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Cornell team proposes new scheme for Lithium-sulfide battery cathodes

31 December 2012

Guo
Discharge capacity (left axis) and Coulombic efficiency (right axis) of the Li2S-C cathode as a function of cycle number. A fixed current density of 200 mA g-1 was used for these measurements. Credit: ACS, Guo et al. Click to enlarge.

Researchers at Cornell University are proposing a new scheme for cathodes for lithium-sulfide batteries (earlier post) to prevent lithium polysulfide dissolution and shuttling during electrochemical cycling. Their approach, described in a paper published in the Journal of the American Chemical Society, creates composites based on lithium sulfide uniformly dispersed in a carbon host, which serve to sequester polysulfides.

Li-sulfur batteries—which conventionally use elemental sulfur (with conductive additives) as the cathode, an aprotic liquid electrolyte, and lithium metal as the anode—are under intensive investigation by research groups worldwide because of the promise for low-cost, high-energy storage. (Earlier post.) Lithium sulfide (Li2S) is a promising cathode material for high-energy lithium ion batteries because, unlike elemental sulfur, it obviates the need for metallic lithium anodes.

Li2S also has a theoretical capacity of 1,166 mAh g-1—nearly 1 order of magnitude higher than traditional metal oxides/phosphates cathodes. If paired with Si anodes with 2,000 mAh g-1 capacity, the specific energy of a Li2S-based lithium-ion battery could be 60% higher than the theoretical limit of metal oxide/phosphate counterparts. (Earlier post.)

As with elemental sulfur, however, a successful lithium sulfide cathode requires a mechanism for preventing lithium polysulfide dissolution and shuttling during electrochemical cycling.

Lithium sulfide (Li2S), the fully lithiated sulfur product, is already under active investigation for its promise as a cathode. Because the cathode is lithiated, it can be paired with high capacity anode materials other than metallic lithium. Also, unlike sulfur that sublimes at a modest temperature, Li2S has a high decomposition temperature above 900 °C, which improves its processing in carbon composites. The particular property of Li2S we utilize in our synthesis is the capacity of the lithium ions to strongly interact with electron-donating groups in carbon-precursor polymers such as polyacrylonitrile (PAN).

Specifically, lone pair electrons in the nitrile group of PAN are capable of interacting with lithium through a coordination bond-like interaction. Thus, when lithium sulfide is mixed with PAN in a homogeneous solution, Li2S may function as a cross-linking agent, which interconnects the PAN network via lithium sulfide net-nodes. We hypothesize that, in addition to stiffening the PAN framework, such linkages favor uniform dispersion of Li2S in the PAN matrix. We show that the resultant lithium sulfide-PAN cross-linked matrix can be carbonized at elevated temperature in an inert environment to obtain an ideal Li2S-C composite cathode material in which Li2S is uniformly and completely dispersed in carbon.

—Guo et al.

The Cornell team’s synthesis methodology makes use of interactions between lithium ions in solution and nitrile groups uniformly distributed along the chain backbone of a polymer precursor (e.g., polyacrylonitrile), to control the distribution of lithium sulfide in the host material.

The method involves the co-dissolution of Li2S3 salt (easily created from Li2S) and PAN in dimethylformamide (DMF). The co-dissolution promotes uniform dispersion in a high-dielectric constant DMF medium, which favors ion pair dissociation of Li2S3 and cross-linking of the polymer.

The cross-linked polymer was then treated at 100 °C for 48 h under vacuum to remove the DMF. The resultant solid material was pulverized by mechanical ball milling to yield a fine powder, which was heated in an argon-filled furnace at 300 °C for 2 h.

They evaluated the synthesized material as cathode materials in a half-cell lithium battery. Under a charge/discharge current of 200 mA g-1, the materials showed stable reversible capacities of 500 mA g-1 and Coulombic efficiencies of nearly 100%. This indicated the effectiveness of the dispersed Li2S architecture in sequestering sulfur and inhibiting shuttling reaction, the researchers suggested.

We believe that similar approaches can be used to control the distribution of other metal salts in polymer- or carbon-based composites. Preliminary results indicate that carbon-Li2S composites created using the new approach offer superior potential, in comparison to other reported methods, as cathode materials for high-energy lithium ion batteries with great cycling stability and excellent Coulombic efficiency.

—Guo et al.

Resources

  • Juchen Guo, Zichao Yang, Yingchao Yu, Héctor D. Abruña, and Lynden A. Archer (2012) Lithium–Sulfur Battery Cathode Enabled by Lithium–Nitrile Interaction. Journal of the American Chemical Society doi: 10.1021/ja309435f

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Comments

"Under a charge/discharge current of 200 mA g-1, the materials showed stable reversible capacities of 500 mA g-1 and Coulombic efficiencies of nearly 100%.".. sounds good..

So, pouch it, include a battery/charge management unit, and sell it. Let's see how it compares with present Li-ion batteries on ebikes.

Show me the battery.

Another variation with higher energy storage potential?

The day may be approaching when somebody will manage to integrate the best three (3) major elements and mass produce an affordable 600 Wh/Kg to 1000 Wh/Kg EV battery?

