The Study Of Synthesis,structure Modifications And Sodium Storage Performance Of Hard Carbon Anode For Sodium Ion Batteries

Hard carbons have received considerable attention as anode materials for sodium-ion batteries（SIBs）.However,it is still a great challenge to improve the plateau capacity and initial coulombic efficiency（ICE）of hard carbons.The sodium storage performance of hard carbon is closely related to its structure.In this work,three different hard carbon materials were synthesized.The influence of the internal structure on the storage performance of sodium was analyzed,and the mechanism of sodium storage was explored.Firstly,the phenolic resin porous carbon（PR）was produced by the reaction of resorcinol and formaldehyde and used as a precursor.PAN was chosen as the coating carbon source to adjusted the specific surface area and closed pore volume of the PR carbon.When the coating ratio was 1:1 and the pyrolysis temperature was 1300℃,the specific surface area of sample 1:1-1300 was as low as 10.76 m² g^-1.The electrochemical tests show that the ICE can reach 83.69%and the reversible specific capacity was 285.5 m Ah g^-1,in which the platform capacity accounts for 71%.Based on the above results,a facile vapor carbon coating method was further developed to modify the surface structure of carbon spheres via the pyrolysis of polypropylene（PP）,aiming to seal the exposed pores and adjust the structure through a high-quality carbon layer.The obtained HC@3%PP exhibits highly improved sodium storage performance which delivers a capacity of 309 m Ah g^-1 and an ICE of 88.72%.Further research shows that the improvement of electrochemical performance is mainly ascribed to the reduction of the surface defects,the low specific surface area（4.4 m²/g）,and the increased closed pores after the PP coating.According to the ex-situ XRD test,no characteristic peak shift is observed,indicating that the platform capacity is not contributed from the Na⁺insertion into graphitized layer,but more depending on the deposition of sodium clusters in the closed pores.Thirdly,a hard carbon material with low specific surface area was prepared by mixing sucrose with PAN.The specific surface area of the carbon material is only 10.1m²/g when the mixing ratio is 2:1,and the ICE is as high as 86.61%.It can deliver a reversible specific capacity of 283.3 m Ah g^-1 at the current density of 25 m A/g,and retain 82%after 100 cycles.The results in this thesis indicate that the porous structure of hard carbon shows a very important effect on its sodium storage performance.Modification of the surface and interior structure by high-quality hard carbon can reduce its specific surface area and defects,and increase the closed pores volume.Accordingly,it is helpful to improve the ICE,kinetic performance and sodium storage capacity of hard carbon anodes.The work in this thesis provides a facile route to design and synthesize the high-performance sodium hard carbon anodes.