Synthesis Of N-doped Active Carbon Materials And Their Applications In Li-S Battery

With the increasing environmental and climatic problems brought by the using of traditional fossil fuels,the importance of developing the next-generation renewable clean energy such as wind and solar energy has become increasingly prominent.Although conventional lithium-ion batteries play an important role in portable power sources,they cannot meet the high energy density requirement of next-generation energy storage systems.Lithium-sulfur battery based on sulfur cathode and metal lithium anode has been regarded as the most promising next-generation energy storage system due to its ultra-high theoretical specific capacity(1675 mAh g^-1)and gravimetric energy density(2600Wh kg^-1).In addition,Li-S batteries have the advantages of sulfur abundance,low cost and non-toxicity to the environment.However,there are still many problems that hinder the further development of Li-S batteries,such as?i?the inherent poor conductivity of sulfur and Li₂S leading to slow chemical reaction kinetics;?ii?the solubility of polysulfides produced in the charge/discharge process causing the shuttle effect;and?iii?the safety problems caused by the use of the metal lithium anode.Among them,the dissolution of the polysulfides is considered to be the major issue to cause poor performance of the battery.To solve the problems mentioned above,much efforts has been made in the design of various materials that could be served as the host of sulfur,such as carbon materials,conductive polymers,metal oxides/sulfides,metal organic frameworks,etc.Those host materials can not only provide a large number of charge/ion transmission channels to enhance both mass and charge transport while improving the utilization of sulfur,but also adsorb the polysulfides by chemical and physical interconnections,effectively alleviating the shuttle effect of polysulfides,thus improving the coulombic efficiency,cycling stability and rate performance of the battery.In particular,carbon materials have been extensively studied as sulfur host owing to their excellent electrical conductivity,good mechanical extensibility,abundant pore structure and adjustable specific surface area.As a typical nanomaterial,active carbon has received great attention for its application in energy storage systems.In addition,nitrogen-doping has been demonstrated to be an effective approach to improve the overall electrochemical performance of Li-S batteries.However,the traditional preparation process of active carbon materials is mostly complicated and expensive.Therefore,it is of great significance to design some facial methods to prepare hetero-doped active carbon materials.Based on the background mentioned above,the thesis work was conducted and the content of this thesis is as follows:1.A brief overview on the background,history,mechanism and current challenges of the Li-S batteries.The current research progress on the cathode,anode,electrolyte and separator of Li-S battery is briefly surveyed,and the advantages and disadvantages of various materials are analyzed and compared.2.The basic principles of various experimental methods including instruments involved in the characterization of electrode materials and their electrochemical properties are briefly presented and discussed.3.Preparation of the 3D N-doped graphene-like active carbon materials?NGC?by using the melamine and L-cysteine as precursors through a solid-state reaction.The NGC-8,NGC-4 and NGC-2 samples with different specific surface area and internal chemical bond components could be obtained by altering the mass ratio of melamine and L-cysteine.The microstructure,elemental composition and internal chemical bond composition of the three samples were determined by XRD,SEM and XPS.The NGC materials was added with polysulfides to load sulfur for preparing an NGC/PS composite electrode for lithium-sulfur batteries.At a current density of 0.2 C,the initial discharge specific capacities of NGC-8/PS,NGC-4/PS and NGC-2/PS electrodes were 1164,964.2 and823.6 mAh g^-1,respectively,and after 400 cycles,the NGC-8/PS electrode is still maintaining a specific capacity of 910 mAh g^-11 with a decay rate of only 0.05%per cycle.The NGC-8/PS electrode also achieved a high specific capacity of 800 mAh g^-1at a current density of 1 C after 500 cycles.The analysis shows that the material has excellent electrochemical performance mainly because?i?its three-dimensional structure can ensure the effective loading of sulfur;?ii?the existence of mesopores and macropores ensures effective electron/ion transport channels inside the electrode material and physical adsorption of polysulfides;?iii?high content of N doping not only provides more reactive sites to promote the redox process,but also provides strong chemisorption towards polysulfides to effectively inhibits shuttle effect.4.Porous active carbon materials?NPC?with different surface areas were prepared through a template method by using chitosan as carbon source.The structure,composition and other information of the samples were analyzed by XRD,Raman,XPS,etc.The NPC/PS electrode exhibits a good electrochemical performance when using as a cathode in lithium-sulfur batteries.At a current density of 0.2 C,it delivers an initial discharge capacity of 1150 mAh g^-1,and there is still a specific capacity of 894 mAh g^-1 retained after 300 cycles.Even at a high current density of 1 C,it shows a high capacity of 784mAh g^-1 with no obviously decay after 500 cycles.Compared with other preparation methods,it has the advantages of high yield,less expense and environmental friendly.In the end of this thesis,conclusions of my research work are given and perspectives for the future development of Li-S batteries are offered.