KAIST researchers develop nitrogen-doped carbon nanotubes for high-capacity Li-ion energy storage systems
05 April 2012
Researchers in S. Korean have developed nitrogen-doped carbon nanotubes for high-capacity lithium-ion energy storage systems, such as a lithium-ion capacitor. Lithium-ion capacitors represent an intermediate system between Li-ion batteries and supercapacitors, and are designed to take advantage of the benefits of both types of energy storage systems (ESSs)—i.e., higher energy densities and power densities, respectively.
Final cells developed by the team from the Korea Advanced Institute of Science and Technology (KAIST) using the N-doped nanotubes exhibit a capacity as high as 3,500 mAh/g; a cycle life of greater than 10,000 without capacity loss; and a discharge rate capability of 1.5 min while retaining a capacity of 350 mAh/g.
Furthermore, the team noted, the electrodes exhibit some anomalous phenomena such as the fact that capacity increases with cycling, an observation that corresponds to gradual Li-ion diffusion into interwall space. A paper on their work is published in the ACS journal Nano Letters.
As an attempt to take only advantages of both types of ESSs, recently, lithium ion capacitors (LICs) have been designed and demonstrated. In LICs, unlike supercapacitors, Li sources exist in the anodes so that their energy densities are much larger than those of supercapacitors. Similarly, unlike rechargeable batteries, the electrodes of LICs function based on adsorption/desorption and thus facilitate fast kinetics for discharge/charge, which enables higher power densities than those of rechargeable batteries. Furthermore, LICs do not undergo the significant volume expansion of electrodes and can therefore afford to run over a large number of cycles, which could be even comparable to the cycle lives of supercapacitors.
...In this study, we demonstrate an LIC storage with unprecedented electrochemical performance by using N-doped CNTs (NCNTs).
—Shin et al.
Nitrogen doping plays the following pivotal roles, according to the researchers:
The N-doping process generates extrinsic defects in the walls, through which Li ions can diffuse into interwall space, thus allowing for the use of unexplored interwall space for Li storage.
N-doping increases capacity further as a result of more favorable binding of N-doped sites with Li ions.
For a further capacity boost, nickel oxide (NiO) nanoparticles (NPs) as small as 3 nm were grown on the NCNT surfaces. N-doping is critical for stable operations of the NP-integrated electrodes because it assists the good dispersion of the NPs over cycling. The team observed subdivision of the NPs into even smaller features as well as their dynamic diffusion between N-doped sites of the multiwall CNTs without agglomeration.
Weon Ho Shin, Hyung Mo Jeong, Byung Gon Kim, Jeung Ku Kang, and Jang Wook Choi (2012) Nitrogen-Doped Multiwall Carbon Nanotubes for Lithium Storage with Extremely High Capacity. Nano Letters doi: 10.1021/nl3000908
Overall, these N-doping effects explain the outstanding power and cycling performance. While the extraordinary results presented herein indicate that our electrodes could also function as Li battery anodes, the electrochemical properties could be applied for LIC electrodes.
...this investigation establishes that N-doping brings multifold revolutionary effects on the LI storage of CNT-based composite electrodes. The Li storage capability in the interwall space and the dispersed binding of high capacity NPs with CNTs during long and aggressive cycling will be especially useful for future challenging energy storage applications that deal with robust and fast ionic and electric transports within high capacity materials.
—Shin et al.
Resources
Would someone more knowledgeable than I please translate that into an energy density in kwh?
We seem to have a flow rate as well as the amperes, of 1.5 minutes per 350 mAh/g
Am I on the right lines in dividing the minutes in an hour by 1.5, then multiplying by 350, which brings me to 14kwh/kg?
That sounds high, so I have probably entirely misunderstood the case.
Posted by: Davemart | 05 April 2012 at 08:18 AM
At a normal 3 V this cell could have an energy density of about 10400 Wh/Kg and last for more than 10,000 cycles. Assuming 500 Km per cycle, a large battery pack could last about 5,000,000 Km or about 200 years at 25,000 Km/year.
All this seems too good to be true. Assuming that the total battery back would have 1/3 the cell performance, that would still be good for about 66 years at 25,000 Km/year. A 100 Kwh battery pack (for 500 Km range) would weight about a bit less than 30 Kg? I'm I making a mistake here?
