| In recent years,with the increasing demand for energy storage,sodium-ion battery（SIB）is expected to become a substitute for lithium-ion batteries in the field of energy storage due to its cost and resource advantages.Restricted by electrode materials,the current sodium-ion batteries need to be further improved in terms of energy density,power density,and cycle life.The hard carbon material has a larger interlayer spacing,more micropores,small volume changes when sodium ions are inserted,and a relatively stable structure.It is currently one of the most practically valuable sodium storage anode materials.However,the specific capacity of hard carbon materials is still low,and the cycle stability is poor,which greatly limits the practical application of hard carbon.This paper deeply explores the influence mechanism of the microstructure in hard carbon materials such as specific surface area,defect concentration,interlayer spacing,and functional groups on the sodium storage performance,and controls the hard carbon structure from the macroscopic and microscopic perspective to prepare the porous carbon material with high rate performance,the hard carbon material with low voltage platform,and the sulfur-doped carbon material with high capacity.It provides ideas for the design of sodium storage carbon materials with high specific capacity and high power density.The main research content and innovative results of this article are as follows:（1）Aiming at the problem of poor rate performance of hard carbon materials,we combined the sodium chloride template method and spray granulation method to prepare a porous carbon material with a honeycomb morphology and a high specific surface area,which improve the capacity of the slope region with the sodium storage mechanism of fast pseudocapacitance process.By adjusting the ratio of the sodium chloride template and the pyrolysis temperature,the surface area of the carbon material is controlled.At the same time,the corresponding relationship between the micromorphology,specific surface area,and sodium storage behavior of the carbon material is systematically studied.Studies have shown that the template porous carbon G-TC5 exhibits a first cycle charging capacity of 304.1 m Ah g-1 at 0.05 A g-1 and an initial coulombic efficiency of 75.8%.Even at a large current density of 3.2 A g-1,it still possesses a reversible specific capacity of 242.1 m Ah g-1.The results of cyclic voltammetry curves of different scan rates show that fast surface pseudocapacitance behavior is the main reaction process of G-TC5 electrode under high current density.（2）Aiming at the problem of high sodium storage potential and low capacity in the plateau region of hard carbon materials,we adopted a simple strategy of pre-oxidation in the air before calcination to promote the crosslinking and stabilization of the carbon skeleton for phenol-formaldehyde resin（PF）.The hard carbon material with an interlayer spacing of 0.4nm and rich in carboxyl functional groups was synthesized,which exhibits excellent low-voltage platform sodium storage performance.In-depth analysis shows that:pre-oxidation in the air can increase the content of C=O double bonds,hinder the graphitization process during the carbonization process,and increase the interlayer spacing.The pre-oxidized PF-300Air-1200 electrode material exhibits a first-cycle charging capacity of 325.5 m Ah g-1 and an initial coulomb efficiency of 72.3%at a current density of 0.1 A g-1,of which more than 64%of the reversible capacity can be released under an ultra-low voltage platform of 0.1 V,thereby increasing the energy density of the full battery.We studied the diffusion kinetics of sodium ions through GITT and the structure evolution process of hard carbon during sodiation by the ex-situ XRD method.The results together show that the model of sodium storage in hard carbon can be divided into three parts:Defect adsorption-Interlayer insertion-Micropore filling.（3）Aiming at the problem of low capacity of hard carbon materials,we adopted a very novel liquid phase reaction doping technology to synthesize sulfur-doped carbon materials with high doping levels and large interlayer spacing,taking the full advantages of active doping elements,expanding the carbon layer spacing of carbon materials,breaking through the limitation of carbon material capacity,and realizing a high sodium storage capacity at low potential.Experiments show that the carbon material prepared by this method shows a sulfur doping degree of up to 13.38 at%,and the interlayer spacing of the carbon material is expanded to 0.396 nm.At the same time,the relationship between the synthesis mechanism,structure,and performance was explored.When S-Pyr20 was used as the anode of the sodium-ion battery,it exhibits a first-cycle charge capacity of 618.5 m Ah g-1 at a small current density of 0.1 A g-1.The CV test shows that its redox peak potential is lower than that of conventional sulfur-doped carbon materials,which indicates that there is no conversion reaction in presence of sulfur elemental in the sulfur-doped carbon materials,so it has better cycle stability and lower voltage platform,which is expected to achieve a comprehensive balance between energy density and cycle life. |
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