Structural hierarchy is the cornerstone of the biological world, as well as the most important lesson that we have learned from nature to develop ingenious hierarchical porous materials for various applications in energy conversion and storage. Recently, a research group from China, led by Prof. Qiang Zhang in Tsinghua University, has developed a novel kind of hierarchical porous graphene (HPG) via a versatile chemical vapor deposition (CVD) on CaO templates for high-power lithium-sulfur (Li-S) batteries. This work is published in the journal Advanced Functional Materials.
"Due to the urgent demand for sustainable energy systems and portable energy storage devices, the Li-S battery has been cited as the most promising alternative for next-generation energy storage devices, due to its high theoretical energy density of 2600 Wh kg-1, low cost, and eco-friendliness," said Prof. Zhang. "Despite these advantages, the practical application still suffers from a formidable challenge due to the intrinsic insulation of sulfur and lithium sulfides, the dissolution of polysulfides with a shuttle effect, and the huge volume change of cathode materials during operation."
Researchers have been seeking to develop hierarchical nanocarbon materials with tunable structural hierarchies and surface features for use as Li-S cathode scaffolds to address these issues effectively. Hierarchical porous materials exhibit porosities on more than one length scale with different properties and roles, respectively. It is important for the improvement in Li-S battery performances.
"However, strategies with a multistep process or/and multiscale templates are dominantly employed to obtain hierarchical porosities. It is always complicated and unfavorable to the structure regulations," says Cheng Tang, a graduate student and the first author.
For the first time, Cheng proposed the hierarchical porous CaO particles as effective catalytic templates for the facile CVD growth of graphene. CaO is a very common and promising material with a low cost, easy purification, and promising cyclic utilization. Additionally, various hierarchical structures can be readily obtained for CaO, making it a versatile strategy to fabricate HPG materials with tunable structural hierarchy.
Based on this concept, they obtained a hierarchical porous structure of graphene with abundant microsized inplane vacancies, mesosized wrinkled pores, and macrosized strutted cavities. It can serve as a favorable scaffold for cathodes of Li-S batteries with enhanced utilization of sulfur, high discharge capacity and efficiency, superior stability, and excellent rate capability. The small mesopores facilitate the entrapment of sulfur and polysulfides; the micropores and defective graphene layers with a high SSA accommodate a high sulfur loading with intimate affinity; the interconnected large mesopores and macropores shorten the transport distance of ion and electrolyte.
"We hope that the novel fabrication strategy of hierarchical porous graphene materials can improve the properties of cathode scaffolds for the practical application of Li-S batteries." said Qiang. Further improvements are expected with more meticulous design of the hierarchical structure and additional surface modification. The idea presented here opens up new perspectives to develop nanoarchitectured graphene with metal oxide catalysts, which is fresh but versatile towards tunable structures, variable properties, and promising applications.
The ultra-high volumetric energy density lithium-sulfur battery
January 23, 2014
Lithium ion battery technology (LIBs) is one of the most important mobile power sources for laptops, cameras, and smart phones. However, the current energy density of LIBs is approaching the theoretical limit, which underscoring the urgent need for new high energy density battery systems. Among the high-energy density storage systems, lithium-sulfur batteries, with 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. Sulfur possesses a very low electric conductivity of 5x10-30 S cm-1 at room temperature. Therefore, 30-70 wt. percent 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 nanocarbon materials with low stacking density neutralizes the high energy density, especially the 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 percent in cathode materials based on an aligned CNT/S scaffold, which benefits the ultra-high volumetric energy density of lithium-sulfur batteries. A volumetric capacity of 1116 mAh・cm-3 and volumetric energy density of 434 Wh・L-1 were achieved based on the volume of the 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 batteries with high volumetric energy density is crucial for practical applications," said Qiang. "We selected aligned CNTs as the ultra-light scaffold because they demonstrate 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 per kg-1. "These aligned CNTs can be easily dispersed into polymer with an ultra-low conductive percolation threshold of 0.0025 wt percent. Obviously, they can also serve as a high-efficiency conducting scaffold for sulfur materials." Prof. Fei Wei adds, "We have found a scalable, room-temperature, one-step method for the fabrication of an aligned CNT/sulfur cathode. The composite cathode material possesses ultra-high sulfur content of 90 wt percent and a high density of 1.98 g cm-3, which is 2 to 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 points out, this approach sheds some light on building lithium-sulfur batteries with high volumetric energy density by using a high-density composite cathode with high sulfur loading amount. Future work in the development of lithium sulfur batteries may focus on the strategy of relieving the shuttle effect and suppressing the lithium dendrites, and further improvement in gravimetric and volumetric energy density of lithium-sulfur electrochemical systems.
Explore further: A longer life for lithium-sulfur batteries
5 / 5 (1)Jan 23, 2014
with energy density of 2600 Wh kg-1 (nearly 3~5 times than that of the traditional LIBs)
Are you absolutely sure about that? There is no traditional lithium battery on the market with an energy denstiy of 520 Wh/kg or 1/5th of the reported number. The best of the best are around 1/10th of that.
Or is the article talking about primary cells instead of rechargeable cells?
The editing here has become increasingly sloppy and confusing, with increasingly hyperbolic headlines to catch page- and adviews.
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