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Are metal-air batteries the solution to long duration storage?

Jun 01, 2022

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Back in March, in Going With The Flow I discussed what flow batteries are and their potential contribution on the grid storage scene. The purpose of grid storage is to store intermittent power from wind/solar sources for use during off time. Lithium-ion batteries (LIBs) are the best batteries we currently have for just about everything from phones to grid storage, but they scale linearly with cost for longer storage durations. The LIB supply chain will face a tough road forward as well due to its raw material composition (read here, here, and here). In flow batteries the energy is stored in tanks which can be scaled up arbitrarily which is the advantage over LIBs, where more packs are needed to scale up for more storage. Metal-air batteries are another technology that has garnished interest for its potentially advantageous grid storage characteristics. Since electric vehicles will likely suck up any supply of LIBs, keeping them out of the grid picture will be a necessity.

Metal-air

Metal-air batteries have a metal anode and an “air cathode.” There are various metals to choose from including lithium, sodium, magnesium, zinc, iron, and aluminum. Metal ions combine with oxygen from the air to form a metal-oxide compound at the cathode while electrons travel through the external circuit providing power. This means there is an electrolyte and separator to facilitate metal ion (but not electron) transport through the electrolyte, similar to a LIB. In the reverse reaction, oxygen is released, and the pure metal (lithium as seen below) is formed at the anode.

Lithium-ion-Compared-to-Lithium-Air — Li-ion vs Li-air Battery. La Triveneta Cavi

Pros

Metal-air batteries have a theoretical energy density of 3-30 times greater than LIBs which means they can store more charge per unit of mass (or volume). This is the biggest draw as researchers are trying to store as much energy in as little space as possible.

Since the oxygen from the air is nearly infinite, the capacity is constrained mostly by the mass of the metal electrode. In theory, the size of the metal electrode could be scaled up arbitrarily to meet energy demand. This advantage is shared with flow batteries, as cost likely does not scale linearly like it does in LIBs.

Not only cost scaling, but the raw cost itself could be another advantage. The supply of zinc, aluminum, sodium, magnesium, and iron are quite abundant and not likely to cause issues like lithium may. Keeping cost down for these batteries is the most vital component as it not only impacts whether they will be chosen over LIB for grid storage, but in the grand scheme whether deploying a solar/battery system will be cost effective in the first place.

Cons

Power density is a main constraint due to the conversion efficiency of oxygen and reaction kinetics. Basically, getting the battery to charge/discharge faster is a key concern. However, this is more of an issue for an electric vehicle than backup storage at a home/business for example.

Reversibility is vital as you expect your phone battery to last you all day for years. No one wants to install a battery system for it to be useless a year later, so having a long cycle life is a challenge in the metal-air battery space.

Assuming a reversible metal-air battery with adequate power density (charge/discharge rates), cost parity becomes the name of the game. 2021 saw the cost of LIBs came down to around $132/kwh (Published Nov 2021), so metal-air batteries will have to go well below LIB price to displace them. (Note: as of Apr 2022, LIB prices are up 353% YOY).

Companies

Form Energy is working on iron-air batteries and market their product as “reversible rusting,” as their battery consists of primarily iron, water, and air. These three inputs are obviously very cheap, so as long as they can get their battery functioning well, they should be in a good spot to keep costs low. The company is still fairly small as they received their series D funding and have a test project in Minnesota planned to show the ability of their product.

Zinc8, aptly named, is a zinc-air battery company that used zinc, an aqueous electrolyte, and air. As these materials are very abundant, the same analysis applies here. Both companies are still fairly small and not ready to deploy in mass to the country yet. It remains to be seen whether they or any other metal-air battery company will emerge victorious and truly displace LIBs as the top choice for grid applications.

Conclusion

Most of the “western” nations currently view wind/solar/batteries/EVs as the next energy transition and where the bulk of energy will come from. I have argued this vision is inflationary and not a realistic path forward, but nonetheless is the road we are currently moving along. In this world, metal air batteries/flow batteries/other alternative would be vital to displace the supply/demand imbalance posed to LIBs.

In a world that embraces nuclear energy for electricity generation, the market for grid storage would be hampered significantly. A grid built upon nuclear energy would likely provide very cheap baseload power and reduce the need for intermittent wind/solar energy sources. Otherwise-wasted space applications and households/businesses looking to source their own energy would still be a successful market for renewables/grid storage and thus metal-air batteries, but they wouldn’t be relied upon like in the first example. If supply chains are secured and costs can be reduced, renewables/batteries will still be an important source of energy in the decades to come.

-Grayson

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