锂离子电池用Si-M(M=Zr, Ni)合金负极材料嵌锂过程的相图热力学研究

When study of the new lithium-ion batteries, the alloy composed of silicon and the metal with inactive lithium insertion can effectively alleviate the volume effect during the lithium insertion/extraction while maintaining a high lithium storage capacity. Therefore, silicide is attached importance by people when study the anode material. The process of lithium ion inserting into silicide is vital to the performance of lithium-ion battery.So, it is important for revealing the lithium insertion mechanism to study of the reaction routes of lithium insertion, the crystal structure of products and lithium insertion performance. The above work is of great theoretical importance and practical value to the development of high-performance silicon-based alloy anode material.In this work, the Si-M (M=Zr, Ni) alloy anode material for lithium-ion batteries is the study object. Firstly, with the method of mensuration of phase diagram and Computer coupling of phase diagrams and thermochemistry (CALPHAD), the phase relations of Li-Si-M(M=Zr,Ni) ternary systems including metastable information will be studied. Secondly, the crystal structure of some compounds will be measured. Thirdly, with the help of the first-principle, the routes of lithium insertion in Si-M（M=Zr,Ni）alloy anode will be studied. Besides, some lithium insertion performance including volume expansion ration, the maximum lithium insertion content and the voltage will be calculated. Lastly, we'll comprehensive analyze the study results including the routes of phase transformation, the crystal structure of products and the lithium insertion performance. The nature of lithium insertion Si-M(M=Zr,Ni) alloy anode materials will be clarified, and lithium insertion mechanism will be proposed.

在新型锂离子电池负极材料的研究中，将硅与惰性嵌锂金属进行合金化，能在保持较高储锂容量的同时，有效缓解脱嵌锂过程中的体积效应，因而硅化物在负极材料研究中倍受重视。锂离子嵌入硅化物的反应过程对电池的综合性能具有重要影响。因此，研究嵌锂反应路径及产物晶体结构和相关嵌锂性能,进而揭示嵌锂机理，对高性能硅基合金负极材料的研发具有重要理论意义和实用价值。本课题以锂离子电池负极材料Si-M(M=Zr,Ni)合金为对象，采用相图实测和相图计算方法（CALPHAD），研究Li-Si-M(M=Zr,Ni)体系的稳定和亚稳相关系，并测定相关化合物的晶体结构；结合第一原理计算，研究锂嵌入Si-M（M=Zr,Ni）合金负极的路径，并计算相关嵌锂性质（包括体积膨胀率、最大嵌锂量、嵌锂电压等）；综合分析相变路径、产物晶体结构及嵌锂性质等研究结果，阐明锂嵌入Si-M（M=Zr,Ni）合金负极材料的本质，提出嵌锂机理。

为理解Si-M(M=Zr, Ni)合金负极材料的嵌锂机理，对Li-Si-M(M=Zr, Ni)三元相图进行了实验测定和热力学优化计算，并采用第一原理方法对部分合金的嵌锂路径、电化学性质与电子结构进行了计算。. 实验测量了Li-Si-Zr三元系470K等温截面和Li-Si-Ni三元系150℃等温截面。然后，采用 CALPHAD 方法和Thermo-Calc软件，优化计算了Li-Zr和Li-Ni二元系；结合文献中已有的Li-Si、Si-Zr和Si-Ni二元系的热力学参数，并以实测三元相关系和部分化合物形成焓的第一原理计算值为基础，对Li-Si-M(M=Zr, Ni) 三元系进行了热力学优化计算，计算结果与相关实验数据符合得较好，获得了一套合理、自洽的热力学参数，对新型锂离子电池硅基负极材料的研发具有重要理论指导作用。. 采用第一原理方法和VASP软件，计算了Li嵌入ZrSi2中的平均嵌Li形成能。ZrSi2嵌入Li时，首先生成ZrSi和Li12Si7，计算发现ZrSi与Li反应的形成能为负，不能继续嵌锂，而Li12Si7可与Li反应，其嵌锂路径为：Li12Si7→Li7Si3→Li13Si4→Li22Si5。. 计算了ZrSi2嵌锂时反应物与产物的电子结构。结果表明：ZrSi2、ZrSi、Li13Si4和Li22Si5均呈金属性质；差分电荷密度显示Si-Zr之间存在较强的共价键，Li-Si间以共价键为主并有一定的离子性质。. 计算了Li嵌入NiSi2中的平均嵌锂形成能，得到了最可能的三步反应嵌Li路径为NiSi2→NiSi→δ-Ni2Si→LiNi2Si。. 计算了LiNi2Si的电化学性质。发现Li嵌入Ni8Si4的平均嵌锂电压约为0.76V，说明Ni8Si4在嵌Li过程中能保持稳定的电压平台。当Li占据Ni8Si4所有八面体间隙形成Li4Ni8Si4时，体积膨胀率仅为13.495%，说明Ni能有效缓解嵌Li过程中负极的体积膨胀。. 计算了LixNi8Si4 (x=0,1,2,3,4)的电子结构。结果表明：Li嵌入Ni8Si4形成LixNi8Si4都呈金属性质；差分电荷密度显Ni-Si之间形成较强的共价键，能有效抑制体积膨胀。