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New “yolk-shell” design yields scalable, stable, and highly efficient Si anodes for Li-ion batteries

28 June 2012

Yolk
Left. The individual Si nanoparticles contained within the carbon shell expands without breaking the coating or disrupting the SEI layer. Right. Delithiation capacity and Coulombic efficiency of the first 1000 galvanostatic cycles between 0.01−1 V (alginate binder). The rate was C/10 for one cycle, then C/3 for 10 cycles, and 1C for the later cycles. Credit: ACS, Nian et al. Click to enlarge.

Researchers at Stanford University and the Pacific Northwest National Laboratory, led by Stanford professor Yi Cui, have designed and fabricated a novel yolk-shell structure for silicon anodes for lithium-ion batteries. The new material shows high capacity (2800 mAh/g at C/10), long cycle life (1,000 cycles with 74% capacity retention), and high Coulombic efficiency (99.84%).

As they report in a paper in the ACS journal Nano Letters, the team sealed commercially available silicon nanoparticles completely within conformal, thin, self-supporting carbon shells, with a rationally designed void space in between the particles and the shell. The void space allows the Si particles to expand freely with cycling without breaking the outer shell, therefore stabilizing the solid-electrolyte interphase on the shell surface.

Alloy-type anodes (Si, Ge, Sn, Al, Sb, etc.) have much higher Li storage capacity than the intercalation-type graphite anode that is currently used in Li-ion batteries. Among all the alloy anodes, silicon has the highest specific capacity: Experiments have demonstrated an initial specific capacity of >3,500 mAh/g, which is 10 times the capacity of graphite. In addition, silicon is the second most abundant element in the earth’s crust (28% by mass), indicating its potential to be utilized in large quantities at low cost....Despite these advantages, graphite anodes still dominate the marketplace due to the fact that alloy anodes have two major challenges that have prevented their widespread use.

First, alloy anodes undergo significant volume expansion and contraction during Li insertion/extraction. This volume change (∼300% for Si) can result in pulverization of the initial particle morphology and causes the loss of electrical contact between active materials and the electrode framework. Second, due to the low electrochemical potential of Li insertion/extraction (<0.5 V vs Li+/Li), the anode surface becomes covered by a solid-electrolyte interphase (SEI) film, which forms due to the reductive decomposition of the organic electrolyte.

In graphite anodes, a thin passivating SEI forms during the first few cycles, and its further formation is terminated due to the electronically insulating nature of the SEI. In alloy anodes, however, the SEI will rupture due to the volume change during cycling, causing the electrode surface to be cyclically exposed to the electrolyte. This results in continual formation of very thick SEI films, which causes the electrolyte to be continually consumed during cycling. The formation of SEI is further complicated by particle fracture, since fracture creates new active surfaces for SEI growth. The excessive growth of SEI causes low Coulombic efficiency, higher resistance to ionic transport, and low electronic conductivity of the whole electrode, and it will eventually result in the exhaustion of the electrolyte and dry-out of the cell.

—Nian et al.

Much effort has been targeted at devising a solution for silicon anodes that would meet the requirements of commercial application, especially in the automotive sector. A significant amount of this work, especially with nanostructures, has been done by Cui and his colleagues.

The “yolk-shell” structure is intended to support a stabilized and scalable Si anode. The structure has silicon nanoparticles (∼100 nm) as the “yolk” and amorphous carbon (5−10 nm thick) as the “shell”. Each SiNP is attached to one side of the carbon shell, while there is an 80−100 nm void space on the other side. This yolk-shell structure has several advantages for LIB alloy anodes, the authors note in their paper:

  • The carbon shell is a self-supporting framework, and the well-controlled void space between the SiNPs and the carbon shell allows for the SiNPs to expand upon lithiation without breaking the carbon. This in turn allows for the growth of a stable SEI on the static surface of the carbon shell and prevents the continual rupturing and reformation of the SEI.

