摘要
锂离子电池是现代科技的核心技术之一，锂离子电池具有很多优越性能，已在手机、移动充电器等便携式设备中得到了广泛的应用，此外，有望应用到电动汽车、航天航空等其它高新领域。然而随着社会经济的发展，当前的锂离子电池已经无法满足市场的需求，人们对它的实用性要求越来越高，例如对它的功率、能量密度和充放电速率等性能的要求。
最近，锂-硫电池激起了很多研究者的兴趣，原因是硫具有巨大的比容量（1673 mAhg-1）。此外，硫是丰富的、廉价的且环境友好的。同时，锂-硫电
美丽的漓江河简谱）的低电导率（在25℃时的电导率为池的缺点也是明显的，包括长硫链（如S
8
5x10-30 S cm-1）和多硫锂化物的高溶解率（多硫锂化物很容易溶解在电极溶液中，引起活性硫的损耗）。这些问题限制了锂-硫电池的循环寿命，为了防止锂硫化合物在电解液中的溶解，目前，国际上实验研究主要集中在包含硫的复合碳纳米材料上，如介孔和微孔碳材料，石墨烯和氧化石墨烯，碳纳米管，碳纤维，和空心介孔相材料等。虽然在不同类型的碳材料的探索上，大家付出了很大努力，但是对其中的微观机制，目前的理解还非常有限。
由于锂资源并不丰富，它的可利用性受到了很大限制。随着锂离子电池的市场化，原材料价格也相应地提高了，导致成本明显提高。最近，钠离子电池受到广泛关注。钠离子电池与锂离子电池原理相似，钠资源丰富，钠离子电池被认为有希望部分代替锂离子电池。实际研究中，国际上各研究团体发现找到合适的负极材料很困难，原因是钠离子的离子半径(~98-102 pm) 远大于锂离子的半径(76 pm)。锂与石墨复合可形成稳定的嵌层化合物LiC6，因此石墨可作为一个很好的锂离子电池的负极材料，但是对于钠离子，与石墨复合仅可形成稳定的嵌层化合物NaC64，导致石墨只有~35mAh/g的电容量。目前，实验上正在探索硬碳、软碳、膨胀石墨等碳材料，希望寻找一种优秀的钠离子负极材料。
在本论文中，针对上面提到的锂、钠离子电池中的两个问题，我们研究了（1）锂硫团簇与石墨烯及缺陷石墨烯的相互作用，（2）钠离子和钠离子团簇在石墨烯及缺陷石墨烯中的存储。具体研究如下:
I
II 一、通过第一性原理计算，研究了锂硫团簇和石墨烯的相互作用，以及在硫
元宵节菜谱>虾仁拌黄瓜存在的情况下，锂团簇在本征石墨烯和缺陷石墨上的扩散。我们发现引入锂后，硫和石墨烯的相互作
用会被削弱，从而形成锂硫团簇。通过电荷转移，这些锂硫团簇与石墨烯有较强的相互作用。我们发现双空位石墨烯可以让锂离子穿过，而硫的存在会限制锂扩散，这是由于锂硫团簇在空位处的存在。同时发现，随在空位处锂离子浓度提高，锂离子可以顺利地穿越空位石墨烯。因此，空位石墨烯通过较强的吸附作用可以有效地防止锂硫短链溶解于电解液中，同时锂离子通过空位穿越石墨烯，可在硫正极和电解液之间自由迁移。此外，通过锂硫团簇与缺陷石墨烯形成的复合体系的电子结构分析，发现缺陷石墨烯作为包覆层，可以有效地提高硫正极的电导性。
计算机文件二、通过理论计算，同时考虑范德华力效应，分析了钠离子及团簇在本征石墨烯和空位石墨烯上的储存情况。我们发现钠与石墨烯的相互作用较弱，钠离子或团簇不能有效地吸附在本征石墨烯上。但是在有空位存在时，发现钠离子或团簇可以有效地吸附在石墨烯上，同时吸附在双空位的钠团簇会随着钠团簇尺寸的增大而减小，这暗示空位石墨烯作为钠离子电池的负极，可有效地抑止钠枝晶的形成。我们也探索了空位浓度效应，发现随着石墨烯缺陷浓度的增加，钠容量有明显的提高。发现在空穴浓度较高时，可储存在缺陷石墨烯中的钠的含量是石墨的10-30倍。我们预言高浓度空位石墨烯是潜在的优秀的钠离子负极材料。
一平方米等于多少亩关键词：
第一性原理计算，锂硫电池，锂硫团簇，钠离子电池，钠团簇，双空位石墨烯，吸附能，扩散势垒
Abstract
Lithium-ion batteries with superior performances, being widely ud in portable devices, have been one of the cores of modern technologies and life. Now its range of applications is expected to extend to other high-tech areas, such as electric vehicles and aerospace. However, current lithium-ion batteries have failed to meet the needs of the market. People expect lithium-ion batteries have higher performances, such as energy power, energy density, and charge/discharge rate.
