金属锂负极的改性及其电化学性能的研究

发布时间：2018-04-30 02:07

本文选题：金属锂电极 + 二次电池　；参考：《浙江大学》2017年博士论文

【摘要】：随着电动汽车和消费性电子工业的发展,对于高能量密度存储电源的需求越来越强烈。金属锂负极因其拥有高的理论比容量(3860mAhg-1)和较低的电势(-3.04 V)而成为高能量密度电池系统的理想候选。但是,金属锂作为电池负极依然存在两个非常严重的问题:(1)金属锂过于活泼,会与电解质发生副反应,电极消耗严重,库伦效率很低;(2)表面SEI容易破损,且表面电位不均匀,导致反应过程中有“枝晶”和“死锂”的产生,甚至会刺破隔膜,造成安全隐患。库伦效率低以及安全性能差极大地阻碍了金属锂负极的实际应用。目前关于金属锂负极改性方法主要有:电解液改性、隔膜改性、集流体改性以及金属锂负极直接改性。其中,对金属锂负极直接进行改性是研究的重点,也是最有望能够实现商业化应用的方法之一。本论文主要采用以下方法对金属锂负极进行了改性研究:(1)利用原位反应方法在金属锂表面制备Li_3N薄膜,得到Li_3N/Li复合负极材料。在该反应中的主要反应参数有:反应时间、反应温度以及通气流量。利用控制变量法研究不同反应参数对Li_3N/Li复合负极电化学性能的影响。在最优反应参数下制备Li_3N/Li复合负极材料,将其与纯Li负极材料进行比较,表现出了较好的电化学性能,且循环后的枝晶形貌被明显抑制,副反应产物也较少。说明表面包覆Li_3N不仅可以保护金属锂不与电解液接触,减少副反应的发生,还可以抑制枝晶的形成,提高安全性能。(2)采用磁控溅射的方法在金属锂表面沉积了a-C纳米薄膜。控制磁控溅射得到a-C包覆层厚度不同的a-C/Li负极。对其进行电化学性能测试,发现随着沉积时间的延长,a-C/Li负极的阻抗性能下降,锂离子传输受到阻碍。对循环后的电极表面进行SEM观察,发现沉积时间越久,对枝晶生长的抑制作用越明显。因此,沉积时间过长或过短在本实验中都会减弱a-C膜对金属锂负极电化学性能的促进作用。(3)结合Li_3N薄膜的高离子电导率和磁控溅射a-C纳米薄膜的均匀致密性,采用磁控溅射方法在金属锂负极表面沉积掺氮非晶碳(a-CN_x)膜。研究了不同含氮量对a-CN_x薄膜性能的影响,控制氮气分压分别为0 sccm,5 sccm和10 sccm,发现氮气分压为10 sccm时a-CN_x/Li负极的电化学性能最好。通过共聚焦原位测试,分别观察了纯Li负极、a-C/Li负极以及a-CN_x/Li负极(10sccm)在不同电流密度下的枝晶生长情况。其中,纯锂负极在1 C电流密度下即产生明显枝晶,刺破隔膜;a-C/Li负极在5 C电流密度下才观察到有刺猬状的枝晶产生;而a-CN_x/Li负极(10 sccm)所对应的隔膜在10 C电流密度下才被穿透。这一方面说明,表面包覆非晶碳薄膜或者掺氮非晶碳薄膜都可以抑制枝晶的形成;另一方面说明,相对于不掺杂N元素的非晶碳薄膜而言,掺杂了N元素的碳氮复合薄膜对枝晶的生长具有更好的抑制作用。(4)采用抽滤法制备微米级的GO纸膜,在水合肼中还原成为rGO薄膜,利用机械压实法得到rGO/Li复合负极材料。通过循环后的平面和断面形貌观察,纯Li负极的表面有明显的枝晶形貌且断面形貌发生坍塌;而rGO/Li复合负极的平面和断面形貌都保持良好。通过金属锂对称电池以及Li-Cu半电池体系进行电化学性能测试,可以看出rGO包覆对提高金属锂负极的循环稳定性有明显的促进作用。最后对不同次数下rGO/Li负极的SEM形貌图进行观察,描述了锂枝晶的生长过程。(5)采用自动铺展法在金属锂片表面包覆了纳米级GO薄膜。选用合适的有机溶剂来分散石墨烯粉末非常重要,我们对乙醇、乙腈、乙醚以及DMC四种有机溶剂进行研究,其中DMC既可以溶解极性的GO粉末,与金属锂之间也有足够的稳定性。对纯Li负极以及GO/Li负极在金属锂对称电池以及锂硫全电池体系中的电化学性能进行比较,发现GO膜包覆一方面可以保护金属锂负极不与电解液接触,提高其电化学性能;另一方面可以抑制枝晶的形成,提高循环稳定性。(6)采用熔融法制备了垂直石墨烯(VG)与金属锂的复合负极材料。首先利用磁控溅射技术在VG阵列表面沉积一层Si,使“疏锂性”的VG阵列转变成“亲锂性”的Si@VG复合阵列。然后在200℃下将液态锂灌入阵列结构之中,得到Si@VG/Li复合负极材料。VG阵列的多孔结构可以控制金属锂的体积膨胀,并抑制枝晶的形成,使其具有良好的电化学稳定性。除此之外,Si@VG/Li复合负极材料在Li-S全电池体系中也表现出了良好的循环稳定性。
[Abstract]:With the development of electric vehicles and consumer electronics industry, the demand for high energy density storage power is becoming more and more intense. Metal lithium anode is the ideal candidate for high energy density battery system because of its high theoretical specific capacity (3860mAhg-1) and lower potential (-3.04 V). However, the lithium metal is still two as the negative electrode of the battery. A very serious problem: (1) the metal lithium is too active, will have a side reaction with the electrolyte, the electrode consumption is serious, the efficiency of the Kulun is very low; (2) the surface SEI is easily damaged and the surface potential is not uniform, which leads to the production of "dendrite" and "dead lithium" in the reaction process, and even the septum will be pierced to cause the hidden danger. The efficiency and safety of Kulun is low and safe. The performance difference greatly hinders the practical application of metal lithium anode. At present, the main methods for the modification of lithium negative electrode include electrolyte modification, membrane modification, fluid collector modification and direct modification of metal lithium anode. This paper mainly uses the following methods to study the modification of metal lithium anode: (1) the preparation of Li_3N film on the surface of lithium metal by the in-situ reaction method, and the Li_3N/Li composite anode material is obtained. The main reaction parameters in this reaction are reaction time, reaction temperature and ventilation flow. The different reactions are studied by the control variable method. The effects of parameters on the electrochemical performance of Li_3N/Li composite negative electrode. Li_3N/Li composite negative electrode was prepared under the optimal reaction parameters. Compared with the pure Li negative electrode, it showed good electrochemical performance, and the dendrite morphology was obviously suppressed and the side reaction products were less. The surface coating Li_3N not only can protect gold. Lithium does not contact with the electrolyte, reduces the occurrence of the side reaction, inhibits the formation of the dendrite and improves the safety performance. (2) the a-C Nanothin film is deposited on the surface of the lithium metal by magnetron sputtering. The magnetron