  • The carbon shell is uniform and mostly free of pinholes, which prevents the electrolyte from reaching the SiNP surface inside the shell. Lithiation of the Si occurs by Li diffusion through the carbon shell into the Si core. Even if there are some minor imperfections or pinholes in the carbon shell initially, the SEI formed on the carbon shell will fill the holes and isolate the inside of the shell from the electrolyte with cycling.

  • The carbon shell is both electronically and ionically conducting, which allows for good kinetics.

  • Unlike high-aspect-ratio nanotubes or nanowires, the yolk-shell nanostructure is fully compatible with current slurry coating technology.

  • Unlike traditional slurry coated electrodes, the electrode has a well-defined void space around every Si particle, which allows for each particle to expand upon lithiation without deforming the electrode microstructure.

The fabrication is carried out without special equipment and mostly at room temperature.

Among the results of the testing of the material:

  • Upon deep galvanostatic cycling between 0.01 and 1 V, the reversible capacity reaches 2,833 mAh/g for the first cycle at C/10 and stabilizes at ∼1500 mAh/g for later cycles at 1C. The specific capacity of the yolk-shell structure is much higher than other reported carbon coated SiNP structures because the carbon shell only comprises <30% of the total weight of the material.

  • No capacity decay was observed in the first 300 cycles, and the capacity retention values after 500, 750, and 1000 cycles are 88%, 81%, and 74%, respectively.

  • he shape of the voltage profile does not change from the 250th to the 1000th cycle, indicating stable electrochemical behavior.

  • Constant capacity galvanostatic cycling with the lithiation capacity limited to 1000 mAh/g results in stable cycling for 1400 cycles.

  • The average CE for the cell with alginate binder, from the 500th to the 1000th cycles, is as high as 99.84%, owing to the stable, thin, and smooth SEI built on the outside of the carbon shell.

  • The stable and thin SEI on the material enables good rate capability. Even at a rate of 4C, the electrode can still achieve a capacity of 630 mAh/g, almost two times that of the theoretical capacity of graphite.

The yolk-shell structure also performs well with conventional PVDF binder, confirming the successful materials design and fabrication, the team said.

In addition to Si, this yolk-shell structure can also be applied to other high capacity alloy-type anode materials for next generation Li-ion batteries to improve cycle life and Coulombic efficiency.

—Nian et al.

Resources

  • Nian L, H Wu, MT McDowell, Y Yao, C Wang, and Y Cui. (2012) “A Yolk-Shell Design for Stabilized and Scalable Li-Ion Battery Alloy Anodes.” Nano Letters 12(6):3315-3321. doi: 10.1021/nl3014814

June 28, 2012 in Batteries | Permalink | Comments (11) | TrackBack (0)

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Comments

Envia is using a "carbon silicon composite" anode already, but this one may give them a run for the money. In either case, Envia's 400 Wh/kg battery looks more believable.

8% loss per year is a bit high. And 1k cycles is not enough for commercial EV sales. But it is great to see the density increase. 1.8Ah/g is okay. But nowhere near electrochemical equivalence (3.82Ah/g) and energy density (1470Wh/Kg) from lithium anodes.

http://www.emc2.cornell.edu/content/view/battery-anodes.html

1K cycles works if a cycle takes the vehicle 200 miles.

@Reel$$ You're not on the right track. The first paragraph in the article you reference is about primary cells. A primary lithium cell could have a pure lithium anode, but it's not rechargable. The article does say "(3.82Ah/g) and energy density (1470Wh/Kg) ", but this is for pure lithium. Li-Ion rechargable batteries need an anode made of another material, such as graphite, which can absorb lithium ions coming from the cathode.

The article goes on to say that "The maximum amount of lithium that can be intercalated within the graphite structure is 1 per 6 carbon atoms, yielding a specific capacity of 372mAh/g." So you will never get close to 3.82 Ah/g with a 1 to 6 ratio.

The current article says "Experiments have demonstrated an initial specific capacity of "3,500 mAh/g, which is 10 times the capacity of graphite." This agrees with the 372 mAh/g that you referenced, but ignored.

