【作者】 黄小忠；

【导师】 王君；

【作者基本信息】 兰州大学 ， 物理学·凝聚态物理， 2020， 硕士

【摘要】 锂离子电池中针对Si负极问题的解决思路已经比较明确:即通过将纳米级Si颗粒进行包覆后搭建体积膨胀空间,然后再用强化层分割包覆的分级复合结构加以解决。在复合结构中,碳是常用的包覆缓冲层,而TiO₂常用于结构束缚。尽管这种结构设计能使Si的电化学性能大幅提升,但包覆层以及多层包覆结构对Si电化学性能的影响并不是十分明确,多层包覆结构的表面改性研究比较罕见。本论文通过对Si@C,Si@TiO₂和Si@C@TiO₂三种包覆结构的对比研究,探究不同包覆层对Si电化学响应过程的影响;同时鉴于TiO₂结构易调控且缺陷态TiO₂具有更好的电化学性能,因此通过将Si@C@TiO₂分别在H₂和NH₃气氛中处理得到含不同缺陷态TiO₂的包覆结构来探究其对电化学性能的影响;碳包覆层的改性主要以掺杂N的作用较为明显,但Si@C@TiO₂中C层难以进行有效掺杂且掺杂所需的的高温会引起TiO₂的结构改变,通过采用低温热解MF泡沫树脂获得高含N量的C材料,为后期C包覆层的掺N研究做准备。1.基于包覆结构,系统对比了以上三种包覆结构的电化学行为,其目的在于研究不同包覆材料对Si电化学响应过程的影响和包覆结构中存在的界面对电化学储锂方式的影响。实验结果显示:Si@C,Si@TiO₂和Si@C@TiO₂具有不同的脱锂电位和储锂方式,这表明Si/包覆材料界面会影响Si的脱锂电位,C/TiO₂界面存在较明显的嵌入型赝电容特性。2.Si@C@TiO₂中存在的缺陷态影响Si的电化学储锂方式,Si@C@TiO₂和Si@C@TiO₂-Ov赝电容储锂方式明显,Si@C@TiO₂-N试样出现了明显的类Si@C脱锂电位,且比容量和首次库伦效率明显高于前两者。3.MF-600相比其它试样具有较高含氮量和充分降解的结构,用作负极材料时,锂离子扩散系数最高(1.8×10^-13cm²/s)、比容量最高(稳定比容量超过600mAh·g^-1)且储锂方式为电化学储锂和赝电容储锂。更多还原

【Abstract】 The solution to the problem of Si anode in lithium-ion batteries has been more clear.That is,it is solved by coating the nano-scale Si particles and then constructing the volume expansion space,and then dividing and covering the graded composite structure with the reinforcement layer.In the composite structure,carbon is a commonly used coating buffer layer,and TiO₂ is often used for structural restraint.Although this structural design can greatly improve the electrochemical performance of Si,the influence of the coating layer and the multilayer coating structure on the electrochemical performance of Si is not very clear,and the surface modification research of the multilayer coating structure is relatively rare.In this paper,through the comparative study of the three coating structures including Si@C,Si@TiO₂ and Si@C@TiO₂,the effect of different coating layers on the electrochemical response process of Si is explored,At the same time,given that the structure of TiO₂ is easy to adjust and the defected TiO₂ has better electrochemical performance,by treating Si@C@TiO₂ in H₂ and NH₃ atmospheres respectively to obtain a coating structure containing different defected TiO₂,explore different defects in TiO₂ influence on the electrochemical performance of Si@C@TiO₂,The modification of the carbon coating is mainly done by doping with N,however,the C layer in the Si@C@TiO₂ structure is difficult to be effectively doped and the high temperature environment required for doping is likely to cause structural changes of TiO₂.C material with high N content was obtained by pyrolyzing melamine-formaldehyde（MF）foam resin by low temperature method,which was prepared for the later N-doping of C coating layer.1.Based on the cladding structure,compares the electrochemical behavior of the above three cladding structures systemly.Its purpose is to study the influence of different coating materials on the electrochemical response process of Si and the influence of the interface existing in the coating structure on the electrochemical lithium storage method.The experimental results show that Si@C,Si@TiO₂ and Si@C@TiO₂have different delithium potentials and lithium storage methods,indicating that the Si/cladding material interface will affect the delithiation potential of Si,C/TiO₂interface has obvious pseudo-capacitance characteristics.2.The defect state in Si@C@TiO₂ affects the electrochemical lithium storage method of Si,the Si@C@TiO₂ samples and the Si@C@TiO₂–Ov samples have obvious pseudo-capacitor lithium storage methods.The Si@C@TiO₂–N samples showed obvious Si@C-like delithiation potential,and the specific capacity and Initial Coulomb efficiency were significantly higher than the former two.3.Compared with other samples,MF-600 has higher nitrogen content and fully degraded structure,when used as a negative electrode,it has the highest lithium ion diffusion coefficient(1.8×10^-13cm²/s),the highest specific capacity(stable specific capacity exceeds 600mAh·g^-1),and the lithium storage methods are electrochemical lithium storage and pseudocapacitor lithium storage.更多还原