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New sulfur-rich copolymer electrodes for Li-S batteries exhibit high-capacity, long-life

28 February 2014

Master.img-004
Cycling performance of Li−S battery from 10% by mass DIB copolymer batteries to 500 cycles with charge (filled circles) and discharge (open circles) capacities, as well as Coulombic efficiency (open triangles). The C-rate capability of the battery is shown in the figure inset. Credit: ACS, Simmonds et al. Click to enlarge.

Researchers from the University of Arizona, Seoul National University and the US National Institute of Standards and Technology (NIST) have developed sulfur-rich co-polymers to create cathode materials for lithium-sulfur (Li-S) battery applications.

As reported in the journal ACS Macro Letters, the materials exhibit enhanced capacity retention (1,005 mAh/g at 100 cycles) and battery lifetimes over 500 cycles at a C/10 rate. These copolymers, based on poly(sulfur-random-1,3-diisopropenylbenzene) (poly(S-r-DIB)) and synthesized via and inverse vulcanization process reported last year (earlier post), represent a new class of polymeric electrode materials that exhibit one of the highest charge capacities reported, particularly after extended charge–discharge cycling in Li–S batteries.

(Vulcanization is the chemical process that makes rubber more durable by adding a small amount of sulfur to rubber. The researchers dubbed their process “inverse vulcanization” because it requires mostly sulfur with a small amount of an additive.)

Lithium−sulfur (Li−S) batteries are considered one of the promising candidates for “beyond Li-ion” technology, given sulfur’s high theoretical specific capacity (1672 mAh/g) and high specific energy (~2600 Wh/kg). While Li-S batteries with capacities of 1200 mAh/g are fairly common, the authors noted, rapid fading of charge capacity is an issue.

The poor long-term performance has been associated with both the shuttling of polysulfides dissolved into the electrolyte medium, in addition to irreversible deposition of solid lithium sulfide (Li2S) and other mixtures of insoluble discharge products on the cathode.

While electrolyte additives have suppressed polysulfide shuttling, repeated cycling ultimately leads to insoluble sulfide deposits encrusted on the carbon cathode framework resulting in both mechanical and electrical detachment from the electrode, leading to failure.

Other researchers have shown that sulfur-based nano-composite materials can improve the performance of Li−S batteries. However, the University of Arizona researchers note, challenges still persist in the creation of chemistry for sulfur-based cathode materials that are inexpensive and amenable to large scale production, while retaining high charge capacity and electrochemical stability.

Last year, the team reported the sulfur copolymers synthesized via inverse vulcanization exhibited high specific capacity (823 mAh/g at 100 cycles).

In this report, we explore for the first time with these sulfur copolymers a direct structure−property correlation of copolymer composition with electro-chemical properties to afford optimal polymeric materials for these battery systems. We further demonstrate improved Li−S battery lifetimes out to 500 charge−discharge cycles with excellent retention of charge capacity.

The enhanced battery performance observed with these polymeric active materials arises from in situ generation of organosulfur additives (from DIB units) and linear polysulfide segments (LixSy) via electrochemical fragmentation of the initial poly(S-r-DIB) copolymer. We propose that these organosulfur species suppress irreversible deposition of insoluble discharge products (Li2S3, Li2S2, Li2S) and are mechanistically distinct from recent Li−S battery systems that nanoencapsulate sulfur to suppress dissolution of linear polysulfides. This sulfur based copolymer is also a new addition to an emerging class of electroactive polymers that have been used as polymeric electrodes for Li batteries, examples of which include conjugated polymers and nitrosyl radical functional polymers.

To our knowledge, these novel sulfur copolymers exhibit one of the highest capacities of any wholly polymeric material serving as the active material in batteries cycled to extended lifetimes.

—Simmonds et al.

The Li−S batteries fabricated with poly(S-r-DIB) copolymers as the active cathode material are identical to traditional Li−S batteries using S8, with the exception of soluble organosulfur species generated upon discharge of the copolymer. These organosulfur products co-deposit with other insoluble lower order polysulfides onto the carbon-binder cathode framework at the end of discharge; the researchers propose that this “plasticizes” these insoluble polysulfide discharge products, enabling more efficient battery cycling.

To investigate the effects of composition of the copolymer materials on battery performance, they fabricated a range of poly(S-r-DIB) copolymers into 2032-type battery coin cells and cycled them at a rate of C/10 (167.2 mA/g) with lithium foil employed as the anode.

They found that sulfur copolymers with 1% by mass DIB exhibited cycling perform- ance comparable to elemental sulfur, whereas copolymers with compositions of 20% or greater by mass DIB exhibited little to no improvement over elemental sulfur. However, poly(S-r- DIB) copolymers with compositions of 5, 10, and 15% by mass DIB all exhibited high initial capacities, low initial capacity loss, and consistently reduced capacity loss per cycle.

