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IBM Almaden Researchers Say Li-Air Batteries Offer Promise for Transition to Electrified Transportation, But Face Challenges and Multi-Decade Development Cycle

9 July 2010

Girishkumar
Four different architectures of Li-air batteries, which all assume the use of lithium metal as the anode. The three liquid electrolyte architectures are aprotic, aqueous, and a mixed aprotic-aqueous system. In addition, a fully solid state architecture is also given. Credit, ACS, Girishkumar et al. Click to enlarge.

While the practical energy density of Li-air batteries could approach that of gasoline—after factoring in tank-to-wheel efficiencies—and thereby enable a transition to an electrified road transportation system, there are challenges facing the development of commercial Li-air batteries and the current understanding of their electrochemistry, according to a Perspective by a team of researchers from IBM Research-Almaden published in ACS’ Journal of Physical Chemistry Letters.

IBM and its partners have launched a multi-year research initiative exploring rechargeable Li-air systems: The Battery 500 Project. (Earlier post.) The “500” stands for a target range of 500 miles/800 km per charge, which translates into a battery capacity of about 125 kWh at an average use of 250 Wh/mile for a standard family car.

...the requirements for large capacity automotive propulsion batteries are extensive, but quite well defined. They will serve as guidelines for the research to be carried out on Li-air systems. At present, automotive propulsion batteries are just beginning the transition from nickel metal hydride to Li-ion batteries, after nearly 35 years of research and development on the latter. The transition to Li-air batteries (if successful) should be viewed in terms of a similar development cycle.

—Girishkumar et al.

Basics. The energy density of gasoline is approximately 13,000 Wh/kg. With a current average tank-to-wheel efficiency of 12.6%, the usable energy density of gasoline in an automotive application is about 1,700 Wh/kg.

Since the efficiency of electric propulsion systems (battery-to-wheels) are about 90%, a 10-fold improvement of the current energy densities of Li-ion batteries, which are typically between 100 and 200 Wh/kg (cell level), would bring electric propulsion systems on-par with gasoline, at least as measured by gravimetric energy density. However, there is no expectation that current batteries such as Li-ion will ever come close to the target of 1700 Wh/kg. New chemistries are required to achieve this goal.

The oxidation of 1 kg of lithium metal releases 11,680 Wh/kg, not much lower than that of gasoline...However, practical energy densities for Li-air batteries will be far less. Existing metal-air batteries, such as Zn/air, typically have a practical energy density of about 40-50% of their theoretical density. However, one can safely assume that even fully developed Li-air cells will never achieve such an excellent ratio, because lithium is very light, and therefore the overhead of the battery structure, electrolytes, and so forth will have a much larger impact.

Fortunately, an energy density of 1700 Wh/kg for the fully charged battery corresponds only to 14.5% of the theoretical energy content of lithium metal. It is not inconceivable that such an energy density, at the cell level, may be achievable, given intensive and long-term development work. The energy density of the complete battery system may be only half of the density realized at the cell level.

—Girishkumar et al.

Considerations for Li-air systems include:

  • Power density and cost. While Li-air systems offer the promise of very high energy densities, their power density is currently very low.

    Prototype aprotic Li-air cells deliver current densities in the order of 1 mA/cm2. It will be critical to increase this current density by at least 1 order of magnitude. Even then, the macroscopic surface area to supply the total power for a propulsion battery is very large. For example, a battery with 100 kW power output at a cell voltage of 2.5 V and a current density of 25mA/cm2 will require a total internal surface area of 160 m2, equal to the internal surface of the human lung.

    —Girishkumar et al.

    One way around the power issue would be to utilize a hybrid system where a small capacity but high power battery, for example, provides power for short periods of high demand, such as during acceleration, the authors suggest.

  • Electrical energy efficiency. Current Li-air cells have a charging voltage that is considerably higher than the discharge voltage (overvoltage). This corresponds to a low cycle electrical energy efficiency, currently on the order of 60-70%, the authors note. Practical propulsion batteries should exhibit “round-trip” energy efficiencies of 90%. The detailed mechanisms underlying these high over voltages are currently not understood.

  • Lifetime and Cyclability. Current Li-air cells have been demonstrated with up to about 50 cycles with only moderate loss in capacity. Therefore, the authors suggest, future research efforts need to focus on improving the capacity retention during cycling.

