## Li-air battery research needs as seen by team from US, China and Korea

##### 05 May 2014
 Schematic cell configurations for the four types of Li−air batteries. Credit ACS, Lu et al. Click to enlarge.

A new review of Li-air battery technology by a team from Argonne National Laboratory, Beijing Institue of Technology and Hanyang University focuses on the most critical issues that must be addressed for the successful development and commercialization of high energy density Li-air batteries. The review appears in the ACS journal Chemical Reviews.

Li-air batteries are of great interest (as evidenced by more than 300 research papers on the topic in the past 3 years). The Li−air battery potentially offers densities of up to 2−3 kWh/kg on the cell level. A fully developed Li−air battery system (i.e., with the full balance of plant required) is expected to surpass battery technology under development for deployment in the medium term (400 Wh/kg), and meet the requirements for plug-in vehicle applications. However, the reviewers noted,

Development of a practical Li−air battery will involve overcoming many formidable challenges, including the need for a fundamental understanding of Li−O2 electrochemistry, development of new and improved cell materials, and innovation in the critical aspects of cell design. In the past few years, dozens of reviews on the topic of Li−air batteries have been published. These reviews address the technical issues and challenges facing Li−air batteries at the current stage from different perspectives, including the stability of the electrolytes, importance of the air electrode/electro-catalyst, and oxygen-selective membranes. In this review, we mainly focus on the most critical issues that must be addressed, with the hope that it will help to advance a truly rechargeable Li−air battery toward its practical application.… Covering the immense body of all the work published in this field is, however, beyond the scope of this review.

—Lu et al.

Li−O2 batteries are based on the oxidation of lithium at the (lithium) metal electrode and reduction of oxygen at the air electrode to induce current flow. On the basis of the oxidation of 1 kg of lithium metal, the theoretical energy density of a Li−O2 cell is calculated to be 11,680 Wh/kg—not much lower than that of gasoline (13,000 Wh/kg). Practically, however, the energy density of the batteries is far less.

Note that the usable energy density of gasoline for automotive applications is approximately 1700 Wh/kg, assuming an average tank-to-wheel efficiency (12.6%) of the US fleet. Fortunately, such energy density accounts only for 14.5% of the theoretical energy content of a fully charged Li−O2 battery, so it is not inconceivable that such a high energy density may be achievable at the cell level, given intensive research effort and long-term development.

—Lu et al.

There are four types of Li−O2 batteries are under development, characterized by the type of electrolyte employed: aprotic; aqueous; solid-state; and hybrid aqueous/aprotic. (Earlier post.) All types of Li−O2 systems require an open system to obtain oxygen from the air; Li metal must also be used as the metal electrode to provide the lithium source for all the systems at the current stage.

In the review, the team focused only on the aprotic and aqueous Li−O2 systems, with a particular emphasis on the former since it has dominated the research effort on Li−O2 batteries for the past decade. “Without a doubt,” they concluded, “substantial challenges exist for each component of the aprotic Li-O2 cells.

• A typical aprotic Li−O2 cell consists of a lithium electrode, an electrolyte consisting of dissolved lithium salt in an aprotic solvent, and a porous O2-breathing electrode that contains carbon particles and, in some cases, an added electrocatalyst.

• Unlike the aprotic electrolyte, the aqueous electrolyte is limited to acidic or basic solutions only. In the aqueous Li−air battery, electrolyte solvent, e.g., H2O, is not a limiting factor in the cell performance, which is its main advantage over the aprotic system. In addition, the incombustible aqueous electrolytes circumvent the safety issue which is a major concern for the organic electrolytes in an open cell configuration. However, due to the different electrochemical reactions involving Li and O2, the gravimetric and volumetric capacities of an aqueous Li−O2 cell are much lower compared to those of an aprotic cell.