Currently, lithium-sulfur batteries have attracted many rearchers becau of the large specific capacity of sulfur (1673 mAhg-1) as the cathode. In addition, sulfur is rich, inexpensive, and environmentally friendly, though it also has some obvious shortcomings including the low specific conductivity of S8 (5x10-30 S cm-1 at 25℃), and high dissolution rate of the short-chain polysulfides, which can make it easily dissolve in the electrolyte, resulting in the loss of active sulfur. The problems limit the cycle life of lithium-sulfur batteries. In order to prevent the dissolution of lithium-sulfur compounds, the current experimental rearches mainly focus on carbon nano-composite materials as the cover layer of sulfur cathode, such as microporous and mesoporous carbon materials, graphene, graphene oxide, CNT, carbon fiber, hollow mesoporous phas, and so on. Though hard work has been done, the understanding of detailed micro-mechanism of the interaction 学生作息时间表
人体生物节律between Li-S and carbon structures in carbon-sulfur composites still is limited.
Since lithium is not abundant on earth, the availability of it has been limited. With the marketization of lithium-ion batteries, the price of raw material has incread, which leads to the high cost. Recently, sodium-ion battery has been regarded as a promising substitute of lithium-ion battery, since it have a similar principle with lithium-ion battery and sodium is rich in our earth. However, many rearchers find that it is difficult to find the proper anode materials, becau the radius of sodium ion (~98-102 pm) is much larger, compared to that of lithium (76 pm). Graphite, a stable nested layer
III
compound in which Li ions can intercalate easily, is an excellent anode material of lithium-ion batteries. But for the Na ions, the most stable structure with the intercalation of Na ions is NaC64, leading to the low electric capacity of ~35mAh/g. At prent, different layered carbon, such as hard carbon, soft carbon, expandable graphite and some other materials is being test in experiments. We look forward to find a new anode material which can absorb sodium ions effectively with high concen
小学英语总复习资料tration.
In this paper, in order to solve the two questions above, we studied (1) the interaction of lithium-sulfur clusters with graphene and defective graphene, (2) the interaction between sodium ions/clusters and graphene and the storage of sodium ions in defective graphene. The detailed introductions are as follows,
1. With first-principles calculations under the existence of sulfur, we studied the interaction between lithium-sulfur clusters and graphene, and the adsorption of lithium clusters on defective graphene. We find that with the introduction of lithium ions, the interaction between sulfur and graphene is weakened, leading to the formation of lithium-sulfur clusters, which interact with graphene by charge transfer. We find that lithium ion can diffu through double-vacancy graphene. However, sulfur can limit the diffusion of lithium. This is becau of the formation of lithium-sulfur clusters near double-vacancy. With the increa of Li concentration near double-vacancy, lithium ions become to diffu easily through double-vacancy graphene. In addition, with the analyzation of electronic properties, defective graphene with the adsorption of Li-S chains has good conductivity.
2. With first-principles methods including van der Waals force, we analyzed the absorption of sodium
ion and its clusters on graphene. It is found that the interaction between sodium and graphene is so weak that sodium ion and its cluster are difficult to absorb on graphene with high concentration. However, with double-vacancy on graphene, sodium ion and its cluster can easily absorb on the graphene. The sodium clusters are adsorbed near double-vacancy. With the increa of the cluster’s size, the adsorption becomes to be weakened. In addition, we find that with the increa of the density of double-vacancy, sodium capacity is enhanced significantly. Under the high density of defects, the content of sodium on defective graphene is 10-30 times more IV
than graphite.
Keywords:
First principles calculation, Li-S batteries, Li n S clusters, Na-ion batteries, Na clusters, Double-vacancy graphene, Adsorption energy, Diffusion barrier
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