sputtering is used to control the a-C/Li negative electrode of the a-C coating with different thickness. The resistance performance of the a-C/Li negative electrode decreased and the lithium ion transmission was hindered. SEM observation on the surface of the electrode after the cycle found that the longer the deposition time was, the more obvious the inhibition effect on the dendrite growth. Therefore, the longer or too short deposition time will weaken the effect of the a-C film on the electrochemical performance of the lithium anode. (3) Combined with the high ionic conductivity of Li_3N films and the uniform densification of a-C Nanothin films by magnetron sputtering, the nitrogen doped amorphous carbon (a-CN_x) films were deposited on the surface of metal lithium anode by magnetron sputtering. The effects of different nitrogen content on the properties of a-CN_x films were studied. The nitrogen partial pressure was 0 SCCM, 5 SCCM and 10 SCCM respectively, and the nitrogen partial pressure was found to be 10 s. The electrochemical performance of a-CN_x/Li negative electrode was the best at CCM. The dendrite growth of pure Li negative electrode, a-C/Li negative electrode and a-CN_x/Li negative electrode (10sccm) was observed at different current densities by confocal in-situ test. The pure lithium negative electrode produced obvious dendrites and pierced the diaphragm at 1 C current density, and a-C/Li negative electrode was at 5 C current density. The hedgehog like dendrites were observed, and the a-CN_x/Li negative (10 SCCM) diaphragm was penetrated at 10 C current density. In this respect, the surface coating of amorphous carbon film or nitrogen doped amorphous carbon film can inhibit the formation of dendrites; on the other hand, the phase is doped for amorphous carbon films without N elements. The carbon and nitrogen composite film of N element has a better inhibitory effect on the growth of dendrite. (4) the micrometer GO paper film was prepared by the extraction method and reduced to rGO film in hydrazine hydrate. The rGO/Li composite negative material was obtained by mechanical compaction. The surface of pure Li negative electrode surface has obvious dendrite morphology through the plane and cross section morphology after the cycle. The plane and surface morphology of the rGO/Li composite negative electrode remained well. The electrochemical performance test of the lithium symmetric battery and the Li-Cu semi battery system showed that the rGO coating could improve the cyclic stability of the lithium anode. Finally, the SE of the rGO/Li negative electrode at different times was SE. M morphologies are observed and the growth process of lithium dendrites is described. (5) the nano scale GO film is coated on the surface of lithium metal sheet by automatic spreading method. It is very important to choose the suitable organic solvent for dispersing the powder of graphene. We have studied the four organic solvents of ethanol, acetonitrile, ether and DMC, in which DMC can dissolve the GO of polarity. There is sufficient stability between the powder and the lithium metal. The electrochemical performance of pure Li negative electrode and GO/Li negative electrode in the lithium and sulfur full battery system is compared. It is found that the coating of the GO film can protect the lithium negative electrode from the electrolyte contact and improve its electrochemical performance; on the other hand, the dendrite can be suppressed. It is formed to improve the cyclic stability. (6) a composite anode material for vertical graphene (VG) and lithium metal is prepared by melting method. First, a layer of Si is deposited on the surface of VG array by magnetron sputtering, and the "lithium sparsely" VG array is transformed into a "lithium-dependent" Si@VG composite array. Then the liquid lithium is poured into the array structure at 200. The porous structure of the Si@VG/Li composite anode material.VG array can control the volume expansion of the lithium metal and inhibit the formation of the dendrite, so that it has good electrochemical stability. In addition, the Si@VG/Li composite negative electrode also shows good cyclic stability in the Li-S full battery system.

【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TM912

【参考文献】