The world could, shortly thereafter, mass produce affordable long range BEVs?

This is a good discovery and publication. Many advances are made this way, it takes time to get those through research to development to an actual product.

I wonder what good discoveries might have been made a couple of years ago and might now be, behind closed doors, working their way to market.

It seems that pure research carried out by educational facilities is made public. And smaller, start up companies speak publicly about their products because they need to attract investors. But larger companies are more likely to keep innovation private so as not to help their competitors.

Just hoping for good news soon...

It remains to be seen if it is a sure path to slash the cost of batteries in the future, increasing energy density can help reduce the cost but as long as it is based on liquid electrolyte, manufacturing in volume will be clumsy. We need solid electrolyte for lost cost high volume battery, so far only Toyota has claimed progress but still showed nothing so I am not sure that the near future is so rosy for EV, the trend doesn't look good actually, only TESLA is getting real traction, but that is not low cost at all...

As a marketing exercise, say we have an EV that sells for $20,000 and has 200 mile range, are they going to sell one million units per year in the U.S. over the next few years. I think that the marketing survey data at the car companies says no.

"an EV that sells for $20,000 and has 200 mile range" should be doable soon(or switch-in better battery later) for a ~1 moving part drive train.

Meanwhile, at $19K, the average Prius C sells in 10 days vs an average 58 days 'on the lot' for new cars.

"sells in 10 days"

Tells us that the market values cars that reduce trips to the gas station.

Imagine what sales would be like for decent range, decent priced car that never needed to visit the gas pump.

"The cross-linked polymer was then treated at 100 °C for 48 h under vacuum to remove the DMF. The resultant solid material was pulverized by mechanical ball milling to yield a fine powder, which was heated in an argon-filled furnace at 300 °C for 2 h."

Ouch, sounds rather energy intensive.

Comparing a Prius C with a LEAF is a good exercise. Which will sell better? I think most would say the Prius C and they would be right.

The Prius C gets 50 mpg, has a long range and is easily and quickly filled. The LEAF has less than 100 mile range, it takes a while to charge.

JRP3

"Ouch, sounds rather energy intensive." No worse than making portland cement.

The Prius C will likely outsell the LEAF. For a while.

Hybrids will likely outsell PHEVs which will outsell EVs. For a while.

But in the long run it's hard to see how it would be possible to build an ICE as inexpensively as batteries.

Think about all the unique parts and energy inputs that ICEs require. Remember, you have to start with mining ores and producing metals. You've got to cast and mill those pieces. That's a lot of work and a lot of energy input.

Get battery chemistry figured out and manufacturing cells should be very cheap. Moving chemicals through well-insulated ovens should take a lot less energy than smelting aluminum or steel.

Bob,

Most metals that go into an ICE car are very common, like iron and aluminium. Much of it is already in circulation and large part of theses metals are recycled, costing very little energy. An EV requires a bit more exotic metals like lithium, copper and rare earths that for a large part still reside in the crust of our planet.

Building batteries apparently requires expensive processes. I am not hopeful that that can easily change as you describe. I would like to think the same as you that bulk processing of these materials could be very cheap, but real life seems to be stubborn. That's why they cost in the hundreds of $ per kWh.

For now and the foreseeable future, the reality is that the price to build a battery + electric motor is higher than that of an ICE + transmission. In terms of TCO however, things look quite a bit better for EV's.

The cost of the lithium that goes into the LEAF batteries is much lower than the platinum required in ICE catalytic converters. There's somewhere between $24 to $35 worth of lithium in LEAF. There can be hundreds of dollars worth of platinum in a catalytic converter.

And we'll never run out of lithium. The oceans are full of it and if we resorted to seawater extraction it would increase the cost of lithium about 5x. Not much more than a $100 increase for the LEAF.

Tesla and Toyota build EVs without rare earth minerals. Both lithium and rare earth minerals can be recycled. As can copper.

The EV and battery industries continue to claim that high battery cost is largely an economy of scale issue.

Recently this site published a report about current battery prices (although they made a transcription error).

"Battery prices will be down to €180–200 per kWh for large-format battery cells in 2014/2015."

http://www.rolandberger.us/media/pdf/Roland_Berger_Li-Ion-Batteries-Bubble_20121019.pdf

That's about $250/kWh. IIRC getting EV batteries to $200/kWh would make EVs purchase price competitive with ICEVs.

An EV (without the battery) should be much easier and cheaper to build than an ICEV and much cheaper than a more complex PHEV.

The low performance, high weight and high cost of current batteries are stalling the sales of EVs.

Solutions are coming and by 2020 or so, EVs performance/range will increase by 3X, cost will go down to equivalent ICEVs and sales will take off.

Another $1T in R & D would do it?

and then gas hits $1 a gallon again

Bob Wallace: reality check, in the 3d quarter of 2012 li ion cells (18650 form factor) reached $120 per kWh.

http://news.cens.com/cens/html/en/news/news_inner_42230.html

I believe Tesla already paying less then $180 for NCA chemistry(per kWh).

If any major automotive manufacturer wanted to mass produce EVs, getting less $150 per kWh already possible.

Mannstein,

"No worse than making portland cement."

Which is very energy intensive.

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