  • Safety. Typical thermal runaway of a Li-ion battery due to overcharging or internal shorts is not a possibility in Li-air batteries because of the rate-limited surface nature of the reaction, i.e., the reactant O2 is not stored in the battery.

    However, there are two other safety concerns to be considered. First, the desired, though not mandatory, use of lithium metal anodes is a well-known safety problem, since lithium metal tends to form dendrites, which can short-circuit the battery and react aggressively with many contaminants. Second, the presumed dominant reaction product of aprotic cells is Li2O2, which is a strong oxidizer. Combined with an organic electrolyte, this could lead to safety issues in an accident. However, preliminary experiments at IBM indicate that no thermal exothermic reactions between Li2O2 and common electrolytes occur at temperatures below the melting point of lithium metal (180 °C). This safety concern does not exist in aqueous cells.

    —Girishkumar et al.

Architectures. There are currently four chemical architectures for Li-air batteries under investigation globally, including three versions with liquid electrolytes—a fully aprotic liquid electrolyte; an aqueous electrolyte; and a mixed system with an aqueous electrolyte immersing the cathode and an aprotic electrolyte immersing the anode—and an all-solid-state battery with a solid electrolyte. Only the aprotic configuration of a Li-air battery has shown any promise of electrical rechargeability; hence, this configuration is attracting the most effort to date, according to the authors.

The fundamental electrochemistry—which is not fully understood in detail—depends upon the electrolyte around the cathode, the authors note.

Aprotic
Schematic operation proposed for the rechargeable aprotic Li-air battery. During discharge, the spontaneous electrochemical reaction 2Li+O2→Li2O2 generates a voltage of 2.96 V at equilibrium (but practically somewhat less due to overpotentials). During charge, an applied voltage larger than 2.96 V (~4 V is required due to overpotentials) drives the reverse electrochemical reaction Li2O2→2Li + O2. Credit: ACS, Girishkumar et al. Click to enlarge.

Aprotic Li-air battery. A “typical” aprotic design would consist of a metallic lithium anode, an electrolyte comprising a dissolved lithium salt in an aprotic solvent and a porous O2-breathing cathode composed of large surface area carbon particles and catalyst particles, bound to a mesh using a binder.

In addition to a detailed discussion of the possible dynamics of the electrochemistry of the cell, the authors note that there are other issues that need to be addressed, including the lithium anode, as well as the question of Li-air or Li-O2 batteries—e.g., whether or not to deploy a membrane that selectively permeates O2 to avoid unwanted parasitic reactions with components such as water, carbon dioxide, carbon monoxide and nitrogen in ambient air.

Recommended key research. To develop and commercialize a practical, rechargeable Li-air battery, the authors recommend research in the following key areas:

  1. Quantitative understanding of the electrochemical reactions and their relationship to the discharge/charge currents. This is the key to quantitatively demonstrating chemical reversibility and understanding Coulombic efficiency of the battery in cycling.

  2. Development of oxidation-resistant electrolytes and cathodes that can withstand high oxidation potentials in the presence of O2. This is also essential for chemical reversibility and Coulombic efficiency in the battery cycling.

  3. Understanding the nature of electrocatalysis for Li-air batteries where insoluble products are formed and the development of cost-effective catalysts to reduce overpotentials for the discharge and charge reactions. This is key to enhancing power density in discharge, electrical efficiency in a discharge-charge cycle, and ultimately in cycle life (due to possible electrolyte oxidation).

  4. Development of new nanostructured air cathodes that optimize transport of all reactants (O2, Li+, and electrons) to the active catalyst surfaces and provide appropriate space for solid lithium oxide products. This is required to maintain capacity at higher power densities. A new realization is that minimizing difficulties due to electron transport through the lithium oxide solid products in the cathode is important.

  5. Development of a robust lithium metal or lithium composite electrode capable of repeated cycling at higher current densities. This will most likely require development of a protective layer that limits the deleterious effects of environmental contamination on the lithium and inhibits dendrite growth.

  6. Development of high throughput air-breathing membranes (or other mechanisms) that separate O2 from ambient air in order to avoid H2O, CO2, and other environmental contaminants from limiting the lifetime of Li-air batteries.

