| Due to the contradiction between continuous utilization of renewable energy and uneven space-time distribution of energy supply,it is urgent to develop large-scale and efficient energy storage devices for voltage and quality regulation and secondary distribution.In recent years,electrochemical energy storage devices,especially lithium-ion batteries,have attracted wide attention because of their high energy density and long cycle life.In the face of the severe situation of lithium resource shortage and uneven geographical distribution,sodium ion battery has become the"top seed"in large-scale energy storage technology due to its advantages of low cost and abundant natural resources.In fact,the application of sodium ion battery depends on innovations of low-cost and high-performance electrode materials.Carbon-based anode materials have become one of the focuses of researchers because of their application in lithium ion batteries and low cost.Coal is the natural carbon source with the lowest cost and the highest carbon content in nature,and the coal with different rank has different molecular structure,which lays a foundation for the multi-direction transformation and high-value utilization of coal.Therefore,for the application of energy storage technology,it is of great significance to develop the conversion technology of coal to high-value carbon materials.The multi-scale nanostructure of carbon anode materials is an important breakthrough point to optimize the performance of sodium storage,which has been widely concerned by researchers.On the one hand,microcrystalline structure of carbon material directly determines sodium ions storage and transport environment.On the other hand,the change of microcrystal may indirectly affect the distribution of pore and marginal active sites.The microcrystalline parameters mainly include three aspects:interlayer spacing,width of crystal plane and thickness of microcrystal,among which the regulation of interlayer spacing is the key to strengthen sodium insertion.The insertion energy of sodium ion is 0.12 e V when that of lithium ion is 0.03 e V.Therefore,sodium ion cannot be stably stored in conventional commercial graphite(35 m Ah g-1)with smaller interlayer spacing（0.335 nm）.In truth,expending interlayer space of microcrystal in carbon materials is an important means to improve sodium storage in graphitic carbon anode.Bituminous coal,with middling molecular structure and abundant reserves,is a potential precursor of hard carbon anode.The thermal conversion process of bituminous coal is accompanied by the violent rearrangement of microcrystals.How to restrain the graphitization trend is the key to regulate microcrystals.Therefore,this article focuses on the following aspects of research:（1）the evolution of intrinsic microcrystals based on bituminous coal-based hard carbon;（2）the regulation of bituminous coal based hard carbon microcrystals and the theoretical analysis of sodium storage based on different pre-oxidation treatments;（3）the construction and performance evaluation of sodium-ion battery devices based on bituminous coal-based carbon anodes.Firstly,to establish a systematic understanding of the evolution of the microcrystalline structure of bituminous coal-based carbon materials,bituminous coal-based carbon materials were prepared at different carbonization temperatures,and investigated specific evolution of microcrystalline structure;investigate the effect of carbonization temperature on crystallites,so as to guide the directional control of crystallite structure.The results show that the microcrystalline structure of bituminous coal presents a development trend of narrowing between interlayers,lateral growth,and vertical stacking during the high-temperature thermal conversion process,and the microcrystalline growth process of modified carbon material is obviously inhibited.To realize the control of crystallites,a series of hard carbon materials with different crystallite sizes have been successfully prepared by adopting a wide range of gas and liquid phase pre-oxidation strategies.The results show that oxygen-containing functional groups,mainly carbonyl groups,are introduced through gas-phase and liquid-phase oxidation to promote the formation of cross-linked structure and inhibit the graphitization process of bituminous coal.Compared with the original bituminous coal,the interlayer spacing is widened from 0.373 nm to 0.380 nm,and the expanded spacing hard carbon material with short-range ordering of microcrystal is obtained.In addition,based on theoretical calculations,the influence of the microcrystalline structure on the sodium storage behavior is predicted.For the interlayer spacing,as the interlayer spacing increases,the energy consumption value of the interlayer sodium storage gradually decreases,and it decreases from+10.55 k J mol-1to-298.99 k J mol-1 at about0.375 nm,and tends to be stable,indicating that 0.375 nm is a suitable interlayer spacing for sodium storage;for the lateral size,under the smaller size（0.7 nm）,sodium storage cannot be achieved(the energy consumption value of sodium storage is+15.55k J mol-1).As the size increases,the energy consumption of sodium storage gradually reduced to-82.25 k J mol-1.At present,the evaluation of electrode materials mostly stays at the level of half battery.Then,this paper evaluates the electrode materials prepared from half battery to full battery and then to pouch battery to discusses their application potential.The results show that the hard carbon material（RC-GO-1200）obtained by gas-phase oxidation,with suitable expanded spacing and short-range ordering,has an optimized sodium storage performance.Compared with the original bituminous coal,the reversible capacity is increased from 222 m Ah g-1 to 274.2 m Ah g-1,and the initial Coulombic efficiency is maintained at 74.8%.Similarly,the modified sample（RC-LO-1200）prepared by liquid-phase oxidation shows great rate performance and cycle performance,maintaining a reversible capacity of 121.3 m Ah g-1 at 0.5 A g-1,and the capacity retention of 82%after 200 cycles at 0.5 A g-1.In addition,the bituminous coal based hard carbon//NVP full cell was constructed.The reversible capacity of 230.1 m Ah g-1 is provided by RC-GO-1200.It is worth noting that compared with the direct carbonization of bituminous coal,the energy density of the full battery has been improved by 60.5%. |
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