【作者】 张旭东；

【导师】 张凌燕；

【作者基本信息】 武汉理工大学 ， 矿业工程， 2020， 硕士

【摘要】 石墨类材料是锂离子电池主流的负极材料,得益于石墨特殊的层状储锂结构与较低的对锂电位,锂离子电池具有工作电压高、能量密度大等优势,广泛应用于新能源汽车、储能、通信等领域。但未处理的天然石墨负极材料工作寿命较短,在当前的电解液体系下活性较高,造成不可逆容量较大,循环充放电过程中锂离子与溶剂分子的共嵌入也使得材料结构的完整性受到破坏,导致循环充放电性能和倍率充放电性能均较差。将不规则的隐晶质石墨和鳞片石墨整形为球形石墨能显著提高材料的振实密度,改善材料的各向异性,有利于提高石墨材料嵌/脱锂离子的性能。为提高石墨材料的电化学性能,实现天然石墨资源的高效利用,论文以两种典型的天然石墨精矿为原料,制备高纯球形石墨,对其基本性质及电化学性能进行研究。以石油沥青为包覆层碳源对高纯球形石墨进行包覆-炭化改性处理,通过SEM、XRD、Raman光谱、比表面积分析及电化学测试等手段进行表征分析,主要内容和结论有:（1）对晶质石墨试样与隐晶质石墨试样的基本性质进行研究。经球形化处理后晶质石墨与隐晶质石墨的粒度更加集中,形状均呈类球形或椭球形,晶质石墨的振实密度由0.62 g/cm³增加至0.93 g/cm³,比表面积为5.7346 m²/g;隐晶质石墨的振实密度由0.84 g/cm³增加至0.91 g/cm³,比表面积为3.9641 m²/g,球形化处理改善石墨的各向异性,提高材料的体积容量密度。电化学测试结果表明:晶质石墨和隐晶质石墨经球形化处理后首次放电容量为393.6 m Ahg^-1和358.1m Ahg^-1,对应的首次充放电效率分别为92.48%和85.20%;（2）对三种不同软化点的沥青进行炭化处理,高温沥青具有比低温沥青更短的热分解反应时间、更高的残炭率,且更易形成致密的包覆层。随着炭化温度的升高,沥青碳由乱序结构趋于有序排列,可逆容量逐渐降低。对软化点为280℃的高温沥青分别在500℃、900℃、1100℃和1300℃下进行炭化处理,电化学测试结果表明:放电容量密度为500℃>900℃>1100℃>1300℃,首次充放电效率为500℃<900℃<1100℃<1300℃;（3）球形化整形和沥青碳包覆联合处理可以有效提高石墨材料的电化学性能。系统研究沥青软化点、沥青包覆用量和炭化温度对试样电化学性能的影响,试验结果表明:采用软化点为280℃的沥青,包覆用量10%,炭化温度1300℃的包覆炭化工艺,改性晶质石墨的可逆容量为352.6 m Ahg^-1,首次充放电效率为91.89%,30次循环充放电后放电容量保持率为97.12%,0.1C电流密度下放电容量保持率为83.09%;采用软化点为280℃的沥青,包覆用量14%,炭化温度1300℃的炭化包覆工艺,改性隐晶质石墨的可逆容量为298.1 m Ahg^-1,首次充放电效率为79.47%,30次循环充放电后放电容量保持率为91.06%,0.1C电流密度下放电容量保持率为56.67%;（4）对三种不同粒度的晶质石墨进行沥青碳包覆改性研究。电化学测试结果表明:粒度较小的石墨倍率性更好,在0.2C、0.5C和1C的循环倍率充放电条件下,对应放电容量保持率为96.14%、86.07%和76.60%;粒度较大的石墨首次充放电效率更高、循环性能更好,首次充放电效率达到92.63%,30次循环充放电后放电容量保持率为96.43%。更多还原

