## Silicon nanowire fabric as high-energy anode material for Li-ion batteries

##### 02 December 2011

Researchers at the University of Texas at Austin have developed a non-woven fabric of silicon nanowires; the material has the look and feel of tissue paper, yet is composed entirely of crystalline silicon (Si).

Thermal annealing of the nanowire fabric in a reducing environment results in good performance of the resulting material as an anode in a Li-ion battery without the addition of conductive carbon or binder. In a paper published in the Journal of the American Chemical Society, the team reports achieving anode capacities of more than 800 mAh g–1.

The work represents another approach to devising a commercially viable solution for practically applying silicon, with its much higher theoretical storage capacity than the graphitic anode materials commonly used, in Li-ion batteries (LIB).

The LIB capacity is limited in part by the intercalation of Li+ by the graphitic anode material; thus, higher capacity batteries require anode materials that can accommodate more Li+. The theoretical capacity of the graphite anode is 372 mAh g-1. Lithium-alloying materials have the potential for significantly higher storage capacities. For instance, Si alloys with Li+ at room temperature to form Li15Si4, which corresponds to a substantially higher theoretical capacity of 3579 mAh g-1. However, Si undergoes an enormous volume expansion of nearly 300% when fully lithiated.

Bulk crystalline Si cannot tolerate the stresses associated with these lithiation/delithiation cycles and crumbles, resulting in battery failure.

On the other hand, Si nanostructures have been found to tolerate extreme changes in volume with cycling. Thin Si films have realized capacities above 2000 mAh g-1. Si nanoparticles have also been explored, but with overall specific capacities that have been limited by the need for a conductive carbon matrix to ensure electrical contact with the electrode. Si nanowires have been promising. For example, Cui and co-workers achieved a capacity of 2725 mAh g-1 with very good stability, retaining more than 1400 mAh g-1 after 700 cycles using an interconnected amorphous Si hollow nanosphere thin-ﬁlm electrode. Unfortunately, these electrode materials are very expensive at present and cannot be produced in the significant quantities needed for commercial LIB applications.

Solvent-based processes for nanowire synthesis, such as supercritical-fluid-liquid-solid (SFLS) and solution-liquid-solid (SLS) growth, can produce large amounts of Si nanowires (SiNWs) at relatively low cost...Here we demonstrate the creation of a Si nanowire fabric and show that it can function as a standalone anode material without the need for additional conductive fillers (activated carbon) or polymeric binders.

—Chockla et al.

The nanowires, made by the supercritical-fluid–liquid–solid process, are crystalline, range in diameter from 10 to 50 nm with an average length of >100 μm, and are coated with a thin chemisorbed polyphenylsilane shell. About 90% of the nanowire fabric volume is void space.

The findings show that the surface composition of Si nanowires plays a critical role in their effectiveness as an anode material in an LIB, the researchers conclude.

Future research needs to focus on understanding the cycling durability and the factors that limit performance, including the formation and stability of the solidelectrolyte interface layer, they suggest.

Another issue is that the since the nanowire fabric has 90% void volume, the volumetric capacity of the nanowire fabric is relatively low. The specific capacity of 800 mAh g-1 corresponds to 186 mAh cm-3, which is significantly lower than the volumetric capacity of 777–867 mAh cm-3 for graphite.

However, they suggest, it should be possible to increase the volumetric capacities of the nanowire fabric by densifying the films with pressure. They estimate that the maximum volumetric capacity of a densified Si nanowire fabric would be 1864 mAh cm-3—more than a factor of 2 higher than that of graphite. However, the influence of volumetric expansion and contraction upon lithiation and delithiation on cycling stability would still need to be understood.

Resources

• Aaron M. Chockla, Justin T. Harris, Vahid A. Akhavan, Timothy D. Bogart, Vincent C. Holmberg, Chet Steinhagen, C. Buddie Mullins, Keith J. Stevenson, and Brian A. Korgel (2011) Silicon Nanowire Fabric as a Lithium Ion Battery Electrode Material. Journal of the American Chemical Society doi: 10.1021/ja208232h

Interesting potential avenue for future improved batteries. A 1000 Wh/kg energy density battery could become a reality for extended range EVs by 2020.

Improved mass production techniques will be developed to lower production cost.

Meanwhile, wireless charging units are also developing at a fast rate.

Much lighter, composites/aluminium bodies, frame and wheels etc will soon contribute to higher efficiency, improved design, lower cost, future vehicles. Fold down, very light weight disappearing doors, could be part of it.

Future weight based progressive registration fees could help to lower operation cost of lower weight vehicles. It could be a bonus-malus budget neutral system, with zero or even negative registration fees for very light vehicles compensated by extra fees for heavy vehicles.

".. high-energy anode material for Li-ion batteries.."

Sites have published such statements for years and years, yet it seems that there's no 'better battery' without a complimentary better cathode.

