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高硫含量正极推动高体积能量密度锂硫电池研究
清华大学绿色反应工程与工艺北京市重点实验室    2014-08-21 16:56:23   

高硫含量正极推动高体积能量密度锂硫电池研究

随着石墨负极的成功商用,锂离子电池在便携式电子设备等方面得到了广泛应用。经过近二十年的发展,传统离子电池几乎已经接近其理论能量密度,但是仍然无法满足高速发展的电子行业以及电动汽车领域对高能电池系统的要求。在各种高能电池体系中,锂硫电池2600 wh kg-1的理论能量密度(接近于传统离子电池的3~5倍)而受到学术和产业界的关注。目前锂硫电池的研究还主要集中在正极,研究对象仅针对基于单位质量硫的容量和能量密度,基于体积的容量和能量密度却鲜少人问津。德国能源署在制定2020年锂电行业计划时,并没有像大部分国家一样一味的追求质量能量密度的提高,而更加关注体积能量密度的提高,他们认为当质量能量密度大于140 Wh kg-1以后,体积能量密度将会是决定电池系统优劣的关键。

锂硫电池的正极硫在室温下的导电率极低(5×10-30 S cm-1),需要向其中添加大量导电剂来构建导电骨架,以增强硫的导电性。目前研究比较成功的导电添加剂主要是各类纳米碳材料,但是纳米碳材料的一个重要问题就是密度过低,其大量添加势必会削弱锂硫电池高能量密度,尤其是体积能量密度的优势。目前,锂硫电池中碳硫复合材料中的硫含量大部分集中在30 ~ 70 %,减少正极导电碳材料的添加量成为锂硫电池商用过程中无法躲避的鸿沟。

清华大学张强课题组在该领域的研究近来有了新进展(相关工作发表在Nano Energy, 2014, 4, 65–72)。该课题组对阵列碳纳米管和升华硫粉在室温下进行球磨处理,以实现碳硫之间的均匀分散。该方法避免了常规正极制备过程中的加热而导致的硫挥发损失,能够制备出可控硫含量的正极材料。另外,阵列碳纳米管是一类具有具有极高导电性的材料,其对聚合物的导电渗流阈值仅为0.0025 wt%,而常规的聚团碳纳米管的值在0.1–4.0 wt%。通过使用阵列碳纳米管来构建锂硫正极的导电骨架,制备出了硫含量高达90%的正极材料,为提高锂硫电池体积能量密度奠定了基础。通过密度测试发现,该正极材料的密度达到1.98 g cm-3(纯硫的密度为2.07 g cm-3),是目前硫正极密度的2~4倍。通过纽扣电池的测试发现,其在167 mA g-1的倍率下得到了高达1116 mAh cm-3的容量。结合器件的模拟计算,其获得了434 Wh L-1的体积能量密度,远远高于目前离子薄膜电池的水平。

基于阵列碳纳米管的硫正极,有效解决了导电剂的大量添加所导致锂硫电池能量密度减小的问题,对于凸显其体积能量密度的优势具有重要的意义。在解决多硫化物飞梭效应和负极枝晶的前提下,高体积能量密度锂硫电池将会在各种高能体系中脱颖而出。


Lithium ion batteries (LIBs) is one of the most important mobile power sources for our laptops, cameras, and smart phones. However, the current energy density of LIBs is approaching the theoretical limit, which arises urgent needs for new high energy density battery system. Among the high energy density storage systems, lithium-sulfur batteries with the energy density of 2600 Wh kg-1 (nearly 3~5 times than that of the traditional LIBs) holds the potential to serve as next generation of high energy battery. As element sulfur possesses a very low electric conductivity of 5×10-30 S cm-1 at room temperature, 30-70 wt.% conductive materials, e.g. carbon nanotubes, graphene, porous carbon, and conductive polymers, have to be added into the electrode for high utilization of sulfur at current processing technology. The addition of the nanocarbon materials with low stacking density neutralizes the high energy density, especially volumetric energy density of lithium-sulfur batteries.

Researchers in Prof. Qiang Zhang’s group from Tsinghua University in Beijing have developed a new strategy to increase the sulfur loading amount up to 90 wt% in the cathode materials based on aligned CNT/S scaffold, which benefits the ultra-high volumetric energy density of lithium-sulfur battery. A volumetric capacity of 1116 mAh·cm-3 and volumetric energy density of 434 Wh·L-1 were achieved based on the volume of total cell, including cathode, current collector, membrane, anode, which was far beyond the lithium thin-film battery. The team has published their findings in a recent issue of Nano Energy (2014, 4, 65-72).

 

“The design of sulfur cathode materials for lithium sulfur battery with high volumetric energy density is crucial for the practical applications,” said Qiang, “We selected aligned CNTs as the ultra-light scaffold for the reason that the aligned CNTs were with well alignment, hierarchical porous architecture, extremely high electrical conductivity, low density, as well as low cost.” In fact, such kinds of aligned CNTs with a length of 20-200 μm have been mass produced in a fluidized bed reactor at a low cost of less than 100 $·kg-1. “Such kind of aligned CNTs can be easily dispersed into polymer with a ultralow conductive percolation threshold of 0.0025 wt%. Obviously, they can also serve as high efficient conducting scaffold for sulfur materials.” as Prof. Fei Wei mentioned, “We have found a scalable, room-temperature, and one-step method for the fabrication of aligned CNT/sulfur cathode. The composite cathode material possess ultrahigh sulfur content of 90 wt % and a high density of 1.98 g cm-3, which is 2~4 times than that of the routine sulfur/carbon composite cathode. Therefore, the volumetric energy density of this research is far beyond the reported result.”

As Prof. Zhang point out, this approach shed some light on building lithium-sulfur batteries with high volumetric energy density by using high-density composite cathode with high sulfur loading amount. The future work in the development of lithium sulfur batteries may focus on the strategy of relieving the shuttle effect and suppress the lithium dendrites, and further improvement in gravimetric and volumetric energy density of lithium-sulfur electrochemical systems.

 

The link for the publication:

Cheng XB, Huang JQ, Zhang Q*, Peng HJ, Zhao MQ, Wei F. Aligned carbon nanotube/sulfur composite cathodes with high sulfur content for lithium-sulfur batteries. Nano Energy, 2014, 4, 65-72.

http://www.sciencedirect.com/science/article/pii/S2211285513002036

 

 

 

Highlight link:

http://phys.org/news/2014-01-ultra-high-volumetric-energy-density-lithium-sulfur.html#nRlv

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