Preperation And Sodium-storage Performance Of Hard Carbon Materials

Energy,information and materials are the three pillars for development of modern social.In the recent three decades,with the prominence of global energy crisis and the deterioration of environmental problems,developing sustainable and clean energy becomes one of the most important parts in the field of science.Although Lithium ion batteries（LIBs）occupied most of the chemical energy storage market duo to its superior electrochemical properties,it is destined to be replaced by the new"non-lithium"batteries because of the limited resources and expensive price of LIBs.Sodium ion batteries（SIBs）are actively pursued as the most attractive batteries for renewable electric power storage,owing to their potential advantages of low cost and widespread availability of sodium resources.Among the negative materials currently studied,hard carbon has been considered as the most promising candidates because of their advantages of abundant resources,low sodium embedded platform and good cycle stability.To find hard carbon anode materials with high performance,in this thesis,we prepared a series of different structure of hard carbon,which derived from sodium alginate and thistle grass.The effects of carbonization temperature,preparation methods and impurity doping on structure,morphology and storage properties of hard carbon materials were systematically studied and the detailed results are summarized as follows:（1）With the loose spongy structure and active element sodium of sodium alginate,three-dimensional porous hard carbon were prepared by pyrolyzation process.The structure,morphology and surface area of hard carbon were analyzed by BET、Raman、XRD、TEM.The results showed that surface area and graphitization degree of hard carbon were gradually increasing when pyrolysis temperature raised from 600oC to 1200oC.Meanwhile,the morphology of hard carbon has a trend from three dimensional porous block structure to two dimensional nanosheets.The product synthesized at 1000oC exhibited best rate performance,when the current density increased up to 0.1 and 1 A·g^-1,the electrode can deliver capacity to 205 and 165 mAh·g^-1;（2）Hard carbon were derived from readily available biomasses thistle grass by a single-step carbonization and two-step hydrothermal-carbonization,respectively.Compared with structure and electrochemical performance of hard carbon prepared by these two methods,two-step hydrothermal-carbonization was confirmed to more suitable for the preparation of hard carbon from biomass.The hard carbon synthesized by two-step hydrothermal-carbonization at 700 ^oC has the first discharge capacity up to 446 mAh·g^-1 at a current density of 0.05 mA·g^-1.Its initial coulomb efficiency was 52%and the discharge capacity would remained at 216 mAh·g^-1 after 100 cycles.Moreover,when the current density increased up to 2 and 10 A·g^-1,the electrode can deliver capacity to 95 and 64mAh·g^-1,respectively;（3）The nitrogen doped,sulfur doped,and nitrogen/sulfur dual-doped hard carbon materials were prepared by mixing the hydrothermal-carbonization thistle grass with melamine and sulphur powders after high temperature carbonization.The comparative research results showed that the heteroatom doped carbon exhibited significantly improved sodium storage performance.The nitrogen/sulfur dual-doped carbon materials possess a high reversible capacity of514 mAh·g^-1.A discharge capacity of 268 mAh·g^-1 could be maintained after 50cycles at 0.1 A·g^-1.Notably,the dual-doped carbon materials has a superior high capability of 120 mAh·g^-1 even at 5 A·g^-1.