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Researchers Develop Electrode Materials for High-Capacity Li-S Battery Cells

Nazar
The galvanostatic discharge and charge profiles of the first cycle of CMK-3 + sulfur; CMK-3/S-145; and CMK-3/S-155. Source: Ji et al. (2009) Click to enlarge.

Researchers at the University of Waterloo in Canada have developed electrode materials for Lithium-Sulfur batteries using a conductive mesoporous carbon framework that have demonstrated reversible capacities of up to 1,320 mAh g-1. A paper on their work appears online in the journal Nature Materials.

The Li-S battery is of interest due a high theoretical specific energy density (~2,600 Wh/kg) that exceeds that of conventional lithium-ion batteries by about a factor of five, good low-temperature performance, and its use of inexpensive and nontoxic raw materials. Last week, for example, BASF and Sion Power Corporation signed a Joint Development Agreement (JDA) to accelerate the commercialization of Sion Power’s proprietary lithium-sulfur (Li-S) battery technology for the electric vehicle (EV) market and other high-energy applications. (Earlier post.)

While lithium-sulfur batteries use the flow of lithium ions in an electrolyte between an anode and a cathode, the way Li-S batteries store the ions is quite different. Li-ion batteries use a process called intercalation to insert the ions between molecules in the electrode. Lithium-sulfur batteries, rely on a multi-step redox reaction with sulfur that results in a number of stable intermediate sulfide ions. This storage process, in theory, reduces limitations of electrode structure—thus enabling higher capacity in similar volumes.

At the negative electrode, lithium is dissolved into solution on discharge and plated out on charge. The solubility of the intermediate sulfide ions depends on the solvent used in the electrolyte, and the voltage vs. discharge capacity profile of the cell thus depends on the solvents used.

Because sulfur is both electrically and ionically insulating, the sulfur must always be in close contact with a conductor. The intermediate sulfur ions can also leak out of the electrodes and into the electrolyte, thereby reducing the electrodes’ active mass. Sion spent a number of years working to increase the sulfur utilization from about 46% to more than 90%.

The University of Waterloo team, led by Dr. Linda Nazar, sought to address those limitations by creating a highly ordered interwoven composite. A conductive mesoporous carbon framework constrains sulfur nanofiller growth within its channels and also generates the essential electrical contact to the insulating sulfur.

The structure provides access to Li+ ingress/egress for reactivity with the sulphur, and we speculate that the kinetic inhibition to diffusion within the framework and the sorption properties of the carbon aid in trapping the polysulphides formed during redox.

Polymer modification of the carbon surface further provides a chemical gradient that retards diffusion of these large anions out of the electrode, thus facilitating more complete reaction. Reversible capacities up to 1,320 mA h g-1 are attained.

The researchers note that the assembly process is simple and broadly applicable, and conceptually can provide new opportunities for materials scientists for tailored design that can be extended to many different electrode materials.

Resources

Comments

HarveyD

Another interesting development for future high energy density lithium batteries for practical PHEVs and BEVs.

BASF-Sion, with their proven capabilities, may manage to use this technology to mass produce a similar affordable end product. with over 500 Wh/Kg.

If they do, and can make it quick charge and long life, the extended range BEV (up to 500 miles between charges) may be around within 5-6 years. Extended range BEVs could certainly compete with various sizes of current ICE vehicles. The future of BEVs, as a better solution for individual transportation, is looking better every day.

Jake

It's too bad they can't seem to get the cycle life up. IIRC as you increase the capacity on LI-S batteries the cycle life drops rapidly. So you may be able to get 1000wh/kg but only with 100-200 cycles there really isn't a practical way to use these in vehicles without replacing them often.

Given the tradeoffs, the short term solution is quick-charge batteries with a lower capacity. If you can charge in under 5 minutes to 80% capacity, it's not the end of the world that you only get a 120 mile range. Then its just a matter of quick-charge infrastructure. With time range will expand and in 10 years we'll have 5 minute recharge to 300 mile range. Really at that point, you probably SHOULD stop to stretch your legs for 5 minutes at that range.

ssintay

With the polymer modification they do address the loss of Sulfur mass into the electrolyte. From reading the paper it looks like they achieve relatively stable capacities at 1,100 mA h g-1 at about 10 charge cycles. This remains relatively stable out at least out to the 20 charge cycles that they provide data for.

yarross

It would be worth research to merge LiS chemistry with solid polymer electrolyte used in LMP battery, no chance to dissolve active material in unwanted way.

Starrfox

Well, I dont see what would be the problem behind a 1000Wh/Kg battery with 200 cycles... In my car, it takes about 54Wh to run 60 miles ( given the efficiency of fuel vs wheel power ) of course this is very approximative and theoretical, but the same will apply to different car size and all... So the reason theres no problem with this, is that with a 1000Wh/kg battery, the car will go 10 times further than with a 100 Wh/Kg one ( probably more due to total weight being less ) but considering that for the same end weight you will go 10 times further, having a battery that you can recharge twice more often is kind of pointless since you just wont have to if it lasts much longer. If you had a battery that has a range of over 1100 miles and had 200 cycles of life, you could run about 232 000 miles on the batterys life, so what would be the use of having one that would last for 1000 cycles?

Roy_H

Unfortunately, this article doesn't explain the most important thing about this research. It is the long life, multi cycle capability. Li-sulfur batteries have been around for years but die after 50 to 200 cycles. No specs given, but this is claimed to have solved that problem.

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