Research On The Performance And Sodium Storage Mechanism Of Biomass-derived Hard Carbon Anode Materials For Sodium Ion Batteries

Biomass-derived hard carbon materials have the advantages of abundant resources,low cost,high reversible capacity and excellent rate performance,which are candidates for commercialization of anode electrode materials for sodium-ion batteries.However,the biomass-derived hard carbon materials have too low carbon yield,the larger specific surface area leads to lower initial coulombic efficiency,and the size of the interlayer distance has a great impact on the long-term reversible insertion/extraction for sodium-ion.Moreover,the mechanism of electrochemical sodium storage of the materials is controversial,these have yet to be further researched in this work.In order to develop biomass-derived hard carbon materials with high performance for sodium-ion batteries,cherry petals and oatmeal are selected as the precursor materials.After a facile preparation,we can obtain biomass-derived hard carbon materials in a large scale.Creatively,the performance and mechanism of sodium storage in the biomass-derived hard carbon materials are also studied in detail.Cherry petals-derived hard carbon material（CP）is a typical hard carbon material composed of disordered graphite microcrystallites with a small amount of stacked-graphite sheet.When performing the electrochemicalperformance measurements,at a current density of 20 mA/g,the CP electrode can provide a high initial reversible capacity of 310.2 mAh/g,with a favorable initial coulomb efficiency of 67.3%,delivering a high retention rate of 99.3%after 100 cycles.Even at a high current density of 500 mA/g,the reversible capacity of the CP electrode can also reach 146.5 mAh/g,exhibiting the excellent electrochemical performance.Physicochemicalmeasurementsshowedthattheexcellentelectrochemical performance of the CP electrode may be attributed to the low specific surface area,the nanoscale mesopores,the oxygen/nitrogen functional groups on the surface,and the expanded interlayer distance.Meanwhile,Cyclic Voltammetry（CV）and X-ray Photoelectron Spectroscopy（XPS）measurements show that the CP electrode is related to sodium-ion insertion into the graphite sheet in the low potential platform region,and the high potential slope region corresponds to the sodium-ion adsorption on the surface of the amorphous carbon materials.Oatmeal-derived hard carbon material（COs）is also a typical amorphous hard carbon material.When being assessed as an anode electrode for sodium-ion batteries,the low specific surface area,the nanoscale mesopores,the presence of oxygen functional groups and the expanded interlayer distance are responsible for the electrochemical performances,which are conducive to the diffusion and reversible accumulation for sodium-ion.The COs electrode can deliver an initial reversible capacity of 272.4 m Ah/g and a capacity retention of 93.3%after 100 cycles at 20mA/g.When performing the rate performance measure（20 mA/g to 1000 mA/g）,the reversible capacity recovery rate is 92.1%.In addition,the transport properties of sodium-ion in the COs electrode were also characterized by Electrochemical Impedance Spectroscopy（EIS）and Galvanostatic Intermittent Titration Technique（GITT）.The values of sodium-ion diffusion coefficient are determined to be 10^–1410^–16and 10^–910^–10cm²/s,and the results verified that the low potential platform region below 0.1 V of the COs electrode corresponds to sodium-ion insertion/extraction in the interlayer of the graphitic microcrystallites.Based on the results of this research,it can be found that the superior properties of the CP electrode and the COs electrode can be attributed to the synergistic effect of sheet morphology.The low specific surface area makes the generated Solid Electrolyte Interface（SEI）film stable,which greatly enhances the initial coulomb efficiency of the materials.The nanoscale mesopores can not only shorten the sodium-ion migration pathway,improve the transmission efficiency,but also provide more sodium-ion diffusion channels,which will promote the electrode material fully contact with the electrolyte.The oxygen/nitrogen functional groups can increase the defective sites that partially improve the storage of the sodium-ion,which contribute to the adsorption of sodium-ion on the surface of the materials.Most importantly,the expanded interlayer distance can enable long-term reversible insertion/extraction of sodium-ion without destroying the structure of the material,which has a great influence on the electrochemical performance.Finally,the results of the sodium storage mechanism measurements show that CP and COs are the typical hard carbon materials,which have the same electrochemical sodium storage mechanism.Indicating that the adsorption behavior of sodium-ion on the surface of the materials corresponds to the high potential slope region,and sodium-ion insertion/extraction between the interlayer of the graphitic microcrystallites in the low potential platform region.