Study On The Preparation Of Pitch-based Carbon Nanosheets By Molten Salt Method And Their Storage Characteristics Of Potassium Ions

With the large-scale commercial application of lithium-ion batteries,the market demand for lithium resources is also rising rapidly.However,the scarcity of lithium resources in the earth crust makes it difficult to be applied in large-scale energy storage systems.Therefore,a new battery system with abundant resources and excellent electrochemical performance is urgently needed.Potassium has attracted extensive attention due to its advantages of large reserves,low cost and similar properties with lithium.However,due to the small interlayer spacing of graphite materials,it is faced with the problems of low capacity and poor cycle performance.Therefore,the research work of this paper is devoted to the design and development of soft carbon anode materials which are suitable for potassium ion storage.In this paper,coal tar pitch was used as carbon source to prepare pitch based carbon nanosheets with different pore sizes by molten salt method.How different molten salt systems would affect the structure and morphology of carbon nanosheets were studied,and the potassium ion storage mechanism of these nanosheets was explored.Then,in the optimized molten salt system,different kinds of heteroatom doped carbon nanosheets were prepared by one-step pyrolysis.The effects of heteroatom doping on the electrochemical properties of carbon materials and the storage mechanism of potassium ions were studied.The main contents and conclusions of this thesis were as follows:(1)Carbon nanosheets were successfully prepared by pyrolysis of coal tar pitch with different kinds of molten salts as the medium.The effects of different kinds of molten salts and carbonization temperature on the structure and morphology of carbon nanosheets were systematically studied,and their potassium ion storage properties were also studied.The results show that the type of molten salt has an important effect on the morphology and structure of carbon nanosheets.Lamellar carbon nanosheets can be effectively formed in the molten salt system of potassium chloride and sodium chloride.The carbon nanosheets carbonized at 750?have excellent cycle stability.At the current density of 100 m A g-1,the reversible specific capacity remains 93.8 m Ah g-1 after 100 cycles.(2)Using coal tar pitch as carbon source,potassium chloride and sodium chloride as hard template and heat transfer medium,the effects of different activation conditions on the structure and morphology of porous carbon nanosheets were studied by High Temperature One-Step pyrolysis and in-situ KOH activation.Research shows:The content of KOH in the precursor has an important influence on the pore structure of porous carbon nanosheets.With the increase of KOH content,proportions of mesoporous and macropores of carbon nanosheets increases gradually.The electrochemical performance test shows that the carbon nanosheets prepared at 750?had the best electrochemical performance when the mass ratio of carbon to alkali was1:4,and the reversible specific capacity keeps 220.3 m Ah g^-1 after 200 cycles at the current density of 100 m A g^-1.(3)Coal tar pitch,urea and sulfur powder were used as carbon source,nitrogen source and sulfur source respectively.In the molten salt system of potassium chloride and sodium chloride,nitrogen doped,sulfur doped and nitrogen sulfur Co doped soft carbon nanosheets were prepared by one pot mixing method.It is found that nitrogen and sulfur co doped can effectively improve the interlayer spacing of soft carbon materials and improve the storage performance of potassium ions into carbon materials.Therefore,nitrogen sulfur Co doped carbon nanosheets have excellent potassium storage capacity and stable cycling performance,the reversible capacity remains at 300 m Ah g^-1 after 200 cycles at a current density of 100 mA g^-1.