【作者】 张永刚；

【导师】 王成扬；

【作者基本信息】 天津大学 ， 化学工艺， 2004， 博士

【摘要】 本论文首次将电极生产过程中产生的人造石墨废品及切割碎屑引入到锂离子电池负极活性材料的研究范围,创新性地提出了以表面化学沉积包覆和氧化成膜的方法分别对其进行表面改性修饰并成功地获得了相应的人造石墨改性样品。通过应用电化学研究手段和现代物理测试技术对改性前后该石墨材料的表面结构、电极性能和锂在其中的嵌入动力学进行了深入的研究; 此外,为获得高比容量和首次低不可逆容量的锂电负极材料,还对低温处理的中间相炭微球（MCMB）进行了表面低温催化热处理修饰的初步探索。研究表明,通过中温煤沥青在人造石墨颗粒表面进行化学沉积反应获得的单颗粒核壳型包覆材料摒弃了文献所述包覆方法中石墨活性表面会部分或全部裸露并导致电极对电解液敏感的不足,经理论分析得出其包覆机理为:中温煤沥青经过热分解和热缩聚反应形成的平面稠环分子吸附在石墨微粒表面并不断层积和继续生长形成中间相沥青包覆物。该石墨的首次放电比容量由包覆改性前的255.5 mAh·g^-1增至305.4 mAh·g^-1,首次库仑效率则从包覆前的80.8%提高到90.2%; 50次循环后放电比容量由改性前的154.1 mAh·g^-1提高至302.3 mAh·g^-1; 同时,改性后的人造石墨提高了对PC系电解液的相容性。不同充放电深度测试表明,人造石墨用作工作电极的不可逆容量主要由溶剂在表面的不可逆分解和石墨内部晶格的缺陷所致; 包覆改性方法不但减少了表面上的不可逆分解,而且其热处理工艺具有消除内部晶格缺陷的作用。双氧水轻度氧化并与Li₂CO₃共混热处理,成功地调整了人造石墨表面官能团的种类及其比例,并在表面形成了化学类SEI膜。50次的恒电流充放电测试证明氧化成膜处理可以明显减少首次不可逆容量,并具有良好的多次充放电循环性能。交流阻抗测试结果证明,改性前后人造石墨的交流阻抗复平面图由高频区的半圆形容阻弧和低频区的一条与实轴约呈45°角的直线组成,具有半无限扩散特征。锂离子在材料中的传输过程由锂离子从电解液穿过膜层和在材料中的嵌入扩散两步组成,其嵌锂过程由固相扩散控制。经Raman光谱和XRD分析,在不降低低温处理中间相炭微球原有的高充电比容量的前提下,证实低温氯化钴催化热处理可以明显地提高MCMB表面碳微晶尺寸,并经恒电流反复充放测试证实,首次放电比容量可达到455 mAh/g,库仑效率也由处理前的52.2%提高至86.7%,循环性能也得到积极的改善。更多还原

【Abstract】 An artificial graphite (AG), obtained from graphite electrode producing process, was studied as anode of lithium ion secondary battery (LIB) for the first time in the dissertation. The innovative surface chemical deposition coating and chemical film formation on AG treated by mild oxidation were respectively applied to AG surface modification, and the corresponding samples were obtained successfully. Their surface structure and groups, negative performance and lithium ion migration in them were deeply studied by utilizing electrochemical and modern physical methods. Moreover, the mesocarbon microbead (MCMB) was heated with catalyst CoCl2 in low temperature for the first time in order to obtain high capacity and lower irreversible carbon material used as anode of LIB. The mechanism of surface chemical deposition coating with coal-tar pitch is proved as follow: the lamellar molecules, formed in pitch pyrolysis and polymerization, continuously adsorbed on AG surface and turned larger. The first discharge specific capacity of AG with coating treatment is 305.4mAh/g, which is higher than that 255.5mAh/g of AG, and its relevant coulombic efficient increased from 80.8% to 90.2%. And the 50th discharge capacity also increased from 154.1mAh/g to 302.1mAh/g. Meanwhile, AG with pitch-coated treatment could work better than untreated AG with electrolyte dissolved in PC solvent. The charge/discharge curves as a function of the state of charge indicate that irreversible capacities loss was caused by solvent decomposition on AG surface and intercalation into irreversible carbon sites inside the AG. The results indicate that surface chemical deposition coating process could effectively reduce irreversible capacity loss and could reduce inner lattice limitation. Mild oxidation and lithium carbonate treatment with AG successfully adjusted the type and ratio of surface groups and formed chemical SEI film. The results of the first 50th charge and discharge tests proved that the method could reduce the first irreversible capacity and have good re-cycling performance. Impedance spectra measurements show that the impedance spectra were composed of nyquist semi-circle and a line with 45℃ slope, and Li~+ migration in the AG working electrode can be divide into two steps, Li~+ charge-transfer in the film and the diffusion of Li+ between AG layers, the latter is the controlling steps. Finally, the Raman spectrum and XRD analysis showed that catalytic heat treatment for MCMB at low temperature could effectively enlarge carbon micro-crystal dimension. And charge and discharge tests identified that catalytic heated-treatment could let MCMB keeping higher discharge capacity and could enhance its coulombic efficient and cycling performance.更多还原