【Abstract】 Graphite materials are the main anode materials of lithium-ion batteries,which benefit from the special layered lithium storage structure of graphite and low potential for lithium.Lithium-ion batteries have the advantages of high working voltage and energy density which are widely used in new energy vehicles,energy storage,communication and other fields.However,the untreated natural graphite anode material has a short working life and high activity under the current electrolyte system resulting in a large irreversible capacity.Moreover,the embedding of lithium ions and solvent molecules in the process of cyclic charging and discharging also destroy the structural integrity of the material leading to poor cycle charge-discharge performance and rate charge-discharge performance.Reshaping irregular aphanitic graphite and flake graphite into spherical graphite can significantly improve the vibration density and anisotropy of the material,thus improving the performance of graphite material intercalation/deintercalation of lithium ions.In order to improve the electrochemical performance of graphite materials and realize the efficient utilization of natural graphite resources,two typical graphite concentrates were used as raw materials to prepare high-purity spherical graphite.The basic properties and electrochemical performances of spherical graphite were also studied.The high-purity spherical graphite was coated and carbonized with petroleum pitch as the coating carbon source,and characterized by means of SEM,XRD,Raman spectroscopy,specific surface area analysis and electrochemical test.The main contents and conclusions are as follows:（1）The basic properties of flake graphite and aphanitic graphite were studied.The experimental results show that the particle size of crystalline graphite and aphanitic graphite is more concentrated after spheroidization and their shapes are almost spherical or ellipsoidal.The vibration density of crystalline graphite increased from 0.62 g/cm³to 0.93 g/cm³ with a specific surface area of 5.7346 m²/g,while the vibration density of aphanitic graphite increased from 0.84 g/cm³ to 0.91 g/cm³ with a specific surface area of 3.9641 m²/g.Spheroidization can help improve the anisotropy of graphite and the bulk density of the material.The electrochemical test results show that the first discharge capacity of flake graphite and aphanitic graphite after spheroidization is393.6 m Ahg^-1 and 358.1 m Ahg^-1,relatively,the first charge and discharge efficiency is92.48%and 85.20%.（2）After carbonization of three kinds of petroleum asphalts with different softening points,it is found that high-temperature asphalt has a shorter thermal decomposition reaction time,a higher carbon residue rate and it is easier to form dense coating compared with low-temperature petroleum asphalt.With the increase of carbonization temperature,petroleum asphalt carbon tends to be arranged in order from disordered structure and reversible capacity decreases gradually.The high temperature asphalt with softening point of 280℃was carbonized at 500℃,900℃,1100℃and1300℃.The electrochemical test results showed that the discharge capacity density according to temperature is:500℃>900℃>1100℃>1300℃and the first charge-discharge efficiency according to temperature is:500℃<900℃<1100℃<1300℃;（3）The combination of spheroidizing and asphalt carbon coating can improve the electrochemical properties of graphite effectively.The effects of softening point,coating content and carbonization temperature on the electrochemical properties of the samples were studied.The results show that using the coating carbonization process with softening point of 280℃,coating content of 10%and carbonization temperature of 1300℃,the reversible capacity of flake graphite is 352.6 m Ahg^-1,the first charge-discharge efficiency is 91.89%,the capacity retention rate after 30 cycles is 97.12%and the capacity retention rate under 0.1C current density is 83.09%;using the carbonization coating process with softening point of 280℃,coating amount of 14%,and carbonization temperature of 1300℃,the reversible capacity of aphanitic graphite is 298.1 m Ahg^-1,the first charge discharge efficiency is 79.47%,the capacity retention rate after 30 cycles is 91.06%and the capacity retention rate under 0.1C current density is 56.67%;（4）Three kinds of flake graphite with different particle size were modified by asphalt carbon coating.The electrochemical test results show that the graphite with smaller particle size has higher rate charge-discharge ability.Under the current density of 0.2C,0.5C and 1C,the corresponding discharge capacity retention rate is 96.14%,86.07%and 76.60%;the graphite with larger particle size has higher first charge-discharge efficiency and better recycle performance.The first charge discharge efficiency reaches 92%after 30 cycles of charge and discharge and the retention rate of discharge capacity is 96.43%.更多还原