GM researchers develop Li-Sulfur cathode material with improved cycling stability and efficiency

17 September 2014
 Discharge capacities and Coulombic efficiency vs cycles for the new composite at 0.6C. Capacity values were calculated based on the mass of sulfur. Credit: ACS, Zhou et al. Click to enlarge.

A team from General Motors Global Research & Development Center in Michigan has developed a new double-layered core–shell structure of polymer-coated carbon–sulfur to confine better the sulfur/polysulfides in the electrode of lithium–sulfur (Li/S) batteries and to improve the batteries’ cycling stability and Columbic efficiency.

In a paper in the ACS journal Nano Letters, they report a stable capacity of 900 mAh g–1 at 0.2 C after 150 cycles and 630 mAh g–1 at 0.6 C after 600 cycles. They also demonstrated the feasibility of full cells using the sulfur cathodes coupled with silicon film anodes, which exhibited significantly improved cycling stability and efficiency.

Despite the promise of high theoretical energy capacity (1673 mAh g−1, 5x that of current commercial cathodes) of Li-S chemistry, practical Li-S batteries have been hindered by poor cyclability, mainly due to (1) the poor conductivity of sulfur; (2) the heavy dissolution of polysulfides; and (3) the large volumetric expansion during lithiation.

A great deal of research is targeting these issues (e.g., earlier post, earlier post, earlier post), with much effort focused on the development of novel nanostructures aiming to confining the sulfur and mitigating the shuttling effect of polysulfides.

Despite successes in stabilizing capacity over the short term, the GM team notes, “some inevitable negative effects in these processes still remain.”

• For mesoporous carbon-sulfur nanocomposites, if sulfur can diffuse into the small pores of the mesoporous carbon, the polysulfides should still be eventually diffuse out since a large amount of sulfur surface is still exposed to the electrolyte, although the weak interactions between sulfur and mesoporous carbon could alleviate the dissolution of polysulfides in the short-term.

• There has been no convincing evidence to support the assumption that the sulfur has diffused into the internal empty space of the porous carbon instead of in/on the superficial carbon pores. If the sulfur was only aggregated in/on the superficial pores of the mesoporous carbon, it would easily be dissolved out during cycling.

• For the approach of a core-shell composite (grown “bottom up” using a sulfur nanoparticle seed coated with a conductive polymer), a practical difficulty is obtaining small-sized sulfur nanoparticle seeds (≤300 nm) in order to enhance the lithium diffusion and improve the power capability, since the nanosized sulfur easily aggregates to bulky particles. Large sulfur particles may exhibit poor conductivity of both sulfur and polysulfides. In addition, the shell can block the direct contact between sulfur and conductive carbon.

Hence, developing new strategy to confine sulfur/polysulfides, control the sulfur particle size, and maintain intimate contact between carbon and sulfur simultaneously is still highly desired and critical for practical applications of sulfur-based cathode materials. In this work, we proposed the preparation of polydopamine-coated, nitrogen-doped, hollow carbon−sulfur in a double-layered core−shell architecture as an optimized design to confine the sulfur/polysulfides and keep intimate contact between sulfur and conductive carbon.

In this structure, the nitrogen-doped hollow carbon served as both carbon shell to control the size of sulfur core and conductive carbon to enhance the conductivity. The coating polymer further helped to confine the sulfur and polysulfides inside the shell. Additionally, the introduction of nitrogen facilitated better immobilizing the lithium polysulfides through the coordination interaction between lithium polysulfides and nitrogen.

… Using this unique sulfur composite, both Li/S half cells and the silicon/sulfur(Si/S) full cells showed highly improved capacity retention and Coulombic efficiency.

—Zhou et al.

They first prepared nitrogen-doped porous hollow carbon spheres, then impregnated them with sulfur under heat treatment to obtain a nitrogen-doped hollow carbon−sulfur (NHC−S) core−shell structure. The NHC−S composite was dispersed in an aqueous solution of dopamine, which would self-polymerize at alkaline pH values and spontaneously deposit on the surface of NHC−S nanoparticles to form a polydopamine-coated NHC−S composite (PDA−NHC−S) in a double-layered core−shell structure.

Scanning transmission electron microscopy (STEM) showed that the sulfur not only successfully penetrated through the porous carbon shell but also aggregated along the inner wall of the carbon shell.

