Microstructure Regulation Of Functional Carbon Materials For Sodium Storage

As a new type of secondary battery,sodium-ion battery（SIBs）has great application prospect in the field of large-scale energy storage due to its abundant resources and low cost.The development of high performance and low cost carbon anode materials is the key to commercialization of SIBs.Carbon materials are widely accepted as promising candidates for anode material of SIBs due to their chemical stability and electrical conductivity,but their capacity and cycling stability are still unsatisfactory.In addition,the precursor selection of carbon anode materials is important.Pitch is an abundant and inexpensive carbon precursor with a high carbon content,making it an excellent choice for carbon materials.However,pitch-based carbon materials tend to graphitize during heating treatment,resulting in a regular crystal structure,small interlayer spacing,and few active sites.In order to develop high-performance,this study aimed to construct highly disordered carbon materials,mainly selecting Magang pitch and ferric citrate as precursors,and controlling the microcrystalline structure disorder of carbon materials through surface modification of pitch,inhibition of graphitization,construction of hybrid microcrystalline structure,and high-temperature heat treatment.A series of high-performance carbon anode electrode materials were prepared,and the main research content and conclusions are as follows:（1）Using Magang pitch as the carbon source,high-performance pitch-based carbon anode electrode materials were prepared by modifying the thermal decomposition carbon of Magang pitch with ammonium hexafluorophosphate.Experimental and theoretical studies reveal that F-element exists in the form of a chemical bond CF_x,which produces coexistence of graphite and defective nanodomains.In addition,the formation of CF_x promotes the generation of hybrid microcrystalline structures and increases the microcrystalline disorder of pitch-based carbon materials.PAHC/NH₄PF₆ exhibits excellent electrochemical sodium storage performance,providing an ultra-high capacity of 450 m Ah g^-1,far exceeding most SIBs carbon anodes currently used,and exhibiting excellent cycling and rate performance.After 1000 cycles at a large current of 2 A g^-1,it can still maintain a high specific capacity of 300 m Ah g^-1,with a capacity retention rate of 96%.Theoretical calculations attribute this high performance to a new Na storage mechanism,in which Na can be accommodated on each F-decorated host site in the form of clusters rather than individual ions,indicating that the pitch-based carbon material prepared by treating with ammonium hexafluorophosphate is a potential sodium-ion battery anode electrode material with no industrial acid-base problems.（2）High-performance pitch-based carbon anode materials（Mn-SGC）were prepared by using Masteel pitch as a carbon source and using metals to inhibit the graphitization of Masteel pitch during heat treatment.The abundant active oxygen-containing functional groups in the pitch were utilized to complex with Mn Cl₂ during the heat treatment process,which in turn inhibited the aggregation of the pitch.Compared with pure pitch carbonization,the layer spacing of Mn-SGC increased from 0.334 nm to 0.39 nm,and the intensity ratio of D-peak to G-peak（I_D/I_G）of Raman spectrum increased from 2.5 to 2.9.As anode electrode material for SIBs,Mn-SGC exhibited excellent electrochemical performance,with a sodium storage capacity increased from 70 m Ah g^-1 for pure pitch thermal decomposition carbon（PAHC）to 390 m Ah g^-1.Additionally,Mn Cl₂ treated pitch-based carbon materials also have excellent cycling stability and rate performance.The capacity retention rate was 91.6%after 1000 cycles at 2A g^-1,which makes it an ideal anode electrode material for SIBs.（3）N-doped porous carbon materials（NDC）were successfully constructed based on the closed pore sodium storage model,and the effects of different carbonation temperatures on the structure,components and properties of the materials for sodium storage were investigated.Among them,NDC-900 has the best performance of 170 m Ah g^-1 at 5 A g^-1 after 1000 cycles.The structure-activity relationship study showed that abundant amorphous carbon structure,graphite microcrystalline region and porous pore structure in NDC-900 material could cooperatively improve the sodium storage performance.This unique microstructure with multiple cavities can be used as a reference for other carbon materials.