Therefore, 2.8 Ah/g (not 1.8) is not just ok, but fantastic, compared to a typical graphite anode, which is only 0.372 Ah/g. This fact is why there is so much excitement about silicon anodes.

The feasibility of cycle lifetime depends on other factors, such as cost. If the battery is very cheap, 100 cycles may be practical, if it's also convenient to replace. The article mentions the fact that silicon is very plentiful. Since it has 10 times the capacity of graphite, the amount of material in the anode would be a lot less, and therefore cost much less.

One day, in the not too distant future, somebody will come up with a 1000+Wh/Kg storage unit capable of 2000+ cycles. How many more years will be required to mass produce affordable ($100/Kwh) units is a difficult question, without a quick answer. Many people think that such unit will be designed by 2020 but low cost mass production may not be around much before 2030.

Quick charging (5 to 10 minutes) 100+ Kwh units will NOT be a major challenge.

long cycle life (1,000 cycles with 74% capacity retention)

I thought it was standard practice to rate battery life down to 80% capacity retention. If they're giving this design a rating of 1,000 cycles to only 74% capacity retention while other batteries are using 80% it would be an unfair comparison.

We should rate this battery at 750 cycles.

"How many more years will be required to mass produce affordable ($100/Kwh) units is a difficult question, without a quick answer."

Nissan has stated that the critical number is around 500,000 units per year.

If what it took to create $100/kWh economy of scale was to subsidize the battery industry at the rate of $5,000 per pack for two years we're looking at $5 billion spread over several countries.

That is not a lot of money. Consider the fact that the US has been spending a billion per day for imported oil.

Five days investing to cut 200+ days cost. Reoccurring return with no additional investment.

I'm guessing that the current $33k cost of a Leaf is going to be dropping. If it comes down to $30k on its own a $5k subsidy would make a very attractive $25k EV, especially in Europe with their much higher fuel costs.

A $25k EV vs. a $20k 40mpg gasmobile would cost only peanuts more to purchase/operate during a five year loan payoff. After that it would save owners more than $100 per month.

We just need a bit better battery capacity and then a couple of years of pump-priming.

Get range up above 140 miles and then let's invest $30 per person for a couple years or a bit more and avoid future oil wars.

@Bob:
AFAIK what Nissan said is that the need to be making 500,000 to 1 million BEVs a year to be fully cost competitive with ICE without subsidy, although it is not clear if they are including exemption from vehicle excise in this.

No mention of $100 kwh batteries in it, and that is anyway a very tough ask as with anything like current technology that is pretty well the cost of the materials alone.

Three watt LED (MR16 with GU5.3, GU10 or E27 base) cost about $39 each three years ago. The new higher efficiency, high power 15W units (with 5X/3W CREE modules each) are currently available from China for about $3 each.

In other words, within 3 short years you can get 2X efficiency (130 lm/W instead of 65 lm/Watt); 5 times the power (15W instead of 3W) at 1/13 the price ($3 from you know where - instead of $39). That's not far from 130 times per unit of light energy. As a bonus, the unit life duration (with the CREE modules) has doubled.

When EV batteries are also mass produced (in the right place) by the millions, the price per Kwh may not drop 130 times but 10 to 20 times is a possibility.

If you're going to get the overall cost of an EV down to that of an ICEV it's all about getting battery price down. (And battery electronics.)

Remember, ICEs with their fuel, cooling and exhaust systems aren't cheap. An electric motor has to be a lot less expensive.

The materials cost of EV batteries? The Leaf has a 24kW pack and uses 4 kilos of lithium. I don't know lithium prices but here's what I find on the web - "Cost, pure: $27 per 100g. Cost, bulk: $9.50 per 100g" That works out to $16/kW?

There's a materials list on page nine of this site. Anyone able to do a quick materials cost?

http://presidioedu.academia.edu/MaxDunn/Papers/177835/Lithium_Battery_Sustainability_Analysis

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