Preparation Of Bagasse-derived Hard Carbon And Study Of Key Influencing Factors Of Sodium Storage Performance

Hard carbon is one of the most promising anode materials for sodium ion battery（SIBs）commercialization.In order to prepare hard carbon materials with excellent performance,it is necessary to find suitable precursor and the best preparation process,so as to achieve high first coulomb efficiency and high capacity cycle stability.In this paper,cheap biomass bagasse from a wide range of sources was selected as the hard carbon precursor.By studying the key preparation conditions affecting the material properties,such as carbonation temperature,holding time and protective atmosphere,we prepared the bagasse hard carbon（BHC）anode material with excellent electrochemical performance for sodium ion batteries.The specific research contents are as follows:（1）Hard carbon anode material was synthesized from biomass bagasse with abundant resources and low price by one step carbonization.The morphology and structure of bagasse hard carbon were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,Raman spectroscopy（Raman）and N₂adsorption and desorption isothermal curves.The electrochemical properties of BHC materials were investigated by constant current charge-discharge,cyclic voltammetry（CV）and electrochemical AC impedance（EIS）.The results showed that the physical properties and electrochemical properties of bagasse hard carbon were greatly affected by the preparation conditions.Using N₂as a protective atmosphere,the hard carbon material was carbonized at the rate of5℃min^-1.After heating to 1300℃and holding for 2 h,the hard carbon material had the best electrochemical performance,with the first effect up to 81.45%.Under the current density of 50 m A g^-1,the specific capacity of reversible charging reaches 237.2 m Ah g^-1,and after 100 cycles,the specific capacity is 205.5 m Ah g^-1,and the capacity retention rate is 86.6%.（2）The effects of different binder and different mass ratio of each component of electrode on hard carbon anode material of sodium ion battery were studied.By comparing PVDF,CMC and SA binders with bagasse derived hard carbon anode material,the effects of different binders on the electrochemical properties of the material were analyzed.The electrode cycling performance of SA binder is better than that of the other two binders.After 100 cycles,Based on BHC-SA,BHC-PVDF and BHC-CMC,the specific capacities of the electrodes prepared by hard carbon materials were 205.5 m Ah g^-1,136.3 m Ah g^-1and 149.8 m Ah g^-1,and the capacity retention rates were 86.6%,67.3%and 79.6%,respectively.The hard carbon active substance,binder and conductive agent were used in the negative electrode of sodium ion battery according to different mass ratio,and the optimal mass ratio was selected.The electrochemical results show that the mass ratio of95:3:2 electrode ratio in the first cycle of the charge-discharge capacity of 243.5 and 300.2m Ah g^-1,in 50 m A g^-1current density of 100 cycles,the capacity remains 220.7 m Ah g^-1.（3）The effects of HCl pickling and ball milling on the morphology,structure and electrochemical properties of hard carbon materials and the mechanism of hard carbon sodium storage were studied.After pickling with HCl solution,it is beneficial to remove impurities such as metal elements which have not been completely removed after carbonization.Through the way of ball milling,the microstructure of the material can be changed to make the particle size smaller.The physical properties of the materials were analyzed by BET,XPS and TEM.The results showed that pickling could effectively reduce the content of metals and other impurities in the materials.After ball milling for 12 h,the particle size of the materials decreased obviously,but the degree of disorder increased.Under the current density of 50 m A g^-1,the untreated hard carbon still maintains a reversible specific capacity of 205.5 m Ah g^-1after 100 cycles,and the capacity retention rate is 86.6%,showing excellent cycling performance.This is related to the natural structure maintained by the material itself.The heteroatom embedded in the carbon structure is conducive to increasing the carbon layer spacing and maintaining the structure.On the contrary,the electrochemical properties of the hard carbon materials treated by pickling and ball milling are inferior to those of the untreated ones.The specific surface area of the treated material will increase,which will lead to the decrease of the first coulombic efficiency of the material.In this experiment,the first coulombic efficiency after HCl pickling was 79.8%,and the specific charging capacity in the first week was213.7 m Ah g^-1.The first coulomb efficiency after ball milling was 67%,and the specific charging capacity in the first week was only 144.5 m Ah g^-1,both of which were not as good as the untreated(81.5%,237.2 m Ah g^-1).In order to explore the mechanism of sodium storage in hard carbon,it is speculated that the storage mechanism of sodium ions in hard carbon is corresponding to the embedding and removal of sodium ions in the graphene-like layer in the low-voltage platform region and the adsorption and desorption of sodium ions in the pores in the high-voltage slope region,based on the phenomenon that the platform capacity of materials decreases and the slope capacity changes little or even increases after ball milling.This is in line with the"adsorption-embedding" mechanism.