  7. Understanding the origin of the temperature dependencies in Li-air batteries and minimizing their adverse effects.

Resources

  • G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson and W. Wilcke (2010) Lithium-Air Battery: Promise and Challenges. J. Phys. Chem. Lett., doi: 10.1021/jz1005384

July 9, 2010 in Batteries | Permalink | Comments (24) | TrackBack (0)

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I think that before the federal government spends billions on railroads, they should be supporting and incentivizing battery r & d on a large scale.

What IBM and other battery researchers need to accept is that overly long development cycles for standard chemical battery designs such as Li-air will spur introduction of disruptive energy technology.

The science is likely not the greatest hurdle in battery evolution. There is foot dragging in energy development mostly due to political and business forces wanting to retard electrification. The foot dragging is to allow big utilities and oils to secure centralized monopolies in "smart grids" and H2 infrastructure.

However the world is much smaller and more transparent now and foot dragging will no longer be tolerated. While IBM paints a 30 year development cycle to get their Li-air batteries to match gasoline energy density - that is NOT the metric for EV transportation. Car owners are well poised to adopt lower energy density batteries due to short drive patterns and recharging habits. Range "anxiety" is addressed by liquid fuel extenders. It is no big deal to recharge a consumer product overnight as billions do it regularly with cellphones and iPods. Plugging in the car overnight is no different.

So, the real question is do big energy players want to hang on to a good piece of the energy market by accelerating electrification now, or lose it to disruptive technology? Foot dragging and out-dated metrics are the fast track to an energy revolution that will mothball old players and their energy monopolies.

If very high energy density batteries are decades away, industrial countries should electrify all their railroads, hybridize all highway ground vehicles to get much better mpg, electrify city vehicles to reduce fuel consumption and GHG to near zero, build clean energy power plants to replace all current polluting coal fired plants by 2030/40.

The world will be pleasently surprised when new technologies or improved current technologies produce lower cost batteries with 1000+ Wh/Kg with 20,000+ cycles for highway type EVs by 2020/2030.

I note that the comparison of energy densities was between gasoline & batteries when it should be between *systems*. Batteries may have less energy per kg than gas but the engine, drivetrain, exhaust system, etc. you need to use the gas weigh a lot more than the electric motor you need to use the batteries.

Electric propulsion *systems* are closer to being on-par with gasoline propulsion *systems* than they want you to know.

Why would you need 20,000 cycles? 1000 Wh/kg battery in the leaf would give about a 500 mile range. 20,000 cycles would be 10,000,000 miles.

50 cycles would be enough for Li-Air, which the author admits is possible now. Batteries typically have about 30% the theoretical capacity, so Li-Air could be expected to have 3,000 wh/kg. A battery like that could be charged once in two months. That means it would last eight years (50 cycle lifetime). Maintaining a good charge on it, instead of letting it discharge, would make it last a lot longer.

The cost of Li-Air is likely to be less than Li-Ion types that use cobalt or titanium. So replacement after four or five years would not be expensive. Being rechargable isn't necessary either. If the Li and electrolyte could easily be replaced once every few months, you have a great replacement for the ICE in airplanes as well as in cars.

@Zhukova,

I like your thinking, but there will be many partial discharges, so at least hundreds of cycles are necessary.

I took as more daunting the low cycle electrical energy efficiency of 60%, and the low power densities. Really big Li-Air batteries, or systems combined with supercaps might mitigate the power density somewhat.

Overall though, this says to me that Silicon nanowires (or silicon encrusted carbon nanotubes, or somesuch) is the best candidate for the next 10 years.

Think about it. The annode is 10x more efficient. You could use everything else the same as today's batteries, but make the annode 1/5th as big, and make the cathode 1.8x as big, and you would get a battery with 80% more capacity.

That should get you over 300 Whr/kg, and off to the races.

Here we go again... comparing tank to wheels in gasoline versus well to wheels in EVs. Plus those number seem to be a bit off, I commonly have seen tank to wheels of gas around 20%, and diesel around 30%. Maybe if you add the well to tank energy expenditure you hit 10-12%.