Aprotic electrolyte. Perhaps the greatest challenge at the current stage for aprotic Li-air cells is the search for stable electrolytes, they concluded. While carbonate-based electrolytes have been widely used in most of the initial research work, these electrolytes decompose in the presence of the superoxide radicals. Despite that, many research projects are using them to investigate the catalytic activities of the air electrode materials.

The catalytic activity of the air electrode materials needs to be re-examined in more stable electrolytes, the team suggested. Ether-based electrolytes seem to be relatively stable in the presence of the reduced oxygen species; however, their stability during charge, especially at high voltage, remains unclear.

Lithium salt deserves much more attention, they suggest, since it may have a positive effect on the electrolyte’s stability in aprotic Li−O2 cells.

Without question, searching for a fully stable electrolyte in the oxygen-rich electrochemical environment is the research priority at present. Design of a robust strategy for effectively screening the stability of various electrolytes would be greatly beneficial to the development of a Li−O2 battery for practical application.

—Lu et al.

Air electrode. Investigation on how the porous air electrode architecture affects the formation of the discharge product, Li2O2, and the specific capacity of the cell is still of great interest, the team found. Researchers need to understand the key limiting factors to determine the capacity, rate capability, and cycling efficiency of aprotic Li−O2 cells.

Porous carbon with or without additional catalyst is the current choice of the air electrode material, although a few non-carbon air electrode materials have been reported. However, the reviewers noted, the mechanism of Li2O2 growth on the porous air electrode during cell discharge and the subsequent decomposition of Li2O2 on charge is still debatable; this matter needs to be further clarified to develop more efficient catalysts for the aprotic Li−O2 cell.

Li metal electrode. The lithium electrode has been a historic problem in any of the Li battery systems, while the long-term cycling of the Li electrode has yet to be demonstrated.

Controlling reactions of the electrolyte at the Li electrode through suitable membranes or passivation films will be essential for achieving good performance with aprotic Li−O2 cells. These membranes should meet the following criteria, the reviewers said:

1. block diffusion of oxygen from the air electrode to the lithium electrode;

2. allow the transport of Li+ to support current flow; and

3. exhibit excellent mechanical flexibility and stability to be compatible with the mechanical flexibility of the supporting polymer membranes and battery design/processing.

Engineering active membranes with a nanometer-scale thickness could potentially meet these criteria, they suggested.

The researchers also noted that the study of electrolyte stability and electrocatalytic process in the aprotic Li−O2 system will require advanced research tools from both experimental and theoretical modeling are necessary.

Aqueous. For aqueous systems, they found, a better understanding of Li−O2 electrocatalysis is required, since the Li−O2 electrochemistry is unique and different from that of conventional electrocatalysis.

The successful development of any aqueous Li−air batteries severely relies on the prevention of direct contact of the lithium metal electrode with water. The most innovative approach to address this issue is the introduction of Li ion conducting glass ceramics. However, these ceramics are generally fragile and highly resistive at low temperature. Moreover, they may not be very stable in strong acidic or basic media. Future research and development of large and more flexible LiC-GC membranes will be greatly beneficial to the aqueous Li−O2 system. Searching for effective catalysts, in particular, with respect to OER, will be a key challenge for rechargeable aqueous Li−air cells.

—Lu et al.

Resources

• Jun Lu, Li Li, Jin-Bum Park, Yang-Kook Sun, Feng Wu, and Khalil Amine (2014) “Aprotic and Aqueous Li–O2 Batteries,” Chemical Reviews doi: 10.1021/cr400573b

Before anyone starts their specific energy arithmetic, projecting 1000 mile EVs with 10-12-15 batteries, fretting about patent restrictions, or postulating the latest Big Oil scheme to own the world's Lithium...

I actually rented this paper because I have been a Li-Air and Zn-Air hopeful since the Carter years. And once again I am disappointed. Save your money: it is basically just a detailed study of the current state of Li-Air technology, with a laundry list of hurdles that (surprise) require more funding.