Posted by: HarveyD | 05 April 2012 at 08:58 AM
Lot of research on NCNT materials in recent years for Li-Ion batteries, supercapacitors, sensors, and other things. Obviously lots of scientists think the impressive properties of NCNTs are worth it. 3,500 mAh/g is exciting. For a final cell to do this is very impressive.
KAIST was working on N-CNT cathodes for Li-Air batteries too. But who needs those with this kind of capacity in a LIC?
Posted by: Zhukova | 05 April 2012 at 10:23 AM
1.5 min = 0.025 hr
350 mAh/g / 0.025 hr = 14,000 mA/g = 14 A/g
That's intantaneous rate, not total.
Posted by: Engineer-Poet | 05 April 2012 at 10:37 AM
Thanks EP.
You seem to be saying that that is power density, not energy density.
Do the figures given allow any estimate of energy density?
Posted by: Davemart | 05 April 2012 at 10:40 AM
Apparently 11-14Wh/kg energy density is acheivable:
http://en.wikipedia.org/wiki/Lithium-ion_capacitor
More on high energy density capacitors here:
http://www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2010/electrochemical_storage/es038_smith_2010_o.pdf
Posted by: Davemart | 05 April 2012 at 10:51 AM
Storage density increasing by factors of 10 with MORE use?
If this is true, scale-able, and affordable - send it to LG or Hyundai.. for mass production pronto.
Posted by: kelly | 05 April 2012 at 10:56 AM
@Davemart. You quoted a typical LIC energy capacity. But you overlooked the nominal LIC operating voltage in the same article, which is 2.2-3.8V. In that case, 3V x 3500 mAh/g = 10,000 mWh/g = 10 Wh/g or 10 kWh/kg. By comparison, gasoline is 13 kWh/kg.
Posted by: Zhukova | 05 April 2012 at 11:18 AM
@Zhukova:
Energy density calculations are above my pay grade.
I simply took the figures they gave on the right in the Wiki article.
Posted by: Davemart | 05 April 2012 at 11:20 AM
Nevertheless, the KAIST LIC must have a similar operating voltage. All the Li-Ion articles for cathodes and anodes give the capacity in mAh/g. Multiply by the voltage and you have energy per gram. That's why their gravimetric capacity of 3,500 is so astounding. Not to forget that it increases the more you use it.
In addition, 10 kWh/kg is more easily compared with the energy of gasoline when you consider efficiency. The ICE is only about 20% efficient, but EV is more like 80%. So the KAIST LIC has about triple the effective energy density of gasoline.
Posted by: Zhukova | 05 April 2012 at 11:37 AM
Well, we might as well go for broke and at 3.8V that is 13.3kwh/kg anyway, about the same as gasoline.
Of course that is at the cell level, so you might end up around 10kwh/kg at the pack level, but that'll do, even for aeroplanes let alone cars.
Posted by: Davemart | 05 April 2012 at 11:46 AM
The wikipedia article talks about 14 Wh/kg, not kWh/kg.
It also states that "the capacity of the anode is several orders of magnitude larger than the capacity of the cathode". Could it be the 3500 mAh/g mentioned in the article is not the energy density of the cell, but that of the the anode. Add a cathode that has 3 orders of magnitude lower capacity and then you obtain 14 Wh/kg.
See also this article.
Posted by: Arne | 05 April 2012 at 11:59 AM
Anne is correct the number quoted in the article is for the anode only. There is not enough information in the release to know what the cell level capacity is. You don't know the cathode, but you also don't know the cell structure and you don't know what the operating voltage is. You also have no idea whether the materials could be made into a workable electrode. Many people in the energy storage arena report mAh/g, but at very thin (impractical) layer thicknesses many things are outstanding electrode materials. However, try to make them into an electrode of reasonable thickness and the become useless. A number that would really tell you something is mAh/cm2. From this you could, given an adequate model and a cathode material assumption, estimate a cell level capacity.
Posted by: Brotherkenny4 | 05 April 2012 at 12:20 PM
I've been misreading my links. Thery are indicating 10-15Whkg, and it is an assumption that this new material has the same voltage.