They concluded that copolymers with 10% by mass DIB performed the best. Cathodes made with this specific copolymer exhibited a specific capacity of 823 mAh/g at 100 cycles. Further optimization of cathode coating methods yielded significant improvement; an initial capacity of 1225 mAh/g was observed in the Li−S batteries fabricated in the present study and low capacity loss was exhibited, with capacities of 1005 mAh/g at 100 cycles and 817 mAh/g at 300 cycles with a Coulombic efficiency of 99% throughout. This system has been extended to 500 cycles while retaining a significant capacity of 635 mAh/g.

The results show that inexpensive, bulk copolymerization can sufficiently modify the properties of sulfur to improve the battery performance without the need for nanoscopic synthesis or processing. The team is exploring with other kinds of sulfur copolymers to further extend cycle life.

Resources

  • Adam G. Simmonds, Jared J. Griebel, Jungjin Park, Kwi Ryong Kim, Woo Jin Chung, Vladimir P. Oleshko, Jenny Kim, Eui Tae Kim, Richard S. Glass, Christopher L. Soles, Yung-Eun Sung, Kookheon Char, and Jeffrey Pyun (2014) “Inverse Vulcanization of Elemental Sulfur to Prepare Polymeric Electrode Materials for Li–S Batteries,” ACS Macro Letters 3, pp 229–232 doi: 10.1021/mz400649w

February 28, 2014 in Batteries, Li-Sulfur | Permalink | Comments (10) | TrackBack (0)

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Comments

Another improved future super battery component.

Who will integrate all the latest top components into a 5-5-5 unit? Will patent rights stop such integration? Will somebody do it in China for the local market?

Well, one things for certain....patent rights will never stop anyone in China from doing ANYTHING. So regardless of what we get, they'll get it. Whatever "it" is and whenever "it" actually exists.

Patent rights, specially in the hands of deep pocket firms, have deprived humanity of cleaner environment, improved health care, lower cost drugs, clean water and better way of life for centuries.

Fortunately, large countries like China and India can and do produce patented good and services for their local markets in order to accelerate their development.

Patent rights is what motivate people and corporations to invest time, money and effort to invent new things. After 20 years, patent rights expires and the intellectual property will be free to everyone. That's how we get low-cost generic drugs and affordable modern technologies to everyone. 20 years can be a very short time for complex and revolutionary technologies. Many important new inventions took 20 or more years to get adequate market penetration, usually well after the patent rights expired, so a lot of money invested were lost.

It was the establishment of patent offices in Europe and USA that resulted in most inventions being made in Europe and USA. After the inventions of paper, gun powder, ink and silk, relatively very few things were invented in Asia for thousands of years until the post modern era, when patent offices were also installed in China and Japan.

@RP: As mass manufacturing, patents are quickly moving East. In 2013, more vehicles and patents came from China than USA & EU. India and the rest of Asia will be next.

Asia will soon need more raw material than available locally. Australia, Africa, Canada, Brazil, Indonesia etc wlll have to fill the gap.

RP: the theoretical purpose of patents is to protect inventors so they can recover investment and profit from the invention. That assumes the inventor will bring the product to market and expand the market. Hopefully they don't sit on it waiting for a market to develop and then lose it after 20 years (like oil companies with battery technology). If sitting on patents becomes the standard procedure, then we need to reconsider the usefullness of patents.

Buying patents and sitting on them to protect your existing market for the next 20 years is an old trick often used.

Unfortunately, humanity is then deprived of the benefits of new inventions for 20 years and often more because humanity wlll normally give up after 10 years or so or pay through the nose to get access to it.

Fortunately, a few large countries (with strong enough armies) can duplicate many of those inventions for the benefit of their own population and businesses. Small countries cannot afford to move against large corporations and have to give in to this type of (patented) commercial blackmail.

Does the negative out weights the positive.

Terrible capacity fade still. I don't see the improvement.

Some things are counter-intuitive. A battery that loses 50% of its capacity after "only" 500 cycles seems like it would not be a commercially competitive product. But consider that these lithium sulfur batteries have about 4x the energy density of current lithium ion batteries. A Tesla Model S gets 265 miles on a charge.

A Model S would get 1,120 miles of range per charge. After putting 212,00 miles on the odometer, it would still work, but only get about 530 miles of range per charge.

That's after 17 years of use at an average of 12,000 miles per year.
During which you would have saved about $31,800 in fuel costs.

I'd take one of these lithium sulfur batteries now. Even after it's full life cycle, it still has 2x the capacity of today's best batteries.

Good observations e-c-i.c.

With 1100+ miles initial range, most owners wouldn't mind mid normal life battery fading to 50% or so.

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