Studies On Amorphous Carbon Anode Materials For Sodium-ion Batteries

Due to the increasingly serious greenhouse effect and environmental pollution caused by the continuous consumption of traditional fossil energy,renewable clean energy(such as solar energy and wind energy)is facing new opportunities and challenges.However,the intermittent and regional defects of renewable energy require the assistance of large-scale energy storage system,which puts forward higher requirements for sustainable and low-cost energy storage technology.Due to the abundant sodium resources,sodium-ion batteries(NIBs)have attracted more and more attention in recent years,and are considered as a beneficial supplement to lithium-ion batteries(LIBs)in the field of large-scale energy storage.However,there is still a lack of anode materials with excellent comprehensive properties to further improve the energy density of sodium ion batteries.Therefore,it is of great significance for the industrialization of sodium ion battery to develop the anode materials with excellent performance and easy availability.Based on this background,this paper developed several high-capacity amorphous carbon anode materials by modified Phenol-Formaldehyde resin(PF),Activated Carbon(AC)and Petroleum Coke(PC)which revolves around"pore-forming",respectively.The paper systematically studied the effect on the microstructure and the sodium storage properties of amorphous carbon by different modification methods,pointed out the relationship of physical index and the capacity of sodium storage,and summed up the storage mechanism of"adsorption-filling in the pore".1.PF is typical hard carbon precursors.Amorphous carbon was prepared from PF with ethanol as pore-forming agent.The effects of ethanol dosage and carbonization temperature on the microstructure and sodium storage properties of PF-based amorphous carbon were systematically studied.Ethanol could improve the crosslinking degree of precursor at the molecular level,resulting in the formation of closed pore structure in the carbonization process of precursor.Macroscopically,the true density decreased and the plateau capacity increased.The true density,as the feedback of the closed pores in the amorphous carbon,changes in the opposite trend to the plateau capacity,which confirms the view that the plateau capacity comes from the filling of sodium ions(Na~+)in the closed pores of the amorphous carbon.Carbonization temperature has a great influence on the microstructure and sodium storage properties of PF based amorphous carbon.With the increase of carbonization temperature,the average carbon layer spacing of amorphous carbon decreases,the local order degree increases,and the capacity of slope region decreases,while the capacity of platform region increases.2.AC contains a large number of pore structures,which can provide enough space for the storage of Na~+.However,when it is directly used as the anode material of NIBs,the initial effect is very low.When the activated carbon is modified by high temperature treatment or surface coating,the open pores of the AC can be changed into closed pores,which is accompanied by a significant reduction in the specific surface area.The reversible capacity and initial coulombic efficiency(ICE)of the modified AC are greatly improved compared with the original sample.The charge-discharge curve of the modified AC is characterized by"long platform,low sodium storage potential".In practical application,this"long platform"carbon has a positive significance for increasing the working voltage of NIBs.Surface coating has a better effect on the modification of microporous activated carbon.3.PC is the main raw material of artificial graphite,with large output,low cost,less impurities,good uniformity and other characteristics.However,it is difficult to prepare amorphous carbon directly because of its liable to graphitization.In this paper,the PC was treated with phosphorus doping and alkali activation,and it was found that different chemical treatment methods would have different effects on the behavior of sodium storage.In this work,PC is doped with phosphorus.The P-O,P-C functional groups are introduced into the carbon layer.It can significantly increase slope capacity,and the capacity growth is associated with phosphorus doping amount.Meanwhile,the plateau capacity is not affected.The results show that the slope capacity is due to the adsorption of Na~+at the defect site.When the petroleum coke is activated by alkali,more closed pores are introduced into the carbon layer structure,and the charge-discharge curve changes from pure"slope"to"slope+platform".With the increase of carbonization temperature,I_D/I_G decreased,the degree of disorder decreased and slope capacity decreased.The true density decreased,the closed pores increased,and the plateau capacity increased.The results indicate that the sloping and plateau capacity relates to surface defects and internal closed pores,respectively.