Herma:
My Leaf battery is three years old now and within the next three years or so I will need to replace the battery. As you suggest, the stuff in this article looks like sky pie.

How about someone just doubling the current battery density so it will be worthy the money to replace it. My needs are minimum. All I want is a 125 mile range at 65 mph at a cost of about 50 cents a Kw. I know..."Good Luck With That!" Eventually, I think it will happen. In fact, it has to happen.

Unfortunatly, small 4 cylinders gasoline car like my neon are still the cheapest way to drive on the roads. Hybrid are costly and cannot be paid by the money you save on gas. Bev do not have sufficient range and you need a dedicated spot to recharge eliminating 50% of potential customers. There is no natural gas cars for sale and there is no nat gas infrastructure.

Im keeping my neon for the next 12 years.

These air batteries are vaporware and serve as a cash cow for dubious studies by mad scientists that make money by exploiting stupid goverments all over the world and if ever they find a good technology they will sale the patents to big oil anyway so we will never have better batteries whatsoever.

I said many time to stop harassing me with tax, subsidies, mad dreams and pollution and high fuel cost. Begin a natural gas infrastructure for cars and start selling nat gas cars with 2 tanks, one for gasoline and one for nat gas immediately. Tomorrow I will check this website.

Lad, I love my Leaf and have been irrationally hoping for some kind of miracle replacement battery when I hit the end of my lease. Very long odds, I fear.

What is Tesla planning for their 2016 battery packs that will supposedly double the energy density of the original roadster?

@gor,

If you are keeping your Neon for the next 12 yrs, and are driving 12,000 miles/year, which is the norm, then you will be driving 144,000 miles, at 27 mpg and at $3.5/gal, you will be spending$18,666 on gasoline cost, assuming no increase in gasoline price. You will be spending a few thousands dollars more for engine and transmission repairs and brake service.

However, if you would trade in your Neon for about ~$4,500 and get a 2007 Prius with about ~90,000 miles for ~$9,000, you will spend only $4,500 more out of pocket. However, with the Prius at 50 mpg, you will be spending only$10,080 on fuel for those 144,000 miles, while spending NOTHING on transmission and brake repair nor service, and the engine is so simple that has no starter nor alternator nor timing belt to replace. The battery of the Prius will last for 250,000 to 300,000 miles, so no worry about having to replace the Prius' battery. So, your saving in repair and maintenance bills from Prius will be almost the $4,500 premium that you're paying for it, allowing to you to pocket almost the entire$8,600 saving in fuel cost, while doing the environment and the USA a big favor of releasing much less CO2 and much less fuel importation.

Not surprisingly, Consumer Report has found the Prius Gen III to be the least expensive vehicle in the USA to own and operate, beating out the much smaller Honda Fit.

@Herman,

Good point. The good news is that the 2,000-Wh/kg battery is already here in the form of H2-FC and is already released NOW as the Huyndai Tucon FCV w/ FREE fuel refill, while Toyota and others will be released by 2015.

The good news for the owners of the Leaf is that better battery tech is already here, in the form of Panasonic NCA 18650 that is capable of 230 Wh/kg which is more than double the energy density of the Leaf battery pack, AND can be cycled 5,000 times at 90% DOD with only 18% loss of capacity after 5,000 cycles. This battery can be found in the hot-selling Tesla model S.

Roger:
I like your calculus but don't understand why, armed with all that battery experience, that Toyota is hawking FCVs, moving marketing and HQ from California to Texas and covering up defects just like every other s**t of an American car company. Sorry Herman but I gotta believe oil money is in there somewhere.

Toyota may have concluded that FCEVs are better suited for colder areas and (specially) for extended range electrified vehicles than BEVs, at least for the next 10+ years.

Secondly, FCs are (and will be) more adapted to long range heavy vehicles for a very long time.

The post-2020 era will probably see a race between FCEVs and BEVs for light vehicles and another race between Diesels and FCEVs for heavy vehicles.

