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Preparation And Electrochemical Performances Of Soft/Hard Carbon Anode Materials

Posted on:2022-01-20Degree:MasterType:Thesis
Country:ChinaCandidate:Z Y GuoFull Text:PDF
GTID:2491306542981989Subject:Chemical Engineering
Abstract/Summary:PDF Full Text Request
Sodium/potassium ion batteries have becoming the one of the most important research topics in recent years due to their cost advantages,similar working mechanism to lithium-ion batteries,and design flexibility.Sodium and potassium are in the same main group as lithium,so they have similar physical and chemical properties to lithium,coupled with a similar standard electrode potential,making them a new energy storage system that"beyond Li-ion batteries".The performance of electrode materials is one of the main factors that affecting the electrochemical performance of batteries.Carbon anode materials have received extensive attention in recent years due to their wide range of sources,good electrical conductivity,and easy production.In this work,different types of carbon sources(biomass waste chestnut shells and coal tar pitch)are used as the precursors of carbon anode materials.By studying the influence of different pretreatment methods and preparation conditions on the microstructure of the materials and comparing their electrochemical performance.The main research contents are as follows:Using wasted chestnut shells as the precursor,the hard carbon anode materials was prepared by a two-step method of hydrothermal pretreatment with Na OH solution and carbonization at 800℃,and then its microscopic morphology and electrochemical performances as sodium ion batteries anode were explored.The results show that hydrothermal pretreatment can bring larger interlayer spacing,more defects and bigger specific surface area.These structures have a positive impact on the electrochemical performances of sodium ion batteries.The experimental results show that the carbon anode material which was hydrothermally pretreated has a high reversible capacity of 286.9 m Ah g-1and good cycle stability.The retention rate after 100 cycles is 83.7%.At the same time,the hard carbon material after hydrothermal pretreatment carbon materials have a smaller charge transfer resistance,which has a positive effect on the electrochemical performance of sodium ion battery electrodes.Second,using coal pitch as the precursor and NaCl as the template,a nano-sheet soft carbon anode material was successfully synthesized by template method,and the influence of different carbonization temperatures on the microstructure of the material was explored in the range of 800-1200℃.The experimental results show that the material synthesized by the template method has more defects,larger interlayer spacing and specific surface area at a carbonization temperature of 1200°C,and exhibits the best electrochemical performances.It has a reversible discharge specific capacity of 321.4 m Ah g-1at 0.1 C in the second cycle.The retention rate exceeds 65.3%after 100 cycles.It maintains a specific capacity of 190.1m Ah g-1at a current of 1 C.After more than 500 cycles,it maintains a specific capacity of190.1 m Ah g-1.The capacity retention rate exceeds 83.2%.Finally,in order to further study the electrode process kinetics of K+deintercalation process in hard carbon material,the insertion process of potassium ion in the first cycle of the hard carbon electrode was explored in three-electrode system(1.0 M KPF6in EC:PC=1:1).The results of electrochemical impedance spectroscopy show that the spectrum consists of two semicircles,which can be attributed to the contact resistance,the diffusion of K+through SEI film and the process of charge transfer,and a oblique line,which is related to the solid diffusion of K+in the hard carbon material particles.By selecting a suitable equivalent circuit and fitting the experimental results,the relationship of the material contact resistance and charge transfer resistance along with the polarization potential of the electrode during the potassium insertion in hard carbon of the first cycle is obtained,and the electrochemical reaction symmetry factor is calculated.
Keywords/Search Tags:hard carbon, soft carbon, pitch, EIS
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