at 0.5 C for lithium-sulfur battery. Even after 150 cycles, a high specific capacity of 615 mAh/g could be obtained, indicating a capacity loss of 0.35% per charge-discharge cycle.Finally, we used tetrabutyltitanate as crosslinking agent, melamine as the carbon source to obtain titanate-crosslinking N-rich carbon hybrids by solid-phase synthesis. The Ti-stable and N-rich framework provides stable 3D channels for lithium-ion transport. Moreover, electrochemical measurements demonstrate that this hybrid can delivered a reversible capacity of 523.3 mAh/g at the high rate of 2.0 A/g, much higher than theoretical value of graphite. This hybrid anode also exhibits superior high-rate capability and cycling performance.