【作者】 郑宇；

【导师】 胡仁宗； 张宏亮；

【作者基本信息】 华南理工大学 ， 材料工程（专业学位）， 2022， 硕士

【摘要】 由于高熵氧化物（HEOs）具备众多特殊性质（晶格畸变效应、缓慢扩散效应、鸡尾酒效应等）,近年来,许多研究人员开发了一系列HEOs并将其应用为锂离子电池负极材料,有些HEOs展示出了出众的电化学性能。然而,由于高熵材料的特殊性以及缺乏有效的表征分析手段,关于HEOs储锂机制的研究进展缓慢并且依然存在争议。有研究人员认为HEOs负极仅基于转化反应储锂,也有人认为HEOs的储锂过程包含不可逆的转化反应和后续的合金化反应。除此之外,HEOs负极还存在诸如相对较低的比容量、较差的循环稳定性和低首次库伦效率等问题仍亟需解决。然而,由于对HEOs基础电化学和对应相变行为认识的欠缺,针对HEOs的改性往往缺乏针对性和有效性。因此,应用新型表征手段对HEOs进行储锂行为和机制进行探究,再基于正确的理解进行针对性的改性优化有着重要的意义。本工作利用新型液相燃烧方法制备了一系列的新型尖晶石型高熵氧化物（SHEOs）材料,并对这些SHEOs的储锂过程进行了深度地物相和电化学分析表征。在理解这些SHEOs负极的根本性质后,对其进行诸如单价金属离子引入、机械球磨预相变等工作。值得注意的是,这些针对SHEOs的改性尝试取得了很好的效果,并且性能提升的原因也得到了合理地解释。主要研究内容如下:利用改良液相燃烧法,一种具有更高构型熵的六组元尖晶石型高熵氧化物负极材料（6-SHEO）得以成功制备。该6-SHEO负极展示出了良好的电化学性能,在0.2 A g^-1的电流密度下的长循环可逆容量可以达到657 m Ah g^-1。后续通过XPS、in-situ XRD、ex-situ TEM、TEM-EDS等一系列物相元素表征,发生在SHEO储锂前期的“尖晶石-岩盐-尖晶石”相变现象得以揭示。更重要的,结合电化学分析,该相变背后蕴含的信息得到充分地展示。随后,一种SHEOs全新的储锂机制被提出。不仅如此,为了更充分地认识SHEOs和理解其中非活性组元的性质和作用,一系列Mg O组元含量不同的SHEOs被制备。并且,他们的电化学性质也得到了详尽地分析。这些工作为后续SHEOs负极的改性研究打下坚实的理论基础。在深度认识SHEOs的储锂过程和组元性质后,对SHEOs进行改性研究。首先,通过液相燃烧法成功将Li₂O引入一种SHEO材料中,制备出了一种掺Li的尖晶石型高熵氧化物（SHEO（+Li））负极材料。该SHEO（+Li）负极较SHEO负极的容量有着显著地提升。通过XPS、CV、充放电、ex-situ XRD等一系列元素价态、电化学分析表征测试,SHEO（+Li）嵌锂的物相转变过程和SHEO（+Li）较SHEO容量上升的原因得到了进一步的探究。结果表明,在SHEO（+Li）储锂的过程中,Li⁺嵌入量的提高和随后转化反应更充分的进行可能共同促进了SHEO（+Li）的容量提升。第二个改性探索针对原位研究观察到的“尖晶石-岩盐-尖晶石”可逆相变现象展开。SHEOs负极循环过程存在循环性能差、容量不稳定波动的问题由来已久。为了解决这个问题,通过高能球磨的方式,一系列SHEOs被诱导发生了类似其储锂过程的“尖晶石-岩盐”相变。最后,通过SEM、粒度分析等手段有效控制实验变量后,进行了预相变处理对SHEO负极电化学性能影响的探究实验。实验结果表明,预相变处理可以有效提高SHEOs负极的循环稳定性。基于之前对SHEOs储锂过程相变行为内涵的深度理解,结合详细的电化学表征,相变处理对SHEOs电化学行为影响及其循环稳定性促进作用得到了充分地说明。更多还原