To test and compare the electrochemical performance of the NHC−S and PDA−NHC−S materials, they fabricated 2032-type coin cells using lithium foil as the anode. The cathode nanocomposites were mixed with carbon black and water-soluble binder poly(vinyl alcohol) to prepare the cathode electrodes. The final sulfur ratios were around 64% and 55% in NHC−S and PDA−NHC−S electrodes, respectively.

They attributed the significantly improved cyclability in the PDA−NHC−S composite noted above to a number of factors:

• The polymer shell helps to encapsulate and confine the sulfur/polysulfides physically inside the shell.

• The unfilled pores/space in the hollow carbon could accommodate the volume expansion of sulfur/polysulfides during the lithiation.

• The thin polydopamine shell would react with the sulfide/polysulfides once they contacted with each other, leading to a three-dimensional, cross-linked polymer shell through interchain disulfide bonds interconnection and introduce some free sulfide anions among the polymer chains. This process would not only increase the mechanical strength of the polymer shells for better tethering the polysulfide species inside the polymer shells, but also facilitate the ionic transportation owing to the formation sulfide anions in/on the shells.

They did note, however, some slow capacity degradation despite the improved cycling stability.

To avoid the formation of lithium dendrites from the lithium anode during cycling—and thus the potential safety issues in the real application—they then used lithiated silicon thin film as the anode (PDA−NHC−S/Si).

They found stable capacities around 840 mAh g−1 at 0.1C and 720 mAh g−1 at 0.2C after 200 cycles in the full PDA−NHC−S/Si cells, with Coulombic efficiencies of more than 99.7% in both C rates. Although the capacity showed a slight decay in the initial 200 cycles from their proof-of-concept Si/S full batteries, the team said that the cycling stability and efficiency were better than other reports on Si/S full batteries.

They did also observe a slight capacity fading, especially in the higher scan rate, which they attributed to (1) the crack of the silicon film and the limited supply of lithium ions with the cycling; (2) the dissolution of the polysulfides during charge/discharge process.

Taking the advantage of this nitrogen-doped hollow−porous carbon and polymer-coated structure, both the Li/S and the Si/S batteries exhibited much improved cycling stability and Columbic efficiency. From the study of STEM images and EDX data, the sulfur not only successfully penetrated through the porous carbon shell but also aggregated into sulfur particles along the internal wall of the hollow carbon, which provided visible and substantial evidence that the sulfur prefer diffusing into the nitrogen-doped hollow carbon.

The polymer coating further helped to confine the sulfur particles inside the hollow carbon spheres, which facilitated to immobilize the polysulfides during the charge/ discharge processes and improved the cycling stability and efficiency relative to the uncoated composite. In the Si/S full batteries, a much improved cycling stability could also be observed with high efficiency. While slight capacity fading still remains in these and premier studies, developing such a strategy is highly desirable due to the fact that these results provide promising insights and novel concepts for future sulfur-based full batteries.

—Zhou et al.

Resources

• Weidong Zhou, Xingcheng Xiao, Mei Cai, and Li Yang (2014) “Polydopamine-Coated, Nitrogen-Doped, Hollow Carbon–Sulfur Double-Layered Core–Shell Structure for Improving Lithium–Sulfur Batteries,” Nano Letters 14 (9), 5250-5256 doi: 10.1021/nl502238b

I losed faith in battery improvements since a long time and this article confirm what I thought. They lose ton of money in researchs paid mostly by state subsidies. This will ruin state budget where each green car sold costed 100 000$approx. with only 2000$ to 3000$of gasoline saved. If you factor in cost of chargers plus installation then it's even worst. Me I choosed a small gasoline car without subsidies and I save more gasoline by driving slow. All the choices made by government have created porverty, increased pollution like a lost of money on batteries, corn ethanol, lack of natural gas vehicles, cash for clunkers and many costly international political meeting and studies like the Kyoto protocol, zev credits, co2 regulation, etc. There are many announcements on battery chemistry improvement that provide limited information regarding specifics. However, what is not said is just as informative as what is said for the person who truly understands the technology. one can improve performance relative to some baseline using low c-rates and a few hundred cycles but it does not mean that commercialization is near. nevertheless, many battery materials performance improvements have been discussed lately however, if there ends up being three or four dominant chemistries, the number of improvements that make it to market by definition has to be very low in number. Simply because there are so few viable cell chemistries. Sulfur will work, and silicon will work and the first people who make it commercial may get very wealthy. Fortunately, there are likely several technological paths (for S and Si) of approximately similar performance and manufacturing cost levels such that trying to tie up the technology through patents, to prevent the inevitable attack on the fossil industry, is likely to be losing strategy. I don't know why a fuel cell advocate would get depressed about battery improvements never seeming to come to commercial fruition. Except that one may hope to disparage others who do at this time support battery and EVs as the future. This is not enough energy density for future Extended range EVs. Nothing to write home about. Late show PR? Panasonic & Tesla do better and their improved larger cyl cells will do better in 2017 or so. Excellent progress, once the remaining cycle life and safety issues are cracked for Li-S, we're heading way below$100 per kWh and above 400 Wh/kg.

