Preparation And Electrochemical Performances Of Hollow Carbon Sphere

The inevitable energy crisis drives people to develop various advanced technology,such as electric transportation and smart grid.Rechargeable high energy density batteries are the key to these cutting-edge energy technologies.The traditional lithium-ion battery based on ion intercalation reaction has been used in portable electronic devices for more than 30 years.With the continuous efforts,the capacity of lithium-ion battery has been close to its theoretical capacity.Therefore,in order to break through the theoretical capacity of existing commercial batteries,people pay more and more attention to the storage system that can replace lithium-ion batteries.Due to its high theoretical specific capacity,low cost and environmental friendliness,lithium sulfur batteries are promising to meet the growing needs of the world for advanced energy storage.However,in order to realize commercialization,there are still many problems to be solved,such as low utilization rate of sulfur,serious volume change and polysulfide shuttle effect.Therefore,in this thesis,we design a hollow carbon material to solve these problems in order to improve the performance of lithium sulfur batteries.The main contents are as follow:（1）Nitrogen doped hollow carbon spheres（NHCS）were prepared by polymerization of PDA,carbonization and NaOH etching process.Nitrogen doped hollow carbon nanosphere coated commercial polyethylene polypropylene separator（PP/PE/PP@NHCS）was obtained by a facile blade-coating approach,which was further employed in lithium sulfur batteries with conventional sulfur/carbon cathode and lithium anode.The unique hollow sphere morphology and rich nitrogen impurity atoms on the NHCS coating have good physical and chemical restrictions toward polysulfide.It can effectively inhibit the shuttle effect of lithium polysulfides and further improve the utilization of sulfur.Furthermore,PP/PE/PP@NHCS separators show excellent ion diffusion and fast reaction kinetics.The lithium sulfur batteries assembled with PP/PE/PP@NHCS separators have an initial discharge capacity of1355 mAh/g at 0.2 C.and display discharge capacities of 461 mAh/g and fading rates of 0.049%per cycle after 1000 cycles at 2 C.（2）Using dopamine hydrochloride as carbon source,nickel acetylacetonate as nickel source and SiO₂ nanospheres as template,nickel/nitrogen co-doped hollow carbon nanospheres（Ni-NHCS）were prepared by self-polymerization,carbonization and etching process.Ni-NHCS/S cathode was prepared by 155℃ melting method.Rich nitrogen impurity atoms and elemental nickel can not only chemically adsorb polysulfides,but also catalyze and accelerate reaction kinetics of sulfur.Therefore,the lithium sulfur batteries assembled by Ni-NHCS/S show good rate performance and cycle performance.The Ni-NHCS/S-assembled Li-S batteries display discharge capacities of 484 mAh/g and fading rates of 0.11%per cycle after 500 cycles at 1 C.