| Lithium-ion batteries are widely used in 3C consumer electronics,new energy electric vehicles and renewable energy storage.But there are two factors limiting the further development of lithium-ion batteries.Firstly,the limited lithium resources,unbalanced global distribution and high cost are not conducive to large scale application in energy storage.Secondly,the single anode of graphite is used in commercial lithium ion battery,and its structural characteristics prevent the further improvement of cycle and rate performance.Sodium-ion batteries are expected to replace lithium-ion batteries and solve the problem of large scale energy storage due to the similar energy storage mechanisms,lower costs and abundant resources of lithium.The development of new carbon-based anode materials with novel structure and controllable composition can further improve the cycle and rate performance of batteries.In this paper we focus on the study of microstructural regulation and structure-activity relationship of high-performance anode materials for sodium ion batteries.A one-step method is used to prepare mesophase pitch(carbon microspheres)sulfur-doped hierarchical porous soft carbon materials using cheap and abundant coal and its by-products pitch as precursors.Coal-based and pitch-based graphites are obtained through high temperature graphitization,and then reduced graphene oxide is prepared by thermal reduction.The microstructure and surface chemical properties of sulfur-doped hierarchical porous soft carbon materials,coal-based(pitch-based)reduced graphene oxide and their composites are systematically studied.Different coal-based carbon materials are used as anode materials for sodium ion batteries to explore the influence of microstructure on the electrochemical performance of anode materials and establish the internal relationship between the structure of anode materials and the performance of sodium storage.(1)Sulfur-doped pitch-based hierarchical porous carbon materials and sulfur-doped carbon microspheres hierarchical porous carbon materials are prepared by high-temperature carbonization under nitrogen atmosphere using coal-based mesophase pitch and mesocarbon microspheres as precursors,Mg SO4 as sulfur source.SO42-in Mg SO4 can mildly react with-CHx and C-C bonds in carbon microspheres or pitch to form a new bond C-S at an appropriate carbonization temperature(700?),which greatly improves the doping amount of sulfur and mainly exists in the form of C-S-C active functional groups(accounting for 81.43 at%of the total sulfur content).The reduction-oxidation reaction between sodium ions and sulfur-containing functional groups on the surface of the anode materials not only achieves the ideal pseudocapacitance,but also maintains the structural stability during the cyclic process,and achieves excellent rate and cycle performance.The capacity retention ratio of anode material with sulfur-doped carbon microspheres hierarchical porous carbon is 31.4%from 0.2 to 10A/g.The capacity is stabilized at 291m Ah/g after 60 cycles at 1A/g,and a high reversible capacity of 148m Ah/g is maintained after 800 cycles at 10A/g.(2)Graphite oxide sheets are prepared by modified Hummers method using graphitized coal-based mesophase pitch as precursors.The multilayer reduced graphene sheets are obtained after thermal reduction treatment.Phosphorus-doped pitch-based multilayer reduced graphene oxide sheet composite nanomaterials are prepared by steam intercalation,cooling condensation of red phosphorus under vacuum,together with hydrogen sulphide etching.The C=C(52.3 at%of the total surface carbon content)and-SOX(63.9 at%of the total surface sulphur content)contents are greatly enhanced by hydrogen sulphide etching,which further enhance the electrical conductivity and improve the wettability of the material to the electrolyte,thus enhancing the sodium storage capacity of the pitch-based multilayer reduced graphene oxide sheet/red phosphorus composites.At current densities of 0.2 A/g,0.5 A/g,1 A/g,2 A/g,5 A/g and10 A/g,the reversible capacities are 623,551,441,400 and 278 m Ah/g,respectively.The anode capacitance control of pitch-based multi-layer reduced graphene oxide sheet/red phosphorus composite materials plays a dominant role,but the diffusion control capacity is beneficial to the intercalation of red phosphorus.The ratio of capacitance control to diffusion control can be adjusted by regulating the thermal reduction time of graphite oxide,so as to prepare doped reduced graphene oxide with high phosphorus intercalation,stable structure and good rate performance.(3)Coal-based reduced graphene oxide with a low number of graphene stacking layers(3-9 layers)and a large number of defects are successfully prepared using coal-based graphite as precursors at 900°C for 10 min.The layer spacing(0.358,0.367,0.375 nm),specific surface area(399,412,472 m2/g)and pore diameter(9.12,9.90,10.21nm)all improve with the increase of coalification and the degree of stripping of graphene sheets is improved.Some defects exist in anthracite-based graphene oxide,which provide fast diffusion channels for sodium ion storage and additional sodium storage sites.Increasing reduction temperature and extending reduction time had adverse effects on the microstructure of coal-based graphene,the structure of graphene sheet becomes incomplete,the oxygen content decreases and the layer spacing is decreased,which are not conducive to the storage of sodium ions.(4)Anthracite-based reduced graphene oxide(FARGO)has excellent sodium storage properties.The first reversible capacities of FARGO are 331m Ah/g at 30m A/g and the reversible capacity of sodium storage is 75m Ah/g at 10 A/g,the reversible capacity of sodium storage can be maintained at 140 m Ah/g(the capacity retention rate is close to 100%)after 200 cycles at 500 m A/g.FARGO electrodes also exhibit excellent high rate cycling performance,and the reversible capacity can still be maintained at 123m Ah/g(the capacity retention rate is 91.8%)even after 1000 cycles at 1 A/g.There are 71 figures,18 tables and 162 references. |
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