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New hybrid Li metal/graphite anode enables high-performance Li-S battery with significantly extended life

10 January 2014

20140109115131846
Schematic of hybrid anode placed in a Li–S battery. The graphite/Li connected in parallel forms a shorted cell where the graphite is always lithiated at equilibrium and maintains a pseudo-equal potential with the Li metal. As such, it functions as an artificial SEI layer of Li metal that supplies Li+ ions on demand, while minimizing direct contact between soluble polysulfides and the metal surface. Huang et al. Click to enlarge.

Researchers at the Department of Energy’s Pacific Northwest National Laboratory (PNNL) have designed a lithium–sulfur battery using electrically connected graphite and lithium metal as a hybrid anode to control undesirable surface reactions on lithium. Lithiated graphite placed in front of the lithium metal functions as an artificial, self-regulated solid electrolyte interface (SEI) layer that actively controls the electrochemical reactions and minimizes the deleterious side reactions, leading to significant performance improvements.

Lithium–sulfur cells incorporating such hybrid anodes deliver capacities of >800 mAh g−1 for 400 cycles (4x the cycle life compared to a conventional anode) at a high rate of 1,737 mA g−1, with only 11% capacity fade and a Coulombic efficiency of more than 99%. In a paper published in Nature Communications, the researchers suggest that this simple hybrid concept may also provide scientific strategies for protecting metal anodes in other energy-storage devices.

Although rechargeable lithium (Li) ion batteries (LIBs) are widely studied for application in portable electronic devices and vehicle electrification, they cannot store sufficient energy for the extended driving range required by electric vehicles. Alternative energy-storage systems with much higher theoretical specific energy are needed. Among these technologies, Li–sulphur (Li–S) batteries hold great promise. Theoretically, a Li–S battery has a specific capacity and energy of 1,675 mAh g-1 and 2,600 Wh kg-1, respectively, much higher than those of traditional LIBs.

Sulphur is also of low cost, and is abundant and non-toxic, making this system attractive for large-scale applications. However, many obstacles in this system still need to be overcome for practical applications. The major issue associated with Li–S cells is the formation of soluble long-chain polysulphides during discharge/charge. The gradual loss of active mass from the cathode into the electrolyte and onto the Li metal anode leads to ‘shuttle reactions’, severe self-discharge, low efficiency and fast capacity decay on cycling, which are commonly observed in Li–S batteries.

—Huang et al.

Most lithium-sulfur battery research to date has centered on stopping sulfur leakage from the cathode. But PNNL researchers determined stopping that leakage can be particularly challenging. Further, recent research has shown a battery with a dissolved cathode can still work. The PNNL team, led by Dr. Jun Liu, thus focused on the battery’s other electrode by adding a protective shield to the anode.

“We speculate that rather than simply using an electrolyte additive or a physical barrier (film) to protect the metal anode, it may be possible to design a completely different anode structure where the interfacial redox reaction is shifted away from the metal surface. ”
—Huang et al.

PNNL’s graphite shield moves the sulfur-side reactions away from the anode’s lithium surface, preventing it from growing the debilitating interference layer. Combining graphite from lithium-ion batteries with lithium from conventional lithium-sulfur batteries, the researchers dubbed their new anode a hybrid of the two.

A graphite film is placed between the Li foil and separator, and then electrically connected with the Li metal to form a parallel anode. Once immersed in the electrolyte, Li ions immediately intercalate into the graphite, which persists in the lithiated state because this hybrid anode is, in essence, a shorted cell.

“Sulfur is still dissolved in a lithium-sulfur battery that uses our hybrid anode, but that doesn’t really matter.”
—Jun Liu

Using this anode concept, the hybrid Li–S cell delivers a reversible capacity of 4900 mAh g-1 at 1,370 mA g-1 (~0.8 C). At a rate of 13,790 mA g-1 (~8 C), cell capacity exceeds 450 mAh g-1, demonstrating exceptional rate performance.

The PNNL team also observed marked improvement in capacity retention after repeated cycling; significantly improved cycling stability was observed at all rates. At 612 mA g-1, the discharge capacity became stable after 50 cycles, maintaining at ~850 mAh g-1 for more than 200 cycles with a small initial capacity loss.

Columbic efficiency is always more than 99.5%, reflecting that the shuttle mechanism is significantly mitigated in this fundamentally different design of Li–S cells, the authors found. Similar performance enhancements were observed at increased rates and over longer cycling times strongly suggesting that this innovative anode concept plays a key role in extending the cycling life of this system.

Huang
Electrochemical behaviors of Li–S cells using the proposed hybrid anode. (a) Charge–discharge curves of hybrid Li–S cells at different rates. (b) Cycling ability of the hybrid Li–S cell at a rate of 1,737 mA g-1. (c,d) Long-term cycling behavior and corresponding Coulombic efficiencies of hybrid Li–S cells at different rates. Huang et al. Click to enlarge.