【Abstract】 Fascinating with many special properties（lattice-distortion effect,sluggish-diffusion effect,cocktail effect,etc.）,a series of high-entropy oxides（HEOs）have been prepared and developing as anode materials for lithium-ion batteries,and some of them could exhibit an excellent electrochemical performance.However,due to the specificity of high-entropy materials and the deficiency of characterization and analysis tools,studies about lithium storage mechanism of HEOs anode have been hindered and still remain controversial.Besides that,HEOs anodes still remain many problems（such as relatively low specific capacity,poor cycling stability,low initial coulombic efficiency）which still need to be addressed.However,due to the absence of underlying electrochemistry and related phase change understanding,modification routes for HEOs anode always ineffective.Therefore,it’s essential to apply novel characterization tools to investigate the lithium storage behavior and mechanism of HEOs.Only after fully understand the lithium storage process of HEOs anodes,modification can be more targeted and effective.In this work,a variety of spinel-type high-entropy oxide（SHEOs）materials were prepared by modified solution combustion（SCS）approach.After fully understand the fundamental properties of these SHEOs,many effective approaches for HEOs anode modification such as“monovalent metal ion introduction”and“pre-phase transformation to enhance cyclic stability”have been proposed.With in-depth characterization,the reasons for the upgraded electrochemical performance have also been analyzed in our works.A six-component spinel-type high-entropy oxide（6-SHEO）with higher conformational entropy was prepared by a novel SCS method.The 6-SHEO anode exhibits good electrochemical performance,delivering a long-cycle reversible capacity of 657 m Ah g^-1 at a current density of 0.2 A g^-1.Subsequently,through a series of elemental and phase characterizations such as XPS,in-situ XRD,ex-situ TEM,and TEM-EDS,we demonstrated that a reversible“spinel-rock salt-spinel”phase transformation occurs in pre-lithium process of SHEOs.After fully analysis,we interpreted the means of phase transformation process and proposed a brand-new lithium storage mechanism for SHEOs anode.Not only that,a series of SHEOs with different Mg O content were prepared and their electrochemical properties were analyzed in detail in order to more fully understand the components of SHEOs.This set of investigations laid a solid theoretical foundation for subsequent studies on modifications for SHEOs anodes.After deeply understand of the lithium storage process and the components properties of SHEOs,modification routes have been carried out afterwards.Firstly,a lithium-contained spinel-type high-entropy oxide（SHEO（+Li））anode material has been prepared by introducing Li₂O into a SHEO material thorough SCS approach.Compared with the SHEO,the specific capacity of the SHEO（+Li）effective enhanced.Through elemental valence and electrochemical analysis such as XPS,CV,charge-discharge,ex-situ XRD,we have not only analyzed the process of physical phase transformation in the lithium-embedded process of SHEO（+Li）,but also fully investigated the reason for the increase in capacity of SHEO（+Li）over SHEO.After fully consideration,we believed that the increasement of the Li⁺embedding in SHEO（+Li）crystal structure and more adequate conversion reaction together contribute to the capacity increase of SHEO（+Li）.The second modification approach was carried out by the“spinel-rock salt-spinel”reversiblephase transition phenomenon that observed in previous in-situ studies.It is known that SHEOs suffer from poor cyclic performance and even capacity fluctuations during cycling process.To address this issue,by high-energy ball milling,we induced a series of SHEOs undergo a"spinel-rock salt"phase transition similar to their lithium storage process for the first time.Surprisingly,pre-phase change treatment can effectively improve the cycling stability of SHEOs anode,which arouse our attention.Based on previous in-depth understanding of the SHEOs and detailed electrochemical characterization,the effect of pre-phase change treatment on the electrochemical behavior of SHEOs and the reasons for cycling stability promotion are well illustrated.更多还原