| In recent decades,lithium-ion batteries(LIB)have been one of the best power supply devices in the field of portable electronics and electric vehicles.However,the reserves of lithium resources are not abundant,and they are unevenly distributed around the world,which casts a shadow over the large-scale application prospects of lithium-ion batteries in electric vehicles and hybrid vehicles.In this case,sodium ion battery(SIB)as an effective alternative to LIB has attracted widespread attention in the industry,because sodium is the fourth most abundant element on the planet and has similar performance to lithium.However,the larger ion radius of sodium ions directly affects the electrochemical process.Therefore,it is very important to choose high-quality materials as SIB anode materials.Hard carbon has high conductivity,low cost and many advantages of structural diversity.The morphological structure of hard carbon(such as hollow structure,nanosheets)is designed to provide abundant active sites and open ion/electron transport paths,thereby promoting good penetration of electrolyte and reduction of polarization process,and can be expected excellent sodium storage performance.In addition,in order to improve the performance of hard carbon,it is an effective method to incorporate dopants into the carbon matrix.Heteroatom doping can not only improve the electronic conductivity,but also improve the storage characteristics of sodium by providing sufficient active sites and improving the diffusion reaction kinetics.In this dissertation,biomass silver willow blossoms and phloxine are used as the raw materials of hard carbon,the length of the carbon layer,interlayer spacing and doping heteroatoms are adjusted by temperature,and then the structural characterization is used to prove the structural characteristics of the carbon materials,and the most suitable ones are screened out.Under the experimental conditions,the relationship between the structural characteristics of carbon materials and battery performance was further analyzed by electrochemical characterization.The specific results are as follows:(1)In order to develop a new type of hard carbon material for sodium-ion batteries,natural micron-sized fibers silver willow blossoms are used as raw materials to prepare N,S,and P co-doped hollow carbon microtubes(HCMTs)by direct high-temperature carbonization.The HCMT has a nanometer-scale wall with several micrometer-sized hollow cavities,which helps to optimize sodium ion transfer.In addition,temperature-dependent,tailorable interlayer spacing plus doped heteroatoms can provide sufficient energy storage sites.By combining these unique structural advantages,HCMT demonstrates impressive sodium storage performance.After sintered at 1300?,the reversible capacity of HCMT can reach 302 m Ah g-1 when charging and discharging at low current,and the reversible capacity remains at 301m Ah g-1 after cycling(100 times)without significant deterioration.When charging and discharging at a current density(Id)of 1 A g-1 for a long period of time,it can still maintain a high capacity of 201 m Ah g-1,showing good cycle performance.(2)In order to further improve battery performance and explore the effect of heteroatom doping on performance,silver willow blossoms is mixed with sulfur powder,pre-carbonized,pickled,and then carbonized to obtain sulfur-doped hollow carbon nanotubes(S-HCMT).Utilizing the characteristics of sulfur melting at low temperature,it will melt and be embedded in the carbon material when the temperature is raised,and the synthesis process is very simple and convenient.S-HCMT still inherits the hollow structure of silver willow blossoms,which makes ion migration easier,and is doped with rich sulfur elements in the carbon material,which improves the electronegativity of the material while increasing ample reaction sites and promotes ion diffusion and transfer.Thereby increasing the sodium storage capacity.S-HCMT-600 has a capacity of 477.47m Ah g-1 after 31 cycles with Id of 0.02 A g-1.With the Id gradually increasing from 0.02 A g-1 to 5 A g-1,the capacity gradually decayed from 507.61 to214.36 m Ah g-1,and then returned to the initial Id,the capacity still remained at 511.02m Ah g-1,showing good rate performance.(3)In order to study the influence of the morphology and structure of carbon materials on the electrochemical performance of sodium ion batteries,hollow carbon nanospheres(HCNS)were prepared using silica spheres as a template and phloxine as a precursor.Phloxine is a staining agent that forms a thin phloxine layer on the surface of the template during dyeing,which becomes a thin carbon layer after being carbonized at high temperature.The hollow structure of HCNS is conducive to electrolyte penetration and ion diffusion.The ultra-thin carbon ball thickness(?5nm)is conducive to the insertion/extraction of sodium ions into the graphite layer.The highly graphitized turbine layer structure provides sufficient ion storage.HCNS-1300 is different from the rule of low initial coulombic efficiency of ordinary sodium ion hard carbon anode for the first time.It reaches 99.55%,providing a solid foundation for commercial applications of high-performance sodium ion batteries. |
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