In summary, a radically different hybrid design has been introduced for Li–S cells to mitigate the loss of active material and harmful parasitic reactions on the anode by using an integrated structure composed of electrically connected graphite and Li metal. Although further studies are required, preliminary findings, combined with available literature results, support the idea that lithiated graphite functions as an electrochemical artificial SEI that decouples the Li+ extraction/reinsertion from side reactions caused by soluble polysulphides. The anode reaction front is shifted from the Li metal to the graphite surface and the interaction with dissolved polysulphide species is primarily confined to graphite. The reduced irreversible sulphur loss on the Li metal anode, as well as the physical barrier benefit provided by the lithiated graphite, collectively leads to the exceptional electrochemical performances observed in Li–S batteries. This simple hybrid concept provides important clues in the future development of Li–S batteries and may be broadly adapted in many energy-storage technologies utilizing metal anodes.

—Huang et al.

This and most other lithium-sulfur battery research is conducted with small, thin-film versions of the battery that are ideal for lab tests. Liu noted tests with a larger battery system would better evaluate the performance of PNNL’s new hybrid anode for real-world applications.

This study was primarily supported by the Department of Energy’s Office of Science (BES), with additional support from DOE’s Advanced Research Projects Agency-Energy, and DOE’s Office of Energy Efficiency and Renewable Energy. Some of this research was performed at EMSL, DOE’s Environmental and Molecular Sciences Laboratory at PNNL.

Resources

  • Cheng Huang, Jie Xiao, Yuyan Shao, Jianming Zheng, Wendy D. Bennett, Dongping Lu, Saraf V. Laxmikant, Mark Engelhard, Liwen Ji, Jiguang Zhang, Xiaolin Li, Gordon L. Graff & Jun Liu (2014) “Manipulating surface reactions in lithium-sulfur batteries using hybrid anode structures,” Nature Communications doi: 10.1038/ncomms/4015

January 10, 2014 in Batteries | Permalink | Comments (7) | TrackBack (0)

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Comments

Im not very hopefull about all of this. With all the people they got and all this budjet, if they not have a 2x battery yet that is working fine without problems then they will never got one. This is probably a wrong design all by itself and there is nothing to do with that. Till that time a lot of desperate folk pay high price for subpar bev like leafs and teslas and are rob of their money paying even higher price then a gasoline equivalent. All in all because of these folks we pay more for gas and electric bevs and for fail researchs like this one. I even loss 10 minutes reading this with some hope that i loss thereafter.


Till something happen i still drive my small gasoline car at slow speed, this is the highest scientific discovery there is.

Gorr

the path to better batteries is a long and hard way, but there is not reason to be desperate, breakthroughs happen, they are not predictable but they do happen. Yes EV cars are still more expensive than their gazoline counterpart, but they do exist now and it shows that the era of EC cars has started. Transition in energy area take 30 years, and the fisrt EV where introduced only 2 years ago, so be patient...

An excellent battery is already on the market which will power Toyota vehicles in 2015. It's called a fuel cell.

This technology may have the potential to be further developed to become the early version of 5-5-5 battery by 2018 or so.

Will it be mass produced by 2020 or so?

Some of the most promising battery research centers around Lithium Sulfur cells. The good news that has been known about for some time now is phenomenally good energy density. Sulfur is far cheaper than any of the elements used in current generation lithium cells and is, in fact, a byproduct of tar sand oil refining. There is - quite literally - mountains of the stuff sitting around. So, there is the potential for a truly low cost battery that performs better than what we've got today and using a waste product that we have in abundance.

What has been a problem with lithium sulfur (Li-S) until recently is lifespan of the battery. It would tend to "eat itself to death" after just a few charging cycles (see article above and elsewhere for a far more technically complete analysis of this effect.) But Oak Ridge National Laboratory, among others, have made great strides in this area. Li-S cells now routinely last past 1000 charge cycles under wide temperature ranges. In fact, Li-S cells actually work best in higher heat environments. All of this was well reported extensively in the news media this past spring and summer.

The next step is to scale up the small batteries currently being tested in laboratories to sizes that will be more useful commercially . . . first in things like portable computers and then to vehicle traction use.

It's interesting work. Too bad the graph only shows 200 charge/discharge cycles. Efficiency and charge left after 2000 charge/discharge cycles would be a lot more interesting to check. The curves do seem to flatten at 200 so that's good news.

This is a good step. Sure, problems may occur when they scale these up to an 85 KwHr system, but it sounds like they've solved the biggest problem with Li-S, and that EV batteries should weigh less and cost less using Li-S, and could potentially still last long enough. Maybe they just scale them up to a 100 KwHr system and only claim 85 KwHr and all is well.

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