Knowledge Management System Of Institute of process engineering,CAS
由于具备理论容量高（3861 mAh/g）、还原电势最低（-3.04 V，相对于标准氢电极）等优点，锂金属是最具潜力的下一代高能量密度电池负极材料。然而由于金属锂化学性质活泼，易与电解质发生副反应形成不稳定的电极/电解质界面，导致不可控的枝晶生长，进而影响电池的使用。LiF是固态电解质界面膜（SEI）的主要无机成分之一，已有实验证实，在SEI形成过程中添加LiF可以调控锂沉积形貌，稳定电极/电解质界面，但其中具体的机理尚不明确。本文利用模拟计算的方法，构建了锂金属和LiF界面，并研究了锂金属电极/LiF界面的相互作用机制。基于第一性原理的计算结果，本文主要得到了以下创新性成果：（1）分别构建了Li、LiF表面模型，初步揭示了该表面模型中电子结构与离子迁移特性。当slab Li(001)层数为5时，表面能为0.48 J/m2，且继续增加层数，表面能趋于收敛。结合文献参考值以及计算时长，slab LiF(001)层数取3。与bulk LiF模型相比，由于尺寸效应的存在，slab LiF(001)电子结构的带隙值从8.87 eV降低到5.81 eV。Li在锂金属表面两个相邻Hollow位点的迁移需要克服0.03 eV的吸附能垒。Li+在LiF体相中以空位机理，间隙机理进行迁移时，迁移能垒分别为0.64 eV与0.74 eV。（2）构建了锂金属负极与LiF界面模型，计算了Li+在Li/LiF界面模型的迁移能垒，并得到了LiF可以稳定锂金属电极/电解质界面、稳定锂金属负极的机理。本文中构建的Li/LiF界面模型晶格失配度为2.17%，满足小于5%的要求。计算得到的Li/LiF界面相互作用能为-3.81 meV/m2。相互作用能为负值，说明构建的界面在热力学上是可行的。Li/LiF界面费米能级附近带隙为零，表明Li/LiF界面体系电导率高，对离子在界面中的迁移有一定影响。Li/LiF模型中靠近界面的Li层Li-2s轨道与靠近界面的LiF层F-2p轨道态密度分布有相互重叠的部分，验证了两者存在相互作用，影响了电荷在界面体系的分布。Li/LiF界面中，电荷重排且电子主要集中在界面处，远离界面的Li层与LiF层则没有明显的电子得失转移。界面的形成促使电荷重排，且主要集中在界面处，使得整个界面体系的电荷浓度分布存在电势梯度，从而诱导电解质中的Li+在界面聚集。Li+在Li/LiF界面的迁移能垒（0.58 eV）小于在金属锂表面的吸附（0.03 eV）与bulk LiF中的扩散能垒（0.64 eV）之和，表明Li/LiF界面阻抗低，促进聚集的Li+在界面处快速迁移，提高了Li+的扩散速率，从而有利于Li+在界面处均匀沉积，得到可控的电极/电解质界面。界面电荷的重新分布、Li+在界面较低的迁移能垒，两者共同作用，使得LiF可以起到稳定锂金属负极/电解质界面、保护锂金属负极的效果。计算结果从原子水平上提出了Li/LiF界面相互作用机理的新见解，并为探索锂金属电池体系中的新型电解液添加剂或界面保护材料设计提供了新的思路。对锂金属负极与LiF界面的深入了解将有利于锂金属电池的应用和进一步发展，并对未来稳定锂金属负极的设计有一定的指导意义。 ;Due to its high theoretical capacity (3861 mAh/g) and the lowest reduction potential (-3.04 V compared to standard hydrogen electrode), lithium metal is expected to be the next generation ideal anode material for energy density batteries. However, lithium metal anode with relatively active characteristics can easily react with electrolytes to form an unstable interface between anode and electrolytes, resulting in uncontrollable dendrite growth and then affecting the performance of batteries. LiF, as one of the main inorganic components of the solid electrolyte interphase (SEI) film, has been experimentally confirmed that the addition of LiF during the formation of SEI can regulate the morphology of lithium deposition and improve the electrochemical performance of batteries, while the specific mechanism has not yet been clarified. In this thesis, the interface between Li metal and LiF were constructed with computation to study the specific interaction mechanism at the interface. Based on the results of first principles calculations, main innovative results of this work are summarized below: （1）Surface models of Li and LiF were constructed and both the electronic properties and ion transport mechamisms of the related models were calculated. When the number of layers in the slab Li(001) model was five, the corresponding value of surface energy was 0.48 J/m2. As the number of layers continued to increase, the surface energy tended to converge. Combined the literature reference value with the calculation duration, the number of layers in slab LiF(001) model were determined to be three. Compared with bulk LiF, the band gap of slab LiF(001) reduced from 8.87 eV to 5.81 eV owing to the size effect. The migration of Li atom at two adjacent stable Hollow sites on the surface of lithium metal needed to overcome an adsorption energy barrier with 0.03 eV. The migration energy barriers were 0.64 eV and 0.74 eV respectively when Li+ migrated in the bulk LiF structure by the vacancy mechanism and the interstitial mechanism. （2）An interface of Li and LiF was built, and both electronic features and ion diffusion properties of the interface were simulated. The mechanism of LiF suppressing the lithium dendrite growth could be obtained simultaneously. The Li/LiF model constructed here had a lattice mismatch of 2.17%, which satisfied the requirement of less than 5%. The interfacial interaction energy is -3.81 meV/m2. The energy value was negative, indicating that the constructed interface model was thermodynamically feasible. The band gap of the interface near the Fermi level was zero, revealing that the interface system of Li/LiF had high conductivity and was beneficial to ion migration. At the interface of Li/LiF, the density of state about the Li-2s orbital in the Li layer and the F-2p orbital in the LiF layer near the interface overlapped each other, verifying that there is interaction between the Li layer and F layer, which had a certain influence on the distribution of electrons. The charge rearrangement was mainly concentrated on the interfacial layer, while the layers which were far from the interface had no obvious electrons gained and lost. Therefore, the charge concertration distribution of the entire interface system has a potential gradient, inducing Li+ in the electrolyte to aggregate at the interface. The migration energy barrier of Li+ at the Li/LiF interface (0.58 eV) is much smaller than the sum of that on lithium metal (0.03 eV) and in bulk LiF (0.64 eV), which indicated a low impedance at the interface, thus it facilitated the uniform deposition of ions aggregated at the interface and induced the formation of a controllable electrode/electrolyte interface. The redistribution of interfacial charge and the lower energy migration barrier at the interface worked together to make LiF stabilize the interface of lithium metal anode/electrolyte and protect the lithium metal anode.The calculation results from the atomic level proposed a new insight into the interaction mechanism of Li/LiF interface, and provided a new idea for exploring new interface protection materials or electrolyte additives in lithium metal batteries. The in-depth understanding of the interface between Li metal anode and LiF would be beneficial to the application and further development of lithium metal batteries, and would had certain guiding significance for the design of a stable lithium metal anode in the future.
|吴小红. 锂金属电极/电解质界面的第一性原理研究[D]. 中国科学院大学,2019.|
|Files in This Item:|
|锂金属电极-电解质界面的第一性原理研究.（4010KB）||学位论文||限制开放||CC BY-NC-SA||Application Full Text|
|Recommend this item|
|Export to Endnote|
|Similar articles in Google Scholar|
|Similar articles in Baidu academic|
|Similar articles in Bing Scholar|
Items in the repository are protected by copyright, with all rights reserved, unless otherwise indicated.