HB - From what I've read, the Li-Ion cathodes that Argonne developed are on the market. The cells are supposed to put out 250 wh/kg. The anodes comprise only 20% of the mass of a typical cell. So if you reduce the anode to zero, you would get 250/.8= 312 wh/kg. I think there is already a cell on the market with silicon anode. If not, there are plenty of researchers claiming they can do it with nanowires, and other methods.

To be clear, I know the Li-Air puts out low current, so you would need a big battery to get enough power. The big battery automatically gets a big range too. You probably can't get enough power, say 75 kw (100 hp) from a 1,000 wh/kg Li-Air, so you would also need a Li-Ion with titanium cathodes. This is mentioned in the article.

The size of the two batteries depends on driving patterns. If you accelerate hard, slow down, accelerate again five or ten time, the little Li-Ion may be depleted faster then the big Li-Air can charge it. On the average you need only 20 hp to go 60 mph (100 kmph). So the Li-Air needs to be maybe 40 hp to keep the Li-Ion charged if there's frequent acceleration.

Zhukova: City driving, as would do city e-taxis and city e-buses etc, would cycle their batteries with hundreds of partial charge/discharge cycles every day. A local research lab own by a large electricity producer/distributor, claims that their new battery hs already been tested to 20,000 cycles and will withstand up to 30,000 cycles and more. Those new batteries will be mass produced in Taiwan or China within 24 months or so. Many EV owners may elect to use smaller batteries and recharge at least twice a day. A 20,000 cycles battery would be good for 20,000/730 = 27 years or about what you should expect from a good e-vehicle.

If the Lithium-Air battery consume Oxygen when discharged, would it give out Oxygen when it is being charged?

HD Actually I've long advocated (probably for 20 years)that the key to practical BEVs is the ability to charge it anywhere. If my company had charging outlets in the parking lot, I would need only a twenty mile range on my battery. Charge it at night with low power ~ 1 kw and charge it while I'm working at ~ 1kw. If I need to go home in the middle of the day, I would be in trouble.

But long trips, and taxis and buses, would require fast charging stations ~ 50 kw. I've been reading a lot about normal Li-Ions that can be charged to 50-80 % in ten or twenty minutes, so it seems like this is already practical. So the only real requirement is a cycle life of 1,000 or more (and relative low cost). Li-Air would be great, but current battery technology can work if we can develope charging station infrastructure. No breakthroughs required for that and it's already underway in some places.

"The science is likely not the greatest hurdle in battery evolution. "

Of course not. It's IBM. Another boogeyman to join the big-3 and big oil and etc.

“The world will be pleasantly surprised when new technologies or improved current technologies produce lower cost batteries with 1000+ Wh/Kg with 20,000+ cycles for highway type EVs by 2020/2030.”

Sadly it’s more like; the world WOULD BE pleasantly surprised.
Same is if pigs flew.

There is general agreement that affordable batteries are not close.
And it is no one’s fault.

TT Your bringing back memories. We used to talk about this in high school (~1973). We didn't know much about technology, but we knew if somebody invented a better battery, the big three auto companies would buy the patents and put the technology on the shelf.

we knew if somebody invented a better battery, the big three auto companies would buy the patents and put the technology on the shelf.

Or sell them to an oil company.

Toppa

I tried to explain this a few times to Harvey, but he is stubbornly stuck in "Alice in the Wonderland of electric miracle" and apparently he has a crystal ball from which he can predict the exact date when the miracle will happen.

Current technologies can reach 200Whrs/kg at his best which is not bad but there is no real breakthrough in sight to make it cheap, so electric car will stay expensive for an undetermined period of time until proven wrong. Asides there is many challenges like fast charging, limited life time, cold weather, recycling, etc. When the industry says Li-Air battery are decades a way it means "we have no clue if it will ever work" so better not to count on it. Car battery is still in its infancy. Hybrid Cars were introduced 13 years ago and they are still only less than 2% of sales, honestly I don't expect electric cars to do much better given the challenges ahead. That's the problem in technology transition in the energy area, it is about infrastructure and low cost so can't be fast.

Sorting out the problems with lithium sulphur batteries (ie short cycle life and safety) would easily lead to 500 Wh/kg batteries with much less R&D effort.

In fact we're almost there with LiS anyway, given the recent advances showing marked improvement in LiS cycle life: http://www.ncbi.nlm.nih.gov/pubmed/19448613?dopt=Abstract

You could then get your 500 miles range with just 250 kg of LiS cells, pretty much what's in a typical EV today anyway.

