Metal-air Batteries

Metal-air Batteries

As discussed in a previous post, wind and solar generators need energy storage mechanisms for a variety of reasons.  This post discusses some promising new battery technology, rechargeable metal-air batteries.  Single use metal-air batteries have been around for years.  The new technology here is to design them so that they are efficiently rechargeable.

There are three sizes of applications to think about:  little button batteries for electrical gadgets (usually single use), batteries for electric vehicles, and batteries to smooth the energy generation of utility-sized wind and solar generators.

The current commercial state-of-the-art, Li-ion batteries, which are used in today’s electric and hybrid vehicles, are really heavy and they cost a lot!  They have some other problems.  First the range today of an EV is 50-200 miles whereas we’re used to say 350 miles.  Second, the charging time is in the order of 7 hours, whereas we’re used to 3-4 minutes to refill a car’s gas tank.  These problems, of course, get worse when scaled to utility sized units; hence, the industry is driven to a new generation of battery.

Metal-air batteries use oxygen directly from the air, which allows for higher total energy density due to unlimited cathode capacity.  This definitely will reduce the weight.  The possible metals investigated by the industry are Zn, Fe,Al, Mg, and Li, with zinc (Zn) and lithium (Li) being the most frequently discussed.  One model predicts that the overall theoretical energy density of polymer electrolyte Li-air battery could be as high as 2790 Wh/kg and 2800 Wh/L, which is comparable to gasoline-air combustion engines [1].  The following table comes from Chemical and Energy News, 11/22/10.  Note that Li-air batteries have 10-11 times the energy storage potential that Li-ion batteries do (per weight).  Zn-air batteries have about four times the energy storage potential as Li-ion batteries.  The potential or theoretical numbers are computed from the energy released from the metal assuming total oxidation from the oxygen.  It is clearly much greater than the energy released from the corresponding metal-air battery today; although this battery technology will improve over time.

This would address the weight and energy storage, but rechargeability is a problem.

Older lithium-air batteries do not have long lasting bi-functional cathodes where the oxygen reduction and evolution both take place.  Effective catalysts are needed to reduce the byproducts of the discharge, such as Li2O2 and Li2O.  These are not soluble in current electrolytes and eventually clog the pores of the cathodes, seizing the cell.  Membranes between the anode and cathode also can clog.  Similar problems occur for older zinc-air and other older metal-air batteries.  Thus to make effective (i.e. having a high rates of oxygen reduction and evolution) rechargeable metal-air batteries, new materials for cathode, catalysts for both the reduction and evolution cycles, electrolytes, and membranes are needed.

There are more problems uncovered by current research on Li-air.  Li-air batteries require more voltage to charge them than one obtains when using them.  This ratio is called “energy efficiency”, and most metal-air batteries today have energy efficiency around 60%.

Another problem is the discharge rate.  The reaction between lithium and oxygen in today’s Li-air batteries proceeds too slowly to generate significant current.  It is a little impractical to gang them up to get adequate current.

Next cost.  While current research tends to favor the performance of lithium-air batteries, zinc is a far more abundant metal than lithium, and hence the cost of zinc-air batteries could be significantly lower than that of lithium-air batteries.  At least two companies are headed in this direction with their zinc-air technologies.  One, ReVolt Technology, has a patented [2, 3] mixture of materials for the anode, cathode, catalyst, electrolyte, and membrane, and the other company, EOS Energy Storage, has a patent-pending mixture.  Both believe that their technologies can scale to utility sized batteries, i.e. batteries with tens to hundreds of MW/h of energy stored, and both believe that they can address the EV market (which would give them the scale to keep the price down at the utility level.)  In other words, both believe they have solved the above mentioned problems with today’s Li-air batteries.  The industry continues with considerable research being done by various organizations.  These organizations all hold a wide variety of related patents.  Lawyers will do well when the industry starts to shake out!

Of course, just as an array of wind or solar generators is needed to produce utility sized amounts of energy, an array of batteries is needed with a capacity sufficient to smooth out the peaks and valleys of the generated electricity.  E.g. the wind might not blow or the sun might not shine for several days in a row.  Allowing for these extremes could drive the cost of the batteries unacceptably high.  As a second level of backup, natural gas generators could also be part of the system, but one would expect very little natural gas to be burned over the course of the year with reasonably sufficient batteries.

-gayn

[1] J.P.Zheng et al, J.Elec.Chem.Soc.155(6)A432-A437 (2008)

[2] Patent US 2007/0166602 A1, “Bifunctional Air Electrode”

[3] Patent US 2008/0096061 A1 “Metal-Air Battery or Fuel Cell”

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