Preparation Of Coal-Based Carbon Materials And Their Sodium Storage Performance

Sodium-ion batteries（SIBs）are expected to become a substitute for lithium-ion batteries（LIBs）in terms of large-scale energy storage.The anode materials are the key to the commercial application of SIBs.This dissertation is based on the development of high-performance SIBs anode materials,starting with low-cost coal,and preparing coal-based graphite with a higher degree of graphitization through high-temperature graphitization.On the basis of coal-based graphite,the preparation of coal-based graphite materials and their composite materials and the relationship between their microstructure and sodium storage performance are studied.The main research contents are as follows:（1）The method of"low-temperature oxidation intercalation+high-temperature expansion reduction"is used to prepare coal-based expanded graphite materials with adjustable interlayer spacing and specific surface area using anthracite-based graphite as raw material.Extending the low-temperature oxidation intercalation time produces expanded graphite with increased interlayer spacing and fewer graphene sheets stacked.The interlayer spacing of anthracite-based graphite is increased from～0.337 nm to～0.370nm after puffing reduction,which provides a fast path for sodiation and desodiation;The specific surface area is increased to 389 m² g^-1,and the average pore volume is 9.09 nm,which improves the wettability of the electrolyte and increases the effective sodium storage active sites.Anthracite-based expanded graphite exhibits excellent rate performance(The reversible sodium storage capacity is 88 m A h g^-1 at a current density of 10 A g^-1)and cycle stability(Under the current density of 1 A g^-1 for 1000 cycles,140m A h g^-1 can still be maintained,and the capacity retention rate is 70%)when storing sodium.（2）Using an improved Hummers method,and extend the low temperature reaction time,the full peeling of graphene sheets is realized,and coal-based reduced graphene oxide with high defects and a small number of stacked layers is prepared.Under the same preparation conditions,the degree of exfoliation,interlayer spacing,specific surface area and pore size of the graphite flakes increased with the increase of coalification.Anthracite-based graphite is used to exfoliate graphene with fewer graphene sheets（3-5layers）and high defects.Anthracite-based reduced graphene oxide exhibits better rate performance(The reversible sodium storage capacity is 91 m A h g^-1 at a current density of 10 A g^-1)and cycle stability(Under the current density of 1 A g^-1 for 1000 cycles,145m A h g^-1 can still be maintained,and the capacity retention rate is 69%)than coal-based expanded graphite.（3）Using H₂S as a sulfur source,the surface of coal-based reduced graphene oxide is modified by high-temperature etching to introduce active sulfur elements.H₂S can react with C-C/O-C=O to form thiophene-type carbon-sulfur covalent bonds（S-C,S=C）.Sulfur can further increase the defects of coal-based graphite materials,enrich the sodium storage sites,and prevent the reduction of the graphite layer spacing after the removal of oxygen-containing functional groups in the form of occupancy.Sulfur doping can significantly increase the reversible sodium storage capacity of coal-based graphite materials.The initial reversible capacity are increased from 326 and 456 m A h g^-1 of anthracite-based expanded graphite and anthracite-based reduced graphene oxide to 486and 591 m A h g^-1,and under the high current density of 10 A g^-1,the original 88 and 91m A h g^-1 are increased to 149 and 171 m A h g^-1.（4）Sodium borohydride（Na BH₄）was used to reduce the nano-sized metal Sb particles on the surface of anthracite-based reduced graphene oxide in situ to prepare an anthracite-based reduced graphene oxide/antimony（ARGO/Sb）composite material.The role of Sb-O-C exists between the nano-sized metal Sb and ARGO,so that the nano-sized metal Sb particles are stably combined with anthracite-based reduced graphene oxide,and the metal Sb is wrapped in the conductive network of ARGO,which prevents the metal Sb particles from falling off due to volume expansion during the sodium storage process,and the sodium storage performance of the metal Sb is released.The antimony particles are loaded on the surface of graphene,reducing its specific surface area and significantly improving the initial coulombic efficiency of the composite material（from21%to 49.7%）.（5）The sodium storage behavior of coal-based graphite materials is dominated by the capacitance control process.Sulfur doping can increase the sodium storage capacity of the capacitance control process and increase the diffusion rate of sodium ions with rapid kinetics.The alloying/dealloying reaction in coal-based graphene composites includes both the capacitance control process and the diffusion control process.In summary,this dissertation uses coal with different degrees of metamorphism as raw materials to prepare a series of coal-based graphite materials with excellent sodium storage performance,including coal-based expanded graphite,sulfur-doped coal-based expanded graphite,coal-based reduced graphene oxide,sulfur-doped coal-based reduced graphene oxide,and coal-based reduced graphene oxide/antimony composite materials.In addition,the sodium storage mechanism and sodium ion diffusion kinetics have been deeply explored,which can provide an important reference for the development of high-performance sodium ion battery anode materials.