Design And Synthesis Of Carbon-Based Hollow Nanostructures For Lithium Storage

Energy and environment issues become the bottleneck problems for sustainable development, particularly in China. Clean energy technologies will be more important in the future than at any time in the past. Among the myriad energy-storage technologies, lithium secondary battery is the most feasible green chemical energy storage technologies. New generations of such batteries will electrify transport and find use in electricity storage. However, even when fully developed, the highest energy storage that Li-ion batteries can deliver is too low to meet the demands of key markets. It requires the exploration of new electrochemistry and new anode materials. Rechargeable Li-S batteries and Li-air batteries with ultra-high theoretical energy density, low cost and environmental friendly make its possible to satisfy this increasing demand for high-performance power sources, which use sulfur and oxygen as the cathode reactants of lithium secondary battery, respectively.Li-S battery is one of the energy storage devices based on the reversible electrochemical reaction between lithium metal and elemental sulfur. When fully utilized, its energy density is 2-4 times higher than that of currently commercial lithium ion batteries. As the Li-air battery cathode reactant, oxygen can be acquired from air directly, which effectively reduces the cost and improves energy density. Given the advantage in energy density, Li-air battery is considered to be the next energy storage system substituting the combustion engine. However, the development of new type of lithium secondary battery technology is at its infancy, various problems needed to be resolved including reaction kinetics, mass transfer kinetics and thermodynamics. In this study, we focus on the design and synthesis of carbon based hollow nanostructures as high performance electrode materials for lithium secondary batteries. Benefiting from the interaction and confinement effect of composites, the electrochemical performance is significantly improved. The detailed work is as follows:SnO2/C hollow nanospheres with uniform size have been succesfully constructed with SiO2 nanospheres templates. The hollow nanostructre with tens of nanometer-thick SnO2/C shell is promising materials for hosting sulfur. The SnO2/C shell can effectively imhibit the diffusion of dissolved polysulfides, and thus increase the lifespan and rate performance of sulfur cathode. At a current rate of 0.5 C, the initial discharge specific capacity can reach 896 mA h g-1. At high rate of 2-5C, the electrode exhibit stable capacities of 300-396 mA h g-1 for as long as 400 cycles.Fe-filled carbon nanotubes (Fe@CNTs) have been developed through one-pot synthesis, which allows scalable synthesis with good reproducibility. Benefiting from the high oxygen reductiong reaction activity causing by the electron transfer from Fe NPs to carbon layer, the morphology and distribution of Li2O2 on the cathode can be optimized. When evaluated as a catalyst in a rechargeable Li-air battery, the Fe@CNTs composite demonstrated high catalytic activity for both the ORR and OER. The cell showed excellent electrochemical characteristic, such as lower overpotential, good rate performance and comparable cycling performance. The specific capacity can reach 2700 mA h g-1 at a current density of 1000 mA g-1. By restricting the discharge capacity of 1000 mA h g-1, the capacity of the Li-air cell can be constant over 50 cycles.