Posted by: Davemart | 05 April 2012 at 12:25 PM
Anne,
Many thanks. You are entirely in the right of it.
I SAID I was not to be trusted with energy density calculations! ;-)
Posted by: Davemart | 05 April 2012 at 12:38 PM
It is a little confusing when the article says "Final cells...exhibit a capacity as high as 3,500 mAh/g"
Also, an LIC is not a battery and the carbon cathode doesn't have to absorb lithium, only electrons, at least that's how I understand it. Even if the other electrode is a bottleneck, KAIST and Maryland University had a goal of gasoline level density (factoring efficiency) in nano electrostatic capacitors in 2009 - http://www.gizmag.com/nanoscale-supercapacitor/11297/
It appears that the researcher's strategy of increasing surface area in the 2009 with atomic fabrication has been replaced by the nitrogen doping method. This is indicated in the paragraph numbers 1 and 3. The atomic fabrication in 2009 was also supposed to improve diffusion, but here in paragraphs 1 and 2, the N doping seems to provide an easier method.
Posted by: Zhukova | 05 April 2012 at 03:31 PM
In any case, LIC-powered cars are already on the market -
http://www.greenoptimistic.com/2011/06/20/electric-vehicle-li-ion-capacitor/
for those who only need to go 5 km between charges!
Posted by: Zhukova | 05 April 2012 at 03:50 PM
I'm not sure what to believe on this one. Like Anne, I first assumed they were just talking some best case scenarios for an electrode in a lab, etc.
But then I saw that quote the Zhukova listed there and now I don't know what to think. I mean, I'd love to believe they really have a working cell with ~10kWh/kg (depending on the voltage of course).
This sounds all so freakin wonderful...that it sounds too good to be true.
Really? Assuming a fairly typical ~3V Li voltage we're talking 42kW/kg worth of power density, 10.5kWh/kg in energy density, 10,000 cycles (my god, did they say 10,000 cycles???)?
Ok, what's the catch? Does it cost $2,500 per kWh? Does it poison baby seals when you charge it? Does it increase sunspot activity and destroy satellites and our ozone?
My real guess: The real world useful energy density is probably closer to that 350mAh/kg with any type of real load, it runs at ~1.5V and it's the electrode only. Don't get me wrong, even those specs would create a phenomenal cell...but not the magic numbers inferred.
Yep, too good to be true. Hope I'm wrong, but I doubt it.
Posted by: DaveD | 05 April 2012 at 06:29 PM
@DaveD I also think the 3500 mAh/g is for the anode only. Many other anodes have been reported like this. For example Dr. Qui's silicon nanowire anode for Li-Ion was supposed to have very high capacity. But the anode is only about 20% of the weight of the whole battery for current Li-Ions. So if a new anode is a million mAh/g, you could make the anode very small and reduce the weight of the battery only about 20%, resulting in an increase in energy density of 25%. The cathode becomes a bottleneck.
At least Envia has an improved cathode and anode, which gives a battery with about 400 Wh/kg and less than $200/kWh. They say they're sampling it now. Life can only get better.
Posted by: Zhukova | 05 April 2012 at 07:34 PM
5 km between charges would be great for a start-stop system; you barely need 1/10 of that. If I could get a capacitor which would hold the kinetic energy of my car at 60 MPH, be the size of a loaf of bread and charge and discharge in 10 seconds, I'd have something that could probably double my fuel economy.
Posted by: Engineer-Poet | 05 April 2012 at 10:47 PM
I suspect that the reason for increased energy density with increasing charge / discharge cycles is the following:
The CNTs are graphene molecules.
IAW Wikipedia, "The van der Waals force (or van der Waals interaction), named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules."
The initial composition of the electrode mass is a chaotic structure. With every charge cycle the structure of the CNT molecules become aligned more and more until after an undefined number of charge cycles the structure looses its chaotic character to become "symmetrical". In this final state, the electrode mass should achieve its lowest resistance or highest conductivity and highest energy density.
It would be exceedingly interesting to determine what life span such an electrode would have after achieving its optimum energy density.
Posted by: yoatmon | 22 April 2012 at 09:16 AM