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New conductive polymer addresses volumetric change issue with silicon anodes for Li-ion batteries; high-capacity and longer cycle life reported

Lbl1
At left, a traditional approach combines Si (blue spheres) with a polymer binder (light brown) plus carbon (dark brown spheres). Repeated swelling and shrinking eventually breaks contacts among the conducting carbon particles. At right, the new conductive polymer (purple) continues to bind tightly to the Si particles despite repeated swelling and shrinking. Click to enlarge.

A team of scientists at the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has developed new polyfluorene-based conductive polymers that address the long-standing issue of volumetric change in high-capacity silicon (Si) anodes for Li-ion batteries. A combination of synthesis, spectroscopy and simulation techniques tailors the electronic structure of the polymer to enable in situ lithium doping.

The tailored conductive polymer continues to bind closely to the lithium-storing silicon particles even as they expand to more than three times their volume during charging and then shrink again during discharge. Composite anodes based on this polymer and commercial Si particles exhibit 2,100 mAh g−1 in Si after 650 cycles without any conductive additive. A paper on their work is published in the journal Advanced Materials.

High-capacity lithium-ion anode materials have always confronted the challenge of volume change—swelling—when electrodes absorb lithium. Most of today’s lithium-ion batteries have anodes made of graphite, which is electrically conducting and expands only modestly when housing the ions between its graphene layers. Silicon can store 10 times more—it has by far the highest capacity among lithium-ion storage materials—but it swells to more than three times its volume when fully charged.

—Gao Liu of Berkeley Lab’s Environmental Energy Technologies Division (EETD)

Many approaches have been proposed to solving the volumetric change issue, which causes rapid capacity fading; some are prohibitively costly. One less-expensive approach has been to mix silicon particles in a flexible polymer binder, with carbon black added to the mix to conduct electricity. Unfortunately, the repeated swelling and shrinking of the silicon particles as they acquire and release lithium ions eventually push away the added carbon particles. What’s needed is a flexible binder that can conduct electricity by itself, without the added carbon.

Conducting polymers aren’t a new idea, but previous efforts haven’t worked well, because they haven’t taken into account the severe reducing environment on the anode side of a lithium-ion battery, which renders most conducting polymers insulators.

—Gao Liu

One such experimental polymer, called PAN (polyaniline), has positive charges; it starts out as a conductor but quickly loses conductivity. An ideal conducting polymer should readily acquire electrons, rendering it conducting in the anode’s reducing environment.

The signature of a promising polymer would be one with a low value of the state called the lowest unoccupied molecular orbital, where electrons can easily reside and move freely. Ideally, electrons would be acquired from the lithium atoms during the initial charging process. Liu and his postdoctoral fellow Shidi Xun in EETD designed a series of such polyfluorene-based conducting polymers (PFs).

When Liu discussed the excellent performance of the PFs with Wanli Yang of Berkeley Lab’s Advanced Light Source (ALS), a scientific collaboration emerged to understand the new materials. Yang suggested conducting soft x-ray absorption spectroscopy on Liu and Xun’s candidate polymers using ALS beamline 8.0.1 to determine their key electronic properties. Soft x-ray spectroscopy can tell the researchers where the ions and electrons are and where they move, notes Yang.

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Transmission electron microscopy reveals the new conducting polymer’s improved binding properties. At left, silicon particles embedded in the binder are shown before cycling through charges and discharges (closer view at bottom). At right, after 32 charge-discharge cycles, the polymer is still tightly bound to the silicon particles, showing why the energy capacity of the new anodes remains much higher than graphite anodes after more than 650 charge-discharge cycles during testing. Click to enlarge.

Compared with the electronic structure of PAN, the absorption spectra Yang obtained for the PFs stood out immediately. The differences were greatest in PFs incorporating a carbon-oxygen functional group (carbonyl).

Lin-Wang Wang of Berkeley Lab’s Materials Sciences Division (MSD) and his postdoctoral fellow, Nenad Vukmirovic, conducted ab initio calculations of the polymers at the Lab’s National Energy Research Scientific Computing Center (NERSC).

The simulation revealed that the lithium ions interact with the polymer first, and afterward bind to the silicon particles. When a lithium atom binds to the polymer through the carbonyl group, it gives its electron to the polymer—a doping process that significantly improves the polymer’s electrical conductivity, facilitating electron and ion transport to the silicon particles.

Having gone through one cycle of material synthesis at EETD, experimental analysis at the ALS, and theoretical simulation at MSD, the positive results triggered a new cycle of improvements. Almost as important as its electrical properties are the polymer’s physical properties, to which Liu now added another functional group, producing a polymer that can adhere tightly to the silicon particles as they acquire or lose lithium ions and undergo repeated changes in volume.

Scanning electron microscopy and transmission electron microscopy at the National Center for Electron Microscopy (NCEM), showing the anodes after 32 charge-discharge cycles, confirmed that the modified polymer adhered strongly throughout the battery operation even as the silicon particles repeatedly expanded and contracted. Tests at the ALS and simulations confirmed that the added mechanical properties did not affect the polymer’s superior electrical properties.

The new PF-based anode is not only superior but economical.

Using commercial silicon particles and without any conductive additive, our composite anode exhibits the best performance so far. The whole manufacturing process is low cost and compatible with established manufacturing technologies. The commercial value of the polymer has already been recognized by major companies, and its possible applications extend beyond silicon anodes.

