The Structure Design And Energy Storage Properties Of S-doped And Sulfide Composite Carbon-based Material

lithium-ion batteries?LIBs?with a lot of advantages,such as high output voltage and high energy density,are used in mobile electronic devices.In recent years,the application ranges of LIBs have been further expanded with the development of electric vehicle industry.However,limited lithium resources and high prices block the development of LIBs.Sodium and potassium are the same alkali metals as lithium,they have similar physical/chemical properties and they are rich in resources.Therefore,sodium-ion batteries?SIBs?and potassium-ion batteries?PIBs?are the new rechargeable batteries that are most likely to substitute for LIBs.Graphite is a successful commercialized anode material for LIBs.However,graphite is not suitable for Na⁺/K⁺storage and receive a low capacity.Amorphous carbon is mostly rich in defects and microporous structures,resulting in reversible storage of Na⁺/K⁺.The heteroatom modification for carbon-based materials is an important factor for achieving high capacity and high reversibility.In addition,transition metal sulfides are also considered as potential electrode materials for replacing carbon-based materials due to their high capacity.However,the sulfides have the low conductivity and the obvious volume effect,resulting in the electrode destruction and the capacity degradation.Therefore,structural modification of sulfides,such as carbon materials coating,heteroatoms doping,and building nano-structures,is effective way to improve their electrochemical performance.In order to synthesize the anode materials with high performance to storage Li⁺/Na⁺/K⁺based on the above considerations,this paper studies the design,synthesis and energy storage mechanism of porous carbon spheres and carbon fiber composites from the sulfur doping and sulfide modification,and explores the performance of the composites as the anodes in full-cells system.The main results are as follows:?1?Carbon materials have poor rate performance and low reversibility as SIBs anodes.In Chapter 2,the carbon microsphere was synthesized by hydrothermal glucose solution,TiO₂ coating,carbonization,and S-doping to obtain TiO₂ coated and S-doped amorphous carbon spheres core-shell structure?CNF-S@TiO₂?.This material showed excellent and reversible capacities of 768 and 480?mAh?g^-1?higher than the theoretical values of both graphite and TiO₂?for Li⁺and Na⁺storage at 100 mA g^-1,respectively.The good electrochemical performance is attributed to the surface ions storage due to the presence of porous structure and sulfur-doing.It is also demonstrated that the S-doping can effectively increase the binding energy of Na⁺on the surface of graphite,and promotes the Na⁺/Li⁺storage on the surface of porous carbon,resulting in a capacitive effect.In addition,TiO₂ nanocoatings are benefit to the storage stability of Li⁺/Na⁺in the S-doped carbon-based composites,resulting in a ultralong cycling life.The synergistic effect of S-doped carbon and TiO₂ dense nanolayers significantly improves the Li⁺/Na⁺storage capacity and cyclic stability of carbon-based materials,which provides a new idea for the research of supercapacitors and related devices.?2?An appropriate structural building and heteroatom doping are the key points to improve the capacity and stability.In order to further improve the energy storage performance of carbon-based materials,in Chapter 3,carbon-based nanofibers were prepared via S-doping and the introduction of ultrasmall TiO₂ nanoparticles into the carbon-based fibers?CNF-S@TiO₂?.It was discovered that the introduction of TiO₂ into carbon nanofibers can significantly improve the specific surface area and microporous volume for carbon materials,which will promote electrolyte infiltration and ions diffusion.In addition,porous structure and high specific surface area will produce surface ions storage effect?capacitive effect?,which can be quantified based on CV scans.By controlling TiO₂ content to obtain the CNF-S@TiO₂-5 material applied in SIBs/LIBs with optimized electrochemical properties.During the charge/discharge process,the S-doping and the incorporation of TiO₂ nanoparticles into carbon-fibers promote the insertion/deinsertion of the ions and enhance the capacity and cyclic life.At the current density of 2 A g^-1,the reversible capacity of CNF-S@TiO₂-5 for Na⁺storage can be maintained at 300 mAh g^-1 after 600 cycles.Moreover,the capacity-retention of such devices is 94%,showing their high capacity and good stability.In addition,the CNF-S@TiO₂-5//Na₃V₂?PO₄?₃ Na-ion full cells were successfully established to promote the practical application of S-doped carbon-based composite materials,and the devices show good performance in energy density cycling stability.?3?Although transition metals sulfide has a high theoretical capacity,they have low electrical conductivity and severe volumetric effects during electrochemical processes.Therefore,the stability and reversibility are the crucial problem to be overcome.Based on the excellent structural properties and good conductivity of carbon materials,in Chapter 4,a simple method was employed to prepare the graphene-coated FeS₂ which was embedded in carbon nanofibers.The dual protection of graphene and carbon fiber can significantly improve the electrical conductivity of FeS₂ and prevent the degradation and inactivation of the electrodes in the electrochemical process,thus improving the reversibility and stability of FeS₂@G@CNF for SIBs and PIBs.When used as SIBs anode materials,the capacity can maintain at 305.5 mAh g^-1 at 3 A g^-1 after 2450 cycles under room-temperature.As for K⁺storage,it displayed a capacity of 120 mAh g^-1 at 1A g^-1 after 680 cycles at room-temperature.Benefitting from the enhanced conductivity and protection on graphene encapsulating,especially,the double carbon protected FeS₂nanoparticles promote good Na⁺storage properties under low temperature?0 and-20°C?.At a current density of 100 mA g^-1,the reversible capacity of FeS₂@G@CNF can reach 420 and 340 mAh g^-1 at 0°C and-20°C,respectively,and it also can tolerate the drastic temperature changes in low temperature and room temperature range.In addition,the Na-ion full-cells based on FeS₂@G@CNF and Na₃V₂?PO₄?₃ can show a reversible capacity of 95 mAh g^-1 at room-temperature.Moreover,the Na-ion full-cells deliver promising capacity and high energy density at low-temperature.DFT calculations implied that graphene encapsulating can effectively decrease the barrier energy between FeS₂ and graphene heterointerface,resulting in an enhanced reversibility of Na⁺storage in FeS₂.?4?Overcoming the disadvantages,such as the pulverization of electrode material and low stability,of sulfides caused by low conductivity and volume change during electrochemical process,the key points are proper structural design and composite of conductive carbon-based materials.In Chapter 5,nickel acetate is used as a catalyst carrier,melamine is used as a carbon source,and a metal-carbon nanotube composite material is grown at a high temperature,and then an in-situ sulfidation is performed to grow carbon nanotube coated NiS nanoparticles?NiS@CNT?.The hollow structure and strong mechanical properties of carbon nanotubes can endure volume change of NiS nanoparticles during charge/discharge process,which overcomes the pulverization and inactivation of NiS nanoparticles.In addition,the high conductivity of carbon nanotubes greatly improves the electron transport of NiS.Moreover,the high specific surface and porous properties of the NiS@CNT materials can promote electrolyte infiltration and ions diffusion,and this structural property also brings an extremely rich interface to promote surface storage effects?capacitive effect?of Na⁺.As a result,NiS@CNT-B displays a reversible capacity of 416.8 mAh g^-1 at 100 mA g^-1,and it also shows good electrochemical performance under low temperature.Especially,the NiS@CNT-B//Na₃V₂?PO₄?₃ Na-ion full cell also shows its good application prospect.