They ARE making progress, if you look back at recent articles about silicon anodes and sulfur cathodes, they have improved capacity retention with cycles significantly.

the problem is that most people don't understand how slow progress on new material could be. A new material that show promising properties can take decades before being qualified for real world applications. Pushing battery capacity beyond 250Whrs/kg is a formidable challenge and will take time, doesn't mean it won't happen.

there are numerous people with Electro Chemistry PhDs, some might think that if you just got a lot of them together, the solution would come in no time...that is NOT the case.

Pellion is working on magnesium ion batteries, they have a computer program that sorts through numerous compounds to come up with the optimal electrolyte. After years the puzzle has not been solved.

The old saying if it was easy everyone would have done it by now.

I expect that Lithium-Sulfide will be the next breakthru in battery technology. There are a number of people working on the problems and they seem to be making good progress. It is also good to see a major automotive company seriously working with Li-Sulfide. Lithium-Air might render the IC engine to dustbin of history but that is an much harder problem.

I agree with SJC and Treehugger on most people not understanding how much time and effort it takes to get a new technology to market. I remember attending a seminar 45 years ago when I was a ME graduate student at MIT. The speaker had a new magnetorheological fluid and demonstrated some of the properties but stated that he had no idea what it could be used for. At the time it was just a laboratory curiosity. It took 30 years or so but now some of the Cadillacs and Corvettes have tunable magnetorheological shock absorbers along with a host of other applications. LED's have been around since 1962 but only in the past few years have they made major inroads in lighting.

I should add that fuel cell research has been going on since 1838 and we still do not have practical fuel cell vehicles.

On the other hand (just to balance the arguements), sometimes breakthroughs are made that do make it to market quite quickly.

For example, Howard Florey and co decided in 1938 to aim to identify the anti-bacterial compound secreted by penicillum mould, managed to do so (a remarkable feat in itself), developed a practical means of isolating it and by 1941 it was being mass produced.

But maybe that's just an example that things tend to happen quicker when there's a real need for something?

Things tend to happen quicker when the military needs it RIGHT NOW.  Among other things, entrenched interests get swept aside.

Historical question:  if the US government had taken oil dependence seriously, would Cobasys have been allowed to stall the NiMH traction battery?

People can say what they want about this formulation, but going from a theoretical capacity of 6x current cathodes to "only" a bit more than 2x as much still looks like progress to me.  I'd take 2x the capacity of the batteries in any of my frequently-used devices in a heartbeat.

@ EP
If you're content with nominal progress, this should be of interest to you.

http://www.innovations-report.com/html/reports/energy-engineering/safe-lithium-batteries-with-a-long-service-life.html

Recent lithium AA batteries will last about 10X good old zinc units at about 8x the price. Secondly, they maintain a much more steady voltage as they discharge. Sales are low.

Recent LEDs use as little as 1 watt per 200+Lumens while old incadescent bulbs did only 15 lumens per watt and CFL about 70. Sales are slowly picking up.

Higher cost, early higher failures and resistance to changes seem to slow the intrduction of new technologies.

Harvey: and real LED products are in 100-150lumen/W range, and thats the best of industry and DC only, not counting any conversion loses

RECENT (2014) LED. Most of the LEDs on the open market place are dated.

Many major manufacturers (Cree, Nichia, Samsung, Osran, Philips, Green Ray etc) have delivered 200 lm/W LEDs in 2014. Cree will soon offer 254 and 276 lm/W units

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