Preparation And Sodium-storage Performanc Of Heteroatom Doped Carbon Hollow Spheres

Lithium-ion batteries（LIBs）alone cannot meet the growing demand for small or medium to large energy storage applications due to the limited and high price of lithium resources.Sodium is widely distributed in the world,low in price,and has similar physical and chemical properties to lithium.Therefore,it is widely believed that sodium ion batteries（SIBs）have the possibility to replace lithium ion batteries in the field of energy storage.Compared with the cathode material of sodium ion battery,the research of anode material is not mature enough and needs further research and exploration.Because of its advantages of environmental protection,abundant reserves,low price and strong chemical stability,carbon has become a promising anode material in the field of electrode materials for sodium ion batteries.In this paper,carbon hollow spheres with special morphology and structure were prepared by template method and modified by heteratom doping with different kinds of ionic liquids.Specific research contents are as follows:（1）Nitrogen-doped carbon hollow spheres（NHCS）were prepared by deposition of polydopamine on the surface of SiO₂spheres using SiO₂spheres as template and dopamine as carbon source and nitrogen source by the spontaneous oxidation polymerization of dopamine.XRD,XPS,Raman spectroscopy,SEM,TEM and EDS were used to characterize its structure and composition.When NHCS is used as the anode material of sodium ion battery,the specific capacity is 159.2 m Ah g^-1after 200cycles under the condition of current density of 0.1 A g^-1;The specific capacity is 71.2m Ah g^-1after 1000 cycles at the current density of 5 A g^-1.When the current density is0.1,0.2,0.5,1,2,5 and 10 A g^-1,the specific capacity of NHCS electrode is 161,149.1,130.9,112.8,101.6,83.5 and 73.8 m Ah g^-1,respectively.（2）By modifying the NHCS with 1-vinyl-3-ethylimidazoldi-trifluoromethane sulfonimide salt as sulfur source and fluorine source,sulfur,fluorine and nitrogen doped carbon hollow spheres（SFNHCS）were obtained.XRD results show that the layer spacing is 0.397 nm,and the extended layer spacing is conducive to the Na⁺embedding/removal process,so as to improve the electrochemical performance of the anode.SEM,TEM and EDS results showed that sulfur,fluorine and nitrogen were uniformly distributed on the surface of the carbon hollow sphere.The first principles were used to calculate and compare the adsorption energy of SFNHCS and NHCS.The results showed that SFNHCS had better cyclic stability and stronger adsorption capacity for sodium ions.The electrochemical test shows that SFNHCS electrode has more excellent sodium storage performance:it still has the specific capacity of 223.0m Ah g^-1after 100 cycles under the condition of current density of 0.1 A g^-1;The specific capacity of 1000 cycles at the current density of 5 A g^-1is 115.6 m Ah g^-1.The specific capacities of SFNHCS electrodes were 217.2,195.2,168.5,147.6,135.4,123.5 and 117.4 m Ah g^-1at current densities of 0.1,0.2,0.5,1,2,5 and 10 A g^-1,respectively.（3）Boron,fluorine and nitrogen co-doped carbon hollow spheres（BFNHCS）were obtained by the modification of NHCS with 1-butyl-3-methylimidazole tetrafluoroborate as boron source and fluorine source.The XRD results show that the layer spacing is 0.421 nm,and the extended layer spacing will help the Na⁺embedding/removal process,so as to improve the electrochemical performance of the carbon anode.SEM,TEM and EDS results showed that boron,fluorine and nitrogen were uniformly distributed on the surface of carbon hollow sphere.The adsorption energy of BFNHCS and NHCS was calculated and compared by first principles.The results showed that BFNHCS had stronger adsorption capacity for sodium ions.The electrochemical test shows that BFNHCS electrode has excellent sodium storage performance:it still has 255.6 m Ah g^-1specific capacity after 100 cycles under the condition of current density 0.1 A g^-1;The specific capacity of 1000 cycles at current density of 5 A g^-1is 136.5m Ah g^-1.When the current is 0.1,0.2,0.5,1,2,5 and 10 A g^-1,the specific capacity of BFNHCS electrode is 255.6,231.5,197.4,178.2,157.8,134.5 and 116.4 m Ah g^-1,respectively.