Preparation And Electrochemical Properties Of Natural Biomass Buckwheat Hull-based Carbon Materials

Electrochemical energy storage devices are gaining importance in reducing the consumption of fossil energy and developing renewable energy sources.The main functions of different energy storage devices are related to electrode materials,and the most commercially valuable anode material in lithium-ion batteries is carbon material.Graphite has become the mainstream material for current commercial rechargeable lithium batteries due to its low cost,high conductivity,good cycling stability and low lithium ion insertion potential.However,the theoretical capacity of graphite is 372m Ah g^-1,which cannot be adapted to the demand for higher specific capacity of future batteries.Biomass,as a renewable energy source,has become a current research hotspot for carbon materials due to its adjustable physicochemical properties,fast regeneration rate,and environmental friendliness.In addition,natural biomass can be used as a new type of carbon material precursor because of its more microstructure.As anode materials for lithium-ion batteries,these biomass-derived carbons are characterized by high capacity,good performance and high cycling stability.In this paper,carbon materials with different morphologies and high specific surface areas were prepared from agricultural waste buckwheat hulls by chemical activation and hydrothermal carbonization,respectively.The material structure of the samples was characterized by X-ray diffraction,Raman analysis,specific surface area and pore size analysis and infrared spectroscopy,and their morphology was observed by scanning and transmission electron microscopy,and the carbon materials were used as anode for lithium-ion batteries for a series of electrochemical performance tests.The main research of this paper is as follows:（1）Porous carbon was prepared by impregnating buckwheat hulls and calcium chloride at mass ratios of 1:2,1:2.5 and 1:3,after pre-carbonization for 3 h and activation at 500°C,600°C and 700°C for 60 min.A single variable was controlled in two experiments to investigate the microstructure of porous carbon and its electrochemical performance as an anode for lithium-ion batteries as influenced by the amount of activator,reaction temperature and other factors.The results show that the specific surface area and average pore size increase with increasing activator.The porous carbon had the best electrochemical performance when the impregnation ratio was 1:2.5 at the activation temperature of 600°C,and the capacity was stabilized at 715m Ah g^-1 after 150 cycles at 0.2C.The electrochemical performance and surface morphology of the carbon material were closely related to the activation temperature.The sample had the largest specific surface area(351.018 m² g^-1)when the activator ratio was 1:2.5 and activated at 600°C.And in cyclic voltammetry test,scanning at a rate of 1 m V s^-1,the carbon material has a superior performance with a capacitance contribution of 90%.（2）Buckwheat hulls and 1 mol/L phosphoric acid were mixed well and hydrothermally reacted at 200°C for 24 h,36 h,48 h,and 60 h,respectively,and then the hydrothermal products were carbonized by calcination at 600°C for 90 min to prepare the carbon spheres.The sodium polyacrylate at a concentration of 4 mg/10 m L was added during the hydrothermal process to improve the dispersibility of the carbon spheres.The effects of hydrothermal time and dispersant on the physical properties and electrochemical properties of the carbon spheres were investigated.At a hydrothermal time of 36 h,the carbon spheres had a uniform size and smooth surface area of 356.441m² g^-1,and the capacity was maintained at 478.5 m Ah g^-1 after 500 cycles even at 5C.The electrochemical properties of the carbon spheres were slightly improved by the addition of the dispersant,and the specific capacity of the sample was 649.4 m Ah g^-1after 100 cycles at 0.2C.