The installation of a reasonable number of H2 Stations is not a real challenge, specially for many industrialized nations with current low employment. It will turn out to be a very good investment.

A progressive pollution fee of a few cents a gallon on liquid fuels or (1/4 of 1%) speculation fee on stock market trades could pay for the initial installation cost over the next 10 years or so.

End users should pay 100% of the on-going operation cost.

Herman, Roger et al,

A big question here. So, Li-Air batteries become reality, nit just a pipe dream. Instead of 200 kilometre ranges, we now have 600, more than adequate. Vuut have you not now tripled the charging time? Even with a supercharger we're looking at big down time and that many kilowatt-hours is going to need a day and a half at typical 220V home installations to get replenished. Or is there some magic I don't know about that repeals basic electrical recharging laws? Edification please!

Good point, and for that, there is already Fuel Cell Vehicle (FCV) available for sale. A fuel cell is a type of flow battery that has range of 300 miles (480km) and can be refilled in a few minutes.

If a person is afraid of Hydrogen, then consider this:

"The new Tesla Model E is intended with a sticker price of $40,000. However, even a$40,000-BEV is competitive in overall cost in comparison to a $20,000 ICEV at 160,000-mile point which is considered to be the life expectancy for ICEV. An ICEV at 25 mpg will consume 6400 gallons of fuel at 160,000 miles. At$3.5/gal, will cost $22,400 in fuel expense alone! Electricity cost at 3.5mi/kWh will consume 160,000/3.5 = 45,700kWh. At$0.12/kWh, the electricity cost will be $5,485. The difference in fuel costs between an ICEV and BEV will be 22,400 - 5,485 = ~17,000 USD. Factoring in oil change, maintenance and repair cost for the engine and transmission and brakes that the BEV has none, will cost at least$3,000 to $6,000, and add that to the$17,000 advantage of the BEV will show that a $40,000-BEV will cost less to own, fuel, and operate than a comparable$20,000-ICEV at 160,000 miles. Now, if the BEV costs $35k and the ICEV costs$25k, then the BEV owner will have over $10k cost advantage over the ICEV owner. However, more importantly, Tesla is a high-end luxury brand. So$40,000 Tesla is comparable to a $40,000 Lexus, Cadillac, Infiniti, Accura, BMW, etc and NOT to a$25,000 regular ICEV. So, a new Tesla Model E owner will be able to laugh all the way to the bank with OVER \$20,000 in overall cost savings at 160,000-mi point.

At that level of cost advantage of a BEV to an ICEV, no one would mind to wait an hour or so for recharging on occasional out-of-town trips at SuperCharger stations within the drive path of the vehicle. Rare trips to remote areas not served by SuperCharger stations can be made by renting an ICEV. Households with a BEV will very likely have at least an ICEV that can be used for out-of-town trips to avoid the cost of car rental!"

The future is indeed bright for EV's!

HealthyBreeze: probably silion anode against an NMC cathode, better energy density matching in electrodes, possibly prelithiated and a good SEI additive to form a stable SEI on the anode. Many battery companies will be approaching this soon. Perhaps a slight voltage increase to 4.2V or something.

Tesla will also likely go to a prismatic cell and are likely speaking only about doubling their pack density rather than a doubling of cell energy density. The cell advances I mention above are probably not out until 2017 or so.

Li-air will never be used in vehicles. It's too much like a fuel cell and most of the researchers are disingenuous about it's potential to be high energy. They talk only about the cathode material and never any other associated add ons that are required for lithium air. It could be very low cost though and might make a good home storage battery for solar and wind.

Toyota's efforts in fool cells are simply a misdirection and stall. They want the prius hybrid to pay off better, before moving to EVs. The fuel cell stuff is silly. If you can split water efficiently, just use it in an ICE. Otherwise, it's still fossil fuels and you will still have the wars, and corruption, and pollution, and it won't be cheap. What's the point?

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