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MIT team synthesizes all carbon nanofiber electrodes for high-energy rechargeable Li-air batteries

Cnf
Gravimetric Ragone plot comparing energy and power characteristics of CNF electrodes based on the pristine and discharged electrode weight with that of LiCoO2. Source: Mitchell et al. Click to enlarge.

A team at MIT, led by Carl V. Thompson and Yang Shao-Horn, has synthesized carbon nanofiber (CNF), binder-free electrodes for lithium-air batteries that yield high gravimetric energies up to ~2500 W h kgdischarged-1 at powers up to ~100 W kgdischarged-1—among the highest values reported for Li–O2 batteries to date (including carbon-only and catalyst containing electrodes).

This translates to an energy enhancement ~4 times greater than the state-of-the-art lithium intercalation compounds such as LiCoO2 (~600 W h kgelectrode-1, the researchers said. They report on their study in a paper published in the RSC journal Energy & Environmental Science.

The carbon nanofiber electrodes are substantially more porous than other carbon electrodes, and can therefore more efficiently store the solid oxidized lithium (Li2O2) that fills the pores as the battery discharges.

In addition, the nanofiber structure allowed for the clear visualization of the morphological evolution of Li2O2 particles as a function of rate and depth-of-discharge and also of the removal of Li2O2 particles during charging.

The visualization of Li2O2 morphologies upon discharge and disappearance upon charge represents a critical step toward understanding key processes that limit the rate capability and low round-trip efficiencies of Li–O2 batteries, which are not currently understood within the field.

—Mitchell et al.

Li-air (or Li-O2) batteries are receiving a great deal of attention and funding as a high-density energy storage solution, especially for electric vehicle applications. Despite their promise, however, Li–O2 batteries face substantial practical challenges, the authors note, including a large voltage hysteresis (70% round trip efficiency); poor rate capability performance at high power (>1 kW kgelectrode-1; and poor cycle life (typically <100 cycles).

The work on the CNF electrode complements extensive recent research focused on the development of catalysts and tunable electrode morphologies to increase round-trip efficiency and discharge capacity respectively, demonstrating that the design of novel all-carbon electrode structures can be an equally promising route for improving the discharge performance of Li–O2 batteries.

In earlier lithium-air battery research that Shao-Horn and her students reported last year, they demonstrated that carbon particles could be used to make efficient electrodes for lithium-air batteries. In that work, the carbon structures were more complex but only had about 70% void space.

In the paper published last year, the team had estimated the kinds of improvement in gravimetric efficiency that might be achieved with lithium-air batteries; this new work realizes this gravimetric gain, Shao-Horn says. Further work is still needed to translate these basic laboratory advances into a practical commercial product, she cautions.

Ji-Guang Zhang, a laboratory fellow in battery technology at the Pacific Northwest National Laboratory, called the CNF work “original and high-quality work.” He added that this research “demonstrates a very unique approach to preparing high-capacity electrodes for lithium-air batteries.

Resources

  • Robert R. Mitchell, Betar M. Gallant, Carl V. Thompson and Yang Shao-Horn (2011) All-carbon-nanofiber electrodes for high-energy rechargeable Li–O2 batteries. Energy Environ. Sci., Advance Article doi: 10.1039/C1EE01496J

Comments

HarveyD

Nano ultra porous fibers are capturing the attention as superior electrode material. At the current rate, much higher performance electrodes should be commercialized by 2015/2016. This should, 5 years latter, translate into EV batteries with 200% to 500% higher energy storage capabilities.

Will we see 100+ Kwh batteries and highway BEVs with 500 to 700 Km range by 2020?

kelly

This sounds great - like most years of breakthroughs..

Engineer-Poet

I find it amusing that two of our biggest problems, excess carbon in the atmosphere and petroleum scarcity, could have a common solution.

Zhukova

You would need 160 kg to get an average 16 hp from these batteries at 100 w/kg. 160 kg * 2500 wh/kg=400kwh At 4 miles per kwh, that's about 1600 miles range. You would need a normal Li-Ion battery or Supercap to give short bursts of 50-100 hp for acceleration.

Zhukova

Actually, 1 hp = 750 watts. 16 * 750 = 12,000 watts. 12,000/100 = 120 kg. Maybe a small car could get by with average 10 hp at 60 mph. Then it would be 7,500 w / 100 w/kg = 75 kg.

4 miles/kwh * 75 kg * 2.5 kwh/kg = 750 miles range. Not bad weight and enough range so the average person only needs to charge about once every three weeks.

Zhukova

"poor cycle life (typically <100 cycles). "

100 cycles sounds good to me, if full discharge happens over a period of two to four weeks because it has a range in a car of 750-1000 miles. It needs to have that range because the available power per kg is small. In that case 100 cycles takes about 4-8 years. The bottom line becomes the replacement cost, not the cycle life.

Engineer-Poet

100 W/kg matches well with the power requirements of powered exoskeletons.

Zhukova

On the other hand, the charging rate is 2500 wh/kg / 100 w/kg = 25 h. So if the battery is depleted by 10 kwh (about 40 miles for a daily commute) it takes 25 hours to recharge. That's not good. There are only 24 hours in a day and it's being driven one or two hours too.

A big battery like 100 kg would require 100 kg * 100 w/kg = 10 kw for 25 hours to charge it completely. But for a car, 100 kg would be about 1,000 miles. Nobody drives that much in a day, so 50% discharged battery could be recharged in 25 hours with about 5 kw easily with 220V supply.

However, in the case of average 40 mile per day commuting, all of the cells in the 100 kg battery would be depleted a little and would require recharging for 25 hours. You could depend on swapping, but at 100 kg, it would be very inconvenient to do this at home every day, and would likely be expensive to have two large batteries.

The only choice is to charge at a faster rate, at night and workplace/shopping mall, which would reduce the cycle lifetime. That may require replacing the battery every two years. That's not often enough to require excessive labor cost and time at the service station. So if the cost is good, a 100 w/kg Li-O2 battery would be practical for cars.

Zhukova

Charging at a faster rate is a definite option. Looking at the red curve for C + Li2O2 on the figure, you can see that at about 100 w/kg discharge rate, you get about 2500 wh/kg.

However, at 500 w/kg discharge rate you get about 1800 wh/kg. In other words, increasing the power output by 400% reduces the gravimetric density by only 30%.

In that case, you could charge in 8 hours instead of 25, while you sleep or work, or at random times during the day, but you would still have about 2000 wh/kg capacity. A 100 kg battery (200 kwh) represents about 800 miles range. Its cycle life may be three or four years. Therefore, cost is still the bottom line, not range, size, power, charging time, or cycle life.

JRP3

"Will we see 100+ Kwh batteries and highway BEVs with 500 to 700 Km range by 2020?"

We'll see that long before then, like maybe last year? Remember DBM Energy had a 98kwh pack in their 600 Km range Audi A2

http://www.engadget.com/2010/10/27/dbm-energys-electric-audi-a2-completes-record-setting-372-mile/

Lithium air is interesting, but we don't even need it.

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