Structural Regulation And Sodium Storage Performance Of Hollow Carbon Sphere Anode Materials For Sodium-Ion Batteries

Lithium-ion batteries（LIBs）will not be able to meet the demand for the future commercialization,with the fast growth of consumer electronics product and electric vehicles.Sodium-ion batteries（SIBs）have thus emerged as a very potential candidate for energy storage system,due to the similarity in the physical and chemical properties of Na element and Li,as well as the advantages of low price and rich resources of sodium.The working principles of SIBs and LIBs are similar,which favors the significant progress that has been achieved in developing cathode materials for SIBs.However,the graphite anode material that has been industrialized in LIBs has a minimal sodium storage capacity when used in carbonate-based electrolytes for SIBs.As a result,there are still several obstacles to overcome regarding SIBs’anode materials towards their commercialization.Hard carbons have been considered the most promising anode materials for novel SIBs because of their high specific Na storage capacity and low Na storage potential,whose electrochemical performance is mainly affected by microstructure such as layer spacing,defects,and pore structure.However,reasonable design of the microstructure of carbon materials still faces enormous challenge.In addition,its poor rate performance limits the development of high-power devices.To solve the problems,hollow carbon spheres are an effective solution for improving the electrochemical performance for SIBs,thanks to their high active surface and short diffusion path for sodium ions.Therefore,this thesis foucuses on the design and preparation of microstructural hollow carbon spheres to improve the electrochemical performance for SIBs.Additionally,the microstructure was analyzed by characterization,and the sodium storage kinetics was invested to explore the impact of the microstructure on the electrochemical performance.The specific details of the thesis are presented in the following sections.（1）Preparation and sodium storage properties of porous hollow carbon spheresPorous hollow carbon spheres（HPCS）were prepared via the hard template method using Pluronic F127 as a pore-forming agent.Through the introduction of an appropriate amount of F127,the material produced a small amount of macroporous structure and possessed more C=O functional groups,providing more active sites for sodium storage.The reversible specific capacity can be retained at 258.5 m Ah g^-1at 0.1 A g^-1after 100 cycles,and a capacity retention rate of 96.62%even at a high current density of 2 A g^-1after500 cycles.In particular,the capacity was maintained at 233.64 m Ah g^-1at a large current density of 30 C(10 A g^-1),and only decays by 10.24%compared to that measured at 0.3 C(0.1 A g^-1),the rate performance is superior to most previously reported hollow carbon spheres anode materials.The GITT,DSCV,and ex-situ Raman results demonstrate that were used to explore the kinetics and mechanism of sodium storage.The excellent electrochemical performance of HPCS was attributed to larger defects and abundant C=O active functional groups,providing more active sites for sodium storage and acting as an elastic buffer for volume change during charge and discharge,respectively.The hollow carbon spheres and macroporous structure also accelerated ion diffusion rate and electron transport,contributing to the impressive performance of the materials.The synthesis route of these HPCS may provide an effective strategy for designing and preparing high-rate-capacity carbon nanomaterials for SIBs.（2）Regulation of shell thickness and sodium storage properties of hollow carbon nano-spheresA series of hollow carbon nano-spheres（HCS）with varying carbon shell thicknesses were synthesized using the hard template method and the carbon precursors.These materials were investigated to examine the relationship between their microstructure and electrochemical performance for sodium storage.A moderate carbon shell thickness resulted in a suitable mesoporous structure with low defect content and a specific surface area of 88.53 m²g^-1,which displayed excellent electrochemical properties.The reversible specific capacity is 227.2 m Ah g^-1,and the initial coulombic efficiency（ICE）is74.35%at 0.1 A g^-1.The capacity retention rate was up to 97.38%after 500cycles at a high current density of 2 A g^-1,and the average capacity decay rate was only 0.00524%.In particular,the HCS with the larger carbon shell thicknesses showed better rate performance,and a reversible specific capacity of 134.7 m Ah g-1 at a current density of 10 A g-1.The kinetics of sodium storage were investigated using GITT,DSCV and EIS,explaining the high-rate performance.The HCS material with larger carbon shell thicknesses showed increased pore volume(2.019 cm³g^-1)and defects,facilitating better electrolyte penetration and Na⁺diffusion.This result demonstrates a useful strategy to regulate and design carbon materials’microstructures.