Perhaps GCC could require electrode pair information and probable power output implications/specs achieved with the latest new high-energy battery articles.

good to read but will it yield superlative batteries in 2020, as wished by some who keep repeating the same thing? I don't think so, that kind of lab finding will takes decades to make to product if ever.

Or. will private industries sit on the fence until public funds come their way? What happened to their good old free enterprise initiative?

Very perceptive Harvey.

Free enterprise has let us down. It’s that simple.

Fortunately Cuba, Iran and North Korea are providing the synergistic advances that we need.

China was coming along well but are now sliding into capitalism - with it's cut throat culture.

I understand China now lets some unsuccessful industries fail and actually let successful ones keep some of their gains instead of confiscating them for the common good..

HarveyD

You should look at the history of the chemistry we use to day in batteries and when they were discovered. Li-ion was discovered in 80s, NaS in the 70s. Pb/PbO in 10s

These somewhat erratic advances may or may not be synergistic.

Our free enterprise system naturally leaves the useless advances to wither (or reemerge as science advances in unexpected directions) and constantly and automatically selects the successful strategy.

Unless, of course YOU know what is best and “the others” must be ignored; but that only works (but not for the best) if you are Emperor or King.

I remain confident that the BEV (or some unexpected alternate) will emerge - but no one knows and if we are disappointed it does not mean someone was delinquent or evil.

Build an affordable battery and the world will pave the road to your factory and make you rich. No one can stop you - in the free world.

That is evolution; that is survival of the fittest and that is free enterprise – but some things will never be.

Because cost is, and MUST remain part of the equation.

The arrogance of picking winners should be done with your OWN money, not other's.

Gosh, Tom, you sounded somewhat reasonable until that last sentence.

Of course the idea that anyone knows what is going to work at the outset means that both free market and "Emperor" will sometimes fail.

What has made this country a world leader is that we have pooled our money and taken chances on long shots, on technologies that took decades to bring to market.

A billion dollars invested in a new battery approach costs each of us $3. Just imagine what our world would be like had we not put a few$3 dollar bills into computers and modern medicine.

