Preparation And Lithium Storage Properties Of Lignin-based Carbon Microspheres

Hard carbon material with more capacity than graphite materials,is considered to be one of the most promise candidate anode materials for lithium ion batteries.In view of the shortcomings of traditional hard carbon material precursors such as non-renewable and serious environmental pollution,the hard carbon microspheres were prepared by hydrothermal method in the thesis by using sodium lignosulfonate as raw material.The studies on nitrogen doping and catalytic graphitization of as-prepared carbon microspheres were also carried out to improve its electrochemical performance.The structure and properties of the prepared materials were characterized by SEM,TEM,XRD,Raman,TG/DTG,charge and discharge test,and AC impedance test.The results are mainly as follows:?1?The lignin-based carbon microspheres can be successfully prepared by hydrothermal method.The hydrothermal reaction temperature and time are the main factors affecting the sphericity,the integrity of graphite microcrystals and properties of carbon microspheres.The preparation conditions are optimized to be hydrothermal reaction of 40 g·L^-1 raw material at 230?for 12h,followed by pyrolysis treatment at600?.The prepared carbon material has a moderate particle size,good sphericity and relatively perfect graphite microcrystalline structure.It exhibits a reversible capacity of389.4 mAh·g^-1 at 0.1C rate,180 mAh·g^-1 at 1C rate and stable cycleability.However,its coulombic efficiency in the first charge-discharge cycle was only 43.6%.?2?The modified carbon microspheres with higher nitrogen content can be successfully prepared by hydrothermal carbonization of melamine and sodium lignosulfonate.The introduced nitrogen atoms contain more graphitic nitrogen and pyridinic nitrogen and a certain amount of pyrrolic nitrogen,which increases the graphitization and conductivity of the material while increasing appropriately the defects,thereby reducing the material/electrolyte interface impedance,and significantly improving its charge and discharge performance.When the raw material/nitrogen source mass ratio is 1.6,the prepared carbon microsphere material shows a reversible capacity of 475.6 mAh·g^-1 with a coulombic efficiency of 53.1%in the first charge-discharge cycle at 0.1C rate,a high capacity of 260 mAh·g^-1 and excellent cycleability at 1C rate.?3?The introduction of iron and boron catalysts during the pyrolysis of hydrothermal products can significantly increase the graphitization degree of the produced carbon microspheres,thereby greatly improving the rate performance and the first charge-discharge coulombic efficiency.The reversible capacity of the carbon microspheres prepared by catalytic graphitization with 6%mass fraction of iron powder reaches 342.3 mAh·g^-1 at 0.1C rate,the first charge-discharge coulombic efficiency is increased to 73.8%,and its capacity still reaches 190 mAh·g^-1 at 2C rate.The boron/iron co-catalyzed graphitization can prolong the graphite layer structure on the surface of the prepared carbon microsphere material,further increases the graphitization degree and reduces the material/electrolyte interface impedance.Its 0.1C reversible capacity is increased to 344.8 mAh·g^-1,the first charge-discharge coulombic efficiency is further increased to 76%,and the capacity at 2C rate is further increased to 200 mAh·g^-1 or more.