Microstructural Regulation Of Anthracite-based Porous Graphitized Carbons And Their Lithium Storage Behavior

The clean and efficient utilization of coal is an important content of the national energy development strategic goals of“Emission Peak”and“Carbon Neutrality”,and the materialization of coal is one of the effective ways to realize its low carbon and high-value utilization.As a green energy storage device,lithium ion batteries(LIBs)are widely used in portable electronic devices,electric vehicles and static energy storage systems.The microstructure of anode materials is a key factor for affecting the electrochemical performance of LIBs.To address the problems of low reversible capacity and poor rate capability for commercial graphite anode,it is of great significance to develop new high-performance carbon materials to replace traditional graphite anode.In this paper,based on the characteristics that anthracite is rich in graphite-like aromatic lamellae and has well-developed native pores,anthracite-based porous graphitized carbons with graphite microcrystals as skeleton were constructed using Taixi anthracite as raw material by reasonably regulating carbonization-graphitization temperature and purposefully introduce nanopores into graphite microcrystal.The microstructural regulation mechanism of various porous graphitized carbon including coal-based porous carbon nanosheets and coal-based graphene nanosheets and the intrinsic influence of microstructure on the eelectrochemical properties for porous graphitized carbon anode are systematically investigated for providing theoretical basis and technical support for the research and development of high-performance anode materials and the low-carbon and high-value utilization of coal resources.The main conclusions are as follows:(1)The evolution behavior of microstructure in anthracite during carbonization and graphitization(1000-2800 ~oC)was systematically studied,and the evolution law of aromatic lamellar structure in anthracite during high-temperature heat-treatment process was revealed.The evolution of aromatic lamellar in anthracite can be divided into four stages:1)graphite microcrystal formation stage(500-1000 ~oC),which mainly involves the removal of aliphatic chains and oxygen-containing functional groups in anthracite,and disordered aromatic lamellar gradually transformed into ordered graphite microcrystals;2)graphite microcrystal stacking stage(1000-2000 ~oC),in which graphite microcrystals are gradually stacked with decreasing layer spacing due to the enhancement of van der Waals forces between the layers,and the stacking thickness and orderliness increases significantly;3)quasi-graphite phase formation stage(2000-2200~oC),carbon-carbon chemical bond appeared,and the lateral size of graphite microcrystals increased rapidly,which promoted the transition of ordered graphite microcrystals into quasi-graphite phase;4)graphite-like phase formation stage(2200-2800 ~oC),under the combined action of van der Waals force and carbon-carbon chemical bond,the stacking thickness and lateral size of quasi-graphite lamellar gradually increase,and eventually evolve into the dense coal-based coal-based graphite with densely stacked microcrystal layers.On this basis,the influence of the defect structure and graphite microcrystals on the lithium ion storage performance is further studied,and it can be found that the reduction of defect structure reduces the"adsorption"lithium ion storage in anode,while the increase of graphite microcrystal lateral size is beneficial to the"embedding"lithium ion storage.(2)Coal-based porous carbon nanosheets(CCNSs)with abundant nanopores and reasonable layer spacing were prepared by modifying coal-based graphite using a liquid-phase oxidation-thermal reduction method.With the synergistic effect of porous structure and reasonable layer spacing,the reversible capacity of CCNSs anode reaches up to 917 m Ah/g,which is 2.6 times of that of coal-based graphite,and the CCNSs anode have excellent rate capability and cycling stability.On this basis,N/P co-doped coal-based porous carbon nanosheets(N/P-CCNSs)were prepared by co-assembling method using melamine and phytate as modifiers.The formation of C-N and C-P bonds could significantly improve the lithium ion storage performance of N/P-CCNSs anode.Its reversible capacity is up to 1170 m Ah/g,and the N/P-CCNSs anode has a high capacity of 481 m Ah/g at 2.0 A/g,and an excellent cycling stability.(3)A new method was developed to prepare coal-based graphite nanosheets(CGNs)by mechanochemical reaction generated by high-energy mechanical ball-milling using coal-based graphite as raw material,which effectively overcomes the problems of complicated process and environmental unfriendliness for the liquid-phase oxidation-thermal reduction method.The CGNs has a main skeleton formed by small-sized cross-linking graphene sheets,supplemented by porous structure with a specific surface area of 76-99 m~2/g,and the regulation of nanopores and layer spacing in CGNs can be achieved by adjusting ball-milling time.The lithium ion storage performance of CGNs anode is improved,and the reversible capacity is up to 726m Ah/g.On this basis,B-doped coal-based graphene nanosheets(B-CGNs)were prepared by high-energy mechanical ball-milling with boric acid as additive.The presence of boric acid not only relieves graphite layers from being damaged by shear force,but also introduces a small amount of B-containing functional groups into graphite layers.With the synergistic effect of porous structure,reasonable layer spacing and B-containing functional groups,the reversible capacity of B-CGNs anode reaches840 m Ah/g,and the reversible capacity can be significantly increased to 459 m Ah/g in the low voltage range(0.01-1.0 V),and it also has excellent long cycle stability after1000 cycles.(4)Coal-based porous graphitized carbon(HPGC-x)was prepared by a combined chemical activation and high-temperature carbonization/graphitization process.By studying the evolution behavior of microcrystal structure in porous carbon during high-temperature heat-treatment(1000-2800 ~oC),graphite microcrystals in porous carbon grew outward along pore framework in an onion shape,and graphite microcrystals are interlaced and stacked with each other.The reversible capacity of coal-based porous graphitized carbon(HPGC-2800)anode is only 278 m Ah/g,which is lower than that of coal-based graphite,but the HPGC-2800 has a higher rate capability,confirming that ordered graphite microcrystals and abundant interlayer nanopores are beneficial to enhance lithium ion insertion/de-insertion process.(5)Based on the results of intrinsic connection between the microstructure and lithium ion storage performance of coal-based porous graphitized carbon and first-principle calculation,the lithium ion storage mechanism of coal-based porous graphitized carbon can be elucidated.The excellent lithium ion storage performance of coal-based porous graphitized carbon is closely related to graphite microcrystal layers,nanopores and containing N,P and B active groups.Reasonably spaced microcrystal layers can reduce the diffusion energy barrier of lithium ions,strengthen the lithium ion insertion/de-insertion process,improve electrical conductivity and enhance ion transport,thus improving the lithium ion storage capacity and rate capability of anode;abundant nanopores can increase the adsorption capacity of lithium ions and provide efficient ion transport channels,thus improving the lithium ions storage capacity and transport kinetics of anode;suitable N,P,B doping can increase electrochemical active sites,improve lithium ion adsorption capacity,strengthen the affinity between electrolyte and material surface,enhance the stability of SEI film,and so improving the cycle stability of anode.