—Gao Liu

This achievement provides a rare scientific showcase, combining advanced tools of synthesis, characterization, and simulation in a novel approach to materials development. The cyclic approach can lead to the discovery of new practical materials with a fundamental understanding of their properties.

—Zahid Hussain, the ALS Division Deputy for Scientific Support and Scientific Support Group Leader

Resources

  • Gao Liu, Shidi Xun, Nenad Vukmirovic, Xiangyun Song, Paul Olalde-Velasco, Honghe Zheng, Vince S. Battaglia, Lin-Wang Wang, and Wanli Yang (2011) Polymers with Tailored Electronic Structure for High Capacity Lithium Battery Electrodes. Advanced Materials DOI: 10.1002/adma.201102421

Comments

Reel$$

Watch the CBS story Dave... or are they part of your conspiracy to change the laws of physics??

http://www.cbsnews.com/video/watch/?id=4955212n

"The most interesting, and promising, at this point, in the farther term, but maybe not so far, is low-energy nuclear [LENR] reactions. This has come out of [22] years of people producing energy but not knowing what it is — and we think we have a theory on it. It’s producing beta decay and heat without radiation. The research on this is very promising and it alone, if it comes to pass, would literally solve both [the] climate and energy [problems.]" Dennis Bushnell, Chief Scientist NASA, Langley Research Center.

Oh, wait a minute - NASA??? THEY'RE trying to debunk the laws of physics too!

Engineer-Poet

This isn't about you needing my money, Reel. It's about you standing up for what you say you believe in, with a commitment that means something.

You fail.

Reel$$

Sorry EP, I am quite happy with the facts I include in my posts. Cheap theatrics are unnecessary;)

DaveD

Reel and Herm,

Look, I'd love for this to actually be true. But there is nothing to indicate it really is. It looks to be EXACTLY like what happened in the Cold Fusion mess. Take your CBS link Reel:

"Scientists today like to call it a nuclear effect rather than cold fusion. At least 20 labs working independently have published reports of excess heat - heat up to 25 times greater than the electricity going in"

This is EXACTLY what happened with Pons and Fleischmann and the original "Cold Fusion". They didn't realize it was just a chemical reaction. Hey, there's nothing wrong with that. It's how we get energy today...by burning things which is just a chemical reaction.

The question is will this "fuel" get used up quickly or not. It's possible and I'd LOVE to be proven wrong.

But for now, I'm with Engineer Poet on this one: I'll gladly bet you guys lunch and I travel all over the US and Europe so I'm sure I can find an excuse to come personally pay up if I'm wrong :-)

Reel$$

Sorry Dave... er, why is it we should believe you and not NASA, NRL, the Chairman of the Swedish Academy of Science’s Energy Committee?

Engineer-Poet

Because NASA, NRL, et al. didn't say what you claim they said.

HarveyD

Imagine what a mini cold fusion box coupled with a mini-ESStor super-cap unit could do for electrified future vehicles. Unlimited range and the surplus energy could supply all power needed for large home and the neighbors fleet etc.

All oil producers, refineries, gas stations, coal mines and power generation plants could close shop?

Reel$$

Come on Harvey, it would be KAOS! Cats and dogs sleeping together! But I'm gonna buy some Orville Redenbacher stock just in case.

Reel$$

And Engineer-Poet, might I suggest you consider the following:

http://www.gamblersanonymous.org/ga/index.php

Engineer-Poet

I only bet on sure things, Reel. Like that out of every 3 assertions you make, 2 are certain to be wrong.

Zhukova

Some things are hard to prove in physics. A lot of independent researchers claim to have evidence for LENR. Maybe they have something, maybe not. Who will prove them wrong?

How about CERN? Physicists there have the highest reputation in the world. However, they have been measuring neutrinos travelling faster than the speed of light. They have been doing this for three years in thousands of tests between Italy and Switzerland. The problem is they can't explain it, so they are releasing their data to the world. But if they can't explain it, who can? If their measurements prove correct, the impact on physics will be much greater than LENR.
http://press.web.cern.ch/press/PressReleases/Releases2011/PR19.11E.html

Reel$$

Zhukova, yes, this is a very interesting result. And should it remain true we will have to rewrite some basic physics. This is IMO natural. Just as evolution. Oddly, there might be some common elements in the apparent nuclear reactions of LANR/LENR and faster than light neutrinos.

Dr. Randall Mills has developed a Ni-H2 over-unity energy source based on his theory of fast H or "hydrinos." His theory extends to application in cosmology where the hydrino plays a role in dark matter and expansion of the universe.

All is highly speculative. But a lower than ground state of Hydrogen would be a fractional sphere of electron motion and release of energy. It may also be a state in which atoms and electrons move temporally rather than by fractional orbitsphere (not Mills'). This would render them potentially undetectable to present instrumentation - thus a theoretical source for dark matter.

If we are to accept Darwin's theory - we can assume that our present understanding of physics and cosmology is minute and evolving.

http://www.blacklightpower.com/theory/overview.shtml

Engineer-Poet

Name-dropping Darwin? That's lame even for you.

We can assume that our present understanding of physics is broadly correct. Anything which has neither mainstream theoretical support nor verifiable experimental results can be assumed to be non-existent for all practical purposes. That include "hydrinos".

HarveyD

Reel$$...that what supposed to be a joke.

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