There are may good examples of excellent new products without massive public $$. 1. Apple Tablets and iPhones with many copy cats. 2. CFL and many copy cats 3. LEDs and many copy cats and many future applications. 4. Image sensors & digital cameras (from Kodak ...not followed up) and many excellent copy cats. 5. 3D multi-layers printers, a near future boom, 2011 onward. . 6. Lithium batteries (on-going) from many sources. 7. Post-lithium batteries (on-going) from many sources. 8. HEVs from Toyota from 1997 to 2011 (2,400,000 units) 9. PHEVs from GM and on-going from many others. 10.BEVs, 1999 from GM/Toyota (failed) from Nissan 2011 (first successful BEV with 25,000 first year sales) plus on-going from 30+ others. 11. Wind mills, worldwide applications. 12. Solar cells, increased efficiency, worldwide. 12. etc etc For the near future, should government invest more, indirectly or directly (hand outs) or as JVs with private industries to promote and accelerate mass production of new products? The public should be getting back part or all the funds invested. Low interest loans and/or JVs could do it. Those of you who keep quoting stats about how long/slow battery development was for the last hundred years, are missing a VERY big point: The battery companies got rich from doing it that way. It was the way they were incented. Why sell someone a cure, when you can sell them treatments for life? Everyready, Duracell, etc: They make the VAST MAJORITY of their profit from selling you disposable batteries every couple of months. Why would they sell you a rechargeable battery that could last years when the public is only willing to pay an extra 50% in price for that rechargeable over a throwaway battery? It doesn't make economic sense for them to do anything other than improve it enough to claim they are "better than last year's model". Nobody was putting huge percentages of the Revenues into advancing battery tech because it was a negative on their bottom line. Do you think Gillette or Schlick would sell you a razor blade that lasted years? Hell no. Everyone who's ever read a business text book knows the story of razors vs. blade cells. Why do you think batteries were any different? But now, suddenly there is a market ONLY if the battery can be counted on to last for years and not cost more than the rest of your car! Suddenly you see tens of billions of dollars from both public and private sources for research and new advances come out of the woodwork every month. Grow up and realize it takes 3-5 years to bring them to market and not all of them workout. HOWEVER, if you're betting against all of them....you're not too bright. A few numbers to chew on: "13.6% R&D spend as a percentage of sales for the software and Internet industry... 1% R&D spend as a percentage of sales for the chemicals and energy industry" Now, guess which industry Batteries fall under? LOL Still wanna talk about how impossible it is for batteries to advance rapidly? http://www.industryweek.com/articles/rd_spending_by_the_numbers_17988.aspx Uhhh, razor vs. blade "cells"...OK, I never claimed I was taking the time to proof read my posts. Sorry. LOL DaveD...yes, designed obsolescence is how many private enterprises got to be very rich and how the consumption economy kept going strong. We were very easy to convince that a razor blade should last only one day; that a light bulb should be only 10% efficient and last less than 1000 hours; that a steel boat on four wheels should weight 3 to 4-tons, guzzle lots of gas, be about 15% efficient and fall apart after 2 or 3 years, etc etc Our local ICEV got better ONLY when superior imported units from Asia and Europe entered the market and moved the Big-3 units overboard. Will that acquired acceptance of very low quality products and junk foods last forever or even get worse? DaveD yes and no, there were industries and customer who were looking for better batteries well before the current craze on EV : military but also space, medical, mining and other niche applications. HarveyD are you so sure that the list you give was without public investment. People who think that big technology move can be done without public money are just blinded by their ideological view, most of the start up that develop new ideas got this new ideas from government lab or university lab which research was often funded by public funds. Private investor don't put money in basic research or development of new material because it doesn't pay off. Only the government can fund basic material research, and that's what we need to find the perfect anode and cathode that some are drooling about. Developing new material for anode and cathode or electrolyte is incredibly deceptive, only 1 in a 1000 or even less will make it trough, so how can you make it without public money? not a chance in a snowball Treehugger, Yes, but notice the common theme of those industries you name: military, medical and space. They all put a total premium on cutting edge function and a very distant after-thought on cost for those cutting edge components. Harvey, when I read your list I see public funds writ large. Trace each of those items back and you find yourself at government sponsored labs where the initial heavy lifting occurred. You may find some points at which a critical discovery was made in a private lab, but even those would have been built on earlier basic research. Then let's look at wind turbines, solar panels, BEVs and PHEVs. All of those have or are using public funds to build enough market to create economies of scale. Back in the 1980s the government provided large support to get the first generation of turbines installed. They did that, people figured out what worked and what didn't and the next generation required less support. Not many years back the government was about the only purchaser of solar panels. Then they provided substantial assistance for private purchase. Building a market has brought the price down from >50/watt to <1/watt. IMO, the government should increase its funding of basic research and continue to be the funding source for projects which are promising but not close enough to market for private money to support. And if there is a need to pump some money into the economy in order to get things moving, like we've needed as we recover from the most recent recession, putting some of that money into creating American industry I can get behind that as well. We get that money back because those investments create jobs. People with jobs do not collect unemployment checks and pay taxes. They spend their salaries and create more jobs which also pay taxes. Until the 80s we were supporting wind, then Reagan eliminated all of that and Denmark became the world leader. BW...nothing wrong with public$$$supporting basic research and early mass production of essential products required for advancement of the nation and to create long term jobs, etc. However, the public should get pay backs proportional to the funds contributed. That would help to cover numerous other loses without hope of any pay backs and would reduce current and future national budget deficits. Hand outs and subsidies (of public ), to favored few, are not very legal and should be declared illegal, unless a direct pay back mechanism is built in. All investors, including the public-governments, should share the profits equitably. Where does the "favored few" come from? Companies apply for grants, contracts and loan guarantees. It's an open application system, all are free to submit. They get objective reviews and some get funding. Are you saying that we should go to a lottery system for government contracts? We do have a payback system in place for loans. As the case of any loaning agency you lose some through default. There are no 100% guarantees when you make loans. Contracts state a specific payback for money received. We contract with XYZ, Inc. for a container of paper clips and they ship a container of paperclips and then we pay them. Grants, no payback is built into grants. It's assumed that the country will profit from discoveries made from research. The payback comes in terms of taxes which will flow from the successful findings and from the overall benefits to society. We do have a subsidy problem. That's because we have an election funding problem. Until we get the influence of big money out of our election system even the most honorable candidates will be forced to seek campaign money from those who have "political needs". That's how the incredibly profitable oil industry hangs on to its subsidies. Bob Wallace I agree with you this discussion is getting ridiculous, what better use there is of public money than funding basic research to develop new technologies that are too far off for private investors ? and anyway there is no alternative so what it is the point of arguing. Of course advanced research is high risk investment, very few programs really yield something that the public will use at the end, but it is a misconception to only count the benefit of a research by necessarily a final product that will hit the market. Even if a program doesn't end on a product it generates knowledge, competence, training, skilled people and brains which will be used for other programs of research. Is it so hard to understand that? Tree....do not forget that each major political party needs about$500+M every second year for election purposes. He who contributes the most gets the most. It has very little to do with R & D, jobs etc. Very simple accounting. To get \$500 B they have to hand out 10 to 100 times mores. If that could be cut out, the national deficit could be reduced by half (and more) in a very short time. Shameful, isn't it?

Harvey,

Are you arguing against the basic research that is funded by the government or just objecting to the WAY it is spent based on political contributions?

Stop allowing politicians to run for re-election and the problem goes away.

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