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碳/硫界面调变提升锂硫电池稳定性
清华大学绿色反应工程与工艺北京市重点实验室    2014-08-21 16:57:14    文字:【】【】【

/硫界面调变提升锂硫电池稳定性

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为满足日益增长的能源及高性能储能器件的需求,新的电化学和新型二次电池技术亟待进一步的发展。得益于以锂-硫电化学对为代表的多电子传输化学以及单质硫低的电化学当量和丰富的储量,锂硫电池具有远超传统的锂离子电池接近四倍的、高达2600 Wh L-1的理论能量密度,因而其近年来正成为储能研究和二次电池技术的热点。然而其实际应用仍然受限于较低的活性材料利用率、较低的效率、不理想的倍率性能和较差的循环寿命。其中,循环寿命则是解决锂硫电池实用性难题的重中之重。通常锂硫电池的低寿命和快速的容量衰减被归咎于以下两点原因:(1)可溶性中间产物多硫化物在电解液中的溶解、扩散及和负极的不可逆反应;(2)放电产物硫化锂从导电骨架中剥离。由于优良的导电性、高比表面积以及纳米孔限域作用,多孔纳米碳材料被认为是储存活性物质硫的优良载体。然而由于锂硫电池多电子传输过程的特性,过程中含硫客体分子呈现与单质硫截然不同的极性,使得含硫分子和碳载体之间的相互作用随电化学进程发生巨大改变。因此传统的碳基介质难以满足不同极性含硫分子在电极表面高效利用的需求。

近期,清华大学张强课题组研究发现,将原有的sp2共轭连接的碳原子部分替换为电负性更强的氮原子,既保证了碳基材料优异的导电性,又调变了碳/硫界面的相互作用。该课题组通过理论计算表明:极性的多硫化物和硫化锂分子倾向于吸附在具有一对孤对电子的吡啶氮位点以及石墨氮位点周围的碳原子表面。掺杂氮之后,碳基体和极性含硫分子之间的相互作用增强,将提升多硫化物的利用率,削弱多硫化物向外溶解的效应,同时增强硫化锂和导电骨架的结合能力。因此,将氮掺杂碳纳米管作为硫正极的导电骨架,锂硫电池的循环性能显著提高。相比未掺杂的碳纳米管,其循环寿命提升了六倍。该研究近期发表于Advanced Material Interfaces10.1002/admi.201400227)上

该工作表明,通过对复杂电化学过程解耦分析、理解符合电池实际需求的界面关系并设计表面性质可调的先进碳材料,高能量密度、长循环寿命的二次锂电池技术将成为可能。这种活性物质分子和导电基体之间相互作用关系的理解和调变还将在金属-空气电池、超级电容器和燃料电池等领域发挥重要的作用。

英文新闻稿:

Heterogeneous carbon hosts enable stable lithium-sulfur battery

 

Sulfur, in tremendous abundance as byproducts of petroleum industry, is one of the most intriguing solution to address the energy dilemma by manifesting the chemistry between sulfur and lithium. Thus, lithium-sulfur battery, employing lithium-sulfur redox couple, theoretically delivers energy density of 2600 Wh kg-1, which is 3-5 times higher than traditional lithium-ion batteries. Although it depict a promising prospect, there are still several obstacles hindering its practical application. One, of the most importance, is the rapid capacity fading.

“The fast capacity decay of lithium-sulfur battery is ascribed to multifaceted aspects. One of the most widely accepted reason is assigned to the intermediate polysulfides. Polysulfides is a variety of transition form of sulfur partially lithiated, which is highly polar and soluble in the typical organic electrolyte here we used. During discharge, they dissolve in the electrolyte, diffuse from cathode to anode, and react with lithium anode. The active materials lose in this way, causing capacity fading undoubtedly,” said Dr. Qiang Zhang, an associate professor at Department of Chemical Engineering, Tsinghua University, this issue gains enormous concern and considerable endeavor is dedicated to address this problem. But what we are interested in is another rarely concerned issue responding the capacity fading. It’s dynamic fluctuation of affinity between different sulfur species and conductive host materials.”

“Because of the multi-electron-transfer process, sulfur species vary from initial elemental sulfur, intermediate polysulfides, and final discharge product of lithium sulfides. Sulfur is unpolar, thus exhibits highest affinity to conventional carbon hosts. While polsulfides and lithium sulfides are highly polar, weakening the interaction between them and carbon. Due to this poor interaction, they easily detach from the carbon host and contribute no capacity. As a result, the performance of a lithium-sulfur battery deteriorates rapidly when only pure carbon hosts is employed,” said Qiang, “Consequently, the key issue lies on how to choose an ideal host material with high affinity to both unpolar sulfur and polar polysulfides, as well as lithium sulfides.”

Herein, nitrogen-doped carbon nanotubes were adopted as host material for sulfur cathode. Nitrogen atoms with higher electronegativity are incorporated into the graphitic lattices of carbon nanotubes, which has been demonstrated capability to tune the electronic structure and surface properties. How does the doping nitrogen atoms affect the electrochemical behavior when nitrogen-doped carbon nanotubes are applied for lithium-sulfur battery?

Hong-Jie Peng, a graduate student and the first author, deliberately answered this question. “Firstly, we conducted a density functional theory (DFT) study and designed three model molecular to illustrate pure carbon, carbon with nitrogen at the edge, which we called pyridinic nitrogen, and carbon with nitrogen substituting the central carbon atom, which we called quaternary nitrogen,” Hong-Jie said, “through theoretical calculation, we found nitrogen-doped carbon nanotubes exhibited stronger interaction with polysulfides and lithium sulfides. This is attributed to the adsorption of these polar sulfur species on the negatively charged nitrogen-doped sites. It revealed that nitrogen-doped carbon nanotube might worth trying”

 “Then, we just prepared nitrogen-doped carbon nanotube/sulfur composites and assembled batteries to check if our theoretical results are reliable. Amazingly, the electrochemical experiment matched theoretical prediction very well. Comparing to raw carbon nanotube-based battery, the cycling life was significantly promoted by six times. Furthermore, delicate electrochemical analysis supported theoretical results and cell performance.” said Hong-Jie. This work proposes the importance of a stable dynamic interface between carbon hosts and sulfur-containing guests and shed a new light on lithium-sulfur battery decay mechanism, which has been published on Advanced Material Interfaces recently.

“In fact, the concept of building heterogeneous cathode scaffold won’t go to an end. More advanced host materials satisfying the demand of amphiphilicity to both unpolar and polar sulfur species is going to be explored” Qiang mentioned.

 

 

 

 

 

 

论文链接:

Peng HJ, Hou TZ, Zhang Q, Huang JQ, Cheng XB, Guo MQ, Yuan Z, He LY, Wei F.

Strongly Coupled Interfaces between a Heterogeneous Carbon Host and a Sulfur- Containing Guest for Highly Stable Lithium-Sulfur Batteries: Mechanistic Insight into Capacity Degradation

Advanced Materials Interfaces, 2014, DOI: 10.1002/admi.201400227

http://onlinelibrary.wiley.com/doi/10.1002/admi.201400227/abstract

评论链接:

http://www.materialsviewschina.com/2014/08/tan-liu-jie-mian-diao-bian-ti-sheng-li-liu-dian-chi-wen-ding-xing/

http://www.nanowerk.com/spotlight/spotid=36790.php

http://phys.org/news/2014-07-heterogeneous-carbon-hosts-enable-stable.html

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