Having said that I am still a big fan and supporter of LiAir.

One problem with Li-Air is the low current required, about 0.1 A/g of cathode, to maintain high energy capacity. However, it's interesting to look at the EV-1 performance -http://avt.inel.gov/pdf/fsev/eva/genmot.pdf

The EV-1 was a 2 seat, 3000 lb car, with lead acid batteries. It had a 90 mile range at a constant 60 mph! This required an average power of 10 kw = 13 hp. For the normal driving cycle, the range was 78 miles at average power of 4 kw = 5.5 hp. No wonder people liked these cars!

Therefore, a Li-Air battery would need to put out an average of 10 hp for good overall performance. If it has 33% of theoretical efficiency, 1 kg of cathode requires 13 kg of anode (Lithium). Including electrolyte and packaging could add another 6 kg. For 1 kg of cathode in a 20 kg battery, we have 1,000g x .1A/g x 3V = 900 W = 1.2 hp. So if you want 10 hp, you need 7,500 W/hp/900W/20kg = 166 kg. At 2.5 kWh/Kg, that's 415 kwh!!

That's not bad for weight and you get 2,000 cheap miles @ 5 mi/kWh. If the driving cycle needs more frequent acceleration, the range might be reduced to 1,500. This is 1.5 months of driving for the average person. So 50 full charge cycles would last about 6 years. But, the battery would last a lot longer if the charge was maintained.

You wouldn't want to discharge it completely anyway because to get max capacity, you need to charge it slowly. A 2,000 mile battery would take (166,000 g/ 20)*.3 W/g = 2,500 Watts maximum. 415 kwh/2 kw = 166 hours = 7 days!

I see lithium air working with lithium ion in a range extended configuration. Lithium ion provides the power and lithium air provides the energy for range.

"Electric propulsion *systems* are closer to being on-par with gasoline propulsion *systems* than they want you to know."

True. And IBM stands to lose little by fast-tracking Li-air rather than foot dragging. What we are seeing is deliberate braking of EV R&D so as to construct a gradual transfer to electrification. Problem is the culture of shelving innovative technology in favor of old monopolies is so militantly entrenched - it will be hard to break new components like Li-air.

This type protectionism of old energy cartels and troikas will directly result in the introduction of disruptive technology that may cause economic upheaval. That is the price the old guard will pay for sandbagging electrification of transport and distributed energy systems.

Zhukova: In the not too distant future suitably equipped PHEVs/BEVs may be able to pick-up decent recharges while driving over selected streets-lanes-highways equipped with wireless linear charging system (LWCS). Efficiency may not be over 80%, but for people in a hurry, that would be very acceptable. The widespread availability of such wireless charging system would negate the need for very large on-board batteries. The technology exist already. Reducing on-board batteries from 100 Kwh to 10 Kwh in every PHEV and BEV may save enough to pay for a decent linear wireless charging system LWCS). Of course, LWCS could be extended to parking places (on and off street) and domestic garages etc. Electronic billing should not be a major challenge.

This could be an excellent national make work project and would certainly make electrified vehicles practical. White glove drivers would like it. Basically, electric vehicle users would no longer have to worry about recharging the batteries. Paying for electricity used on the credit card would be effortless. Banks to that free.

LWCS is another example of how BEVs can become practical without scientific breakthroughs. What it takes is visionary capitalists or gov't support. Since I'm generally against dead-weight in BEVs (like 166 kg Li-Air batteries) LCWS is really interesting.

I used to dream about LWCS twenty years ago, partly because I was a slot car hobbiest before that. It's great to see it happening now even though it's out of my control (like everything else).

There will be EMF and safety/health issues with any induction charging system on public roads. And cost of infrastructure and maintenance may far exceed hauling batteries around, Worth looking at though.

It might not be that expensive. A single machine could dig a narrow and shallow trench in the middle of the HOV lane. No manholes, electric conduits or other obstructions there. The same machine could lay the electric rail and backfill the trench with asphalt. Large numbers of EV commuters could benefit.

Lots of things we COULD do, but the probability of actually having them get done favors the more practical. FFV/M85 is very practical and possible at a reasonable cost and time frame.

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