Research On Construction And Electrochemical Energy Storage Of Novel Carbon Nanomaterials

With the global massive consumption of fossil energy,a lot of problems such as global warming and serious environmental pollution have been brought.Futhermore,fossil energy such as fossil fuel,coal and natural gas is non-renewable resources.So,it is urgent to develop and extensively use clean energy.At present,the main clean energy technologies include secondary batteries,fuel cells,solar cells,supercapacitors and so on.Among them,alkali metal ion batteries have broad application range and improvement space due to their advantages of green,environmental protection,high specific capacity and rechargeability.Currently,lithium ion batteries?LIBs?are widely used in portable electronic devices and electric vehicles,and their performance is highly dependent on the capability of the anode materials.The development of new carbon materials and metal oxides or sulfides anode materials has attracted increasing attention because of the growing demand for equipment portability and new energy vehicles endurance.Comparing to commercial graphite limited by lower theoretical capacity(372 mAh g^-1),pyrite?FeS₂?with a large theoretical lithium storage capacity(894 mAh g^-1)is eco-friendly and low-cost,but its poor conductivity and shuttle effect in charge/discharge process restrict the application.On the other hand,lithium reserves in the earth's crust are limited?only²0 ppm?,while the sodium/potassium resources?2.36%and 2.09%respectively?is much higher than the lithium resource.So that the research on sodium or potassium ion batteries?SIBs,PIBs?is booming grew.However,the radius of K⁺and Na⁺ions is both larger than that of Li⁺ions,it is difficult for K⁺and Na⁺to inset into commercial graphite layers.Therefore,developing nano carbon materials with the appropriate structure is of great significance to improve the performace of alkaline ion batteries.To solve the above-mentioned problems,this thesis focuses on two aspects of work,and the main research contents and results are as follows:1.Based on the novel nitrogen-doped carbon nanotubes?Fe/N-CNTs?with unique structure,which filled with iron nanowires/rods,the FeS₂/N-CNTs composite has been prepared through a simple in-situ confined sulfuration process,due to lots of defects in the walls of N-CNTs.The pyrite in FeS₂/N-CNTs composites exists in two forms,one type is a semi-open N-CNTs coated FeS₂ nanowire,the other is FeS₂ nanoparticles adhered to the outer wall of N-CNTs.The content of FeS₂ in the composites can be easily modulated.When using as an anode material in LIBs,the conductivity of FeS₂ is obviously improved by carbon nanotubes,and the shuttle effect as well as volume expansion of FeS₂ during charge and discharge process are effectively inhibited,owing to the abundant active sites of fractured N-CNTs and firm bonds between N-CNTs and in-situ formed FeS₂ nanoparticles.A high discharge capacity(996 mAh g^-1/0.1 A g^-1),good rate performance and excellent cycle performance has been exhibited for the FeS₂/N-CNTs composite anode.2.Nitrogen-doped carbon nanorings?N-CNRs?with unique hollow nano-structure have been successfully fabricated in large quantities,using graphite phase carbon nitride nanorings?g-CNNRs?as template,nitrogen-containing polymers as precursor.The shell thickness,outer/inner diameter and nitrogen content of N-CNRs can be conveniently adjusted by changing polymer species,coating time,and carbonization temperature.As an anode material for PIBs at room temperature,N-CNRs exhibit an ultra-high depotassication capacity of 420.9 mAh g^-1at 50 mA g^-1,and the capacity retention can be kept at 89%in a long cycle test for 2100cycles.This excellent electrochemical performance is mainly attributed to the synergistic effect of chemical composition and strong hollow structure.The unique hollow nanostructure has an increased interlayer distance and a thinner wall thickness,which relieves large inter-layer expansion/contraction caused by potassium ion intercalation/deintercalation,while greatly enhancing potassium transfer kinetics.Not only the penetration of the electrolyte is facilitated,but also the potassium ion diffusion pathway is shortened due to the hollow structure.The synthetic strategy opens a new way for controllable synthesis of hollow nanostructures,which will provide insights into the development of highly active materials for energy storage or catalysis related applications.