Zinc-Air and Iron-Air Batteries

A (slightly) deeper dive

In my previous post, I cited some crazy low prices for zinc-air and iron-air batteries: $20/kWh for iron-air and $60/kWh for zinc-air.

Is this for real?

Can you actually buy those batteries at those prices today, or are they raw materials costs or speculative future projections?

Iron-Air

The $20/kWh number comes from Form Energy, an iron-air battery startup, and they are indeed claiming that their current (not future!) commercial product is “less than 1/10 the cost of lithium-ion”.

It’s a press release from a startup, so it’s worth taking with a grain of salt, but that’s what they say.

They have a pilot project with Minnesota-based Great River Energy, planned to open in 2023 and providing 1 MW/150 MWh of power. The price of the facility is undisclosed, however.

What is public information, however, is the raw materials cost of different battery chemistries, which allows us to make apples-to-apples comparisons.

The materials costs provided in this 2017 paper by MIT researchers computes a materials cost (anode, cathode, and electrolyte) that puts zinc-air batteries at $5.8/kWh, much less than the materials cost of lithium-ion batteries ($44/kWh for LFP, $68.3 for NCA, etc).

The very lowest materials costs in this cost comparison are found in sodium-sulfur batteries, a type of high-temperature molten-salt battery, produced solely by Japanese ceramic company NGK and used in stationary power storage applications totaling over 4 GWh worldwide to date.

A paper by Form Energy researchers estimates the chemical costs of their iron-air battery (including only the iron electrode and aqueous electrolyte, not the battery pack or air delivery system) at $1.50/kWh.

The “cathode” in a metal-air battery is the air electrode; the active ingredient is oxygen, which is free from the air, but the whole electrode is obviously not costless. It is a porous structure optimized to facilitate the diffusion of air and liquid electrolytes into reaction sites where oxygen can react with the metal cathode.

A 2021 preprint estimates the materials cost of iron-air battery cells as $4/kWh; if you include materials cost for the whole battery pack and air delivery system, it rises to $25/kWh. This estimate does include the cost of the air electrode. And it shows iron-air battery materials costs comparing favorably to lithium-ion materials costs: ($97/kWh for LFP, $141/kWh for NMC811, $143/kWh for NMC622, and $120/kWh for NCA.) That’s not a factor of 10 the way Form Energy claims, but it’s still a big gap.

Another paper, from 2012, has a more conservative estimate of $59/kWh for iron-air battery materials costs; it assumes higher costs for the air electrode and the air delivery system.

I’m not prepared to evaluate whose models are more credible here; everyone seems to agree that iron-air batteries should be much cheaper per kilowatt-hour than lithium-ion, but there’s a wide range of possibilities.

Zinc-Air

The number I cited for zinc-air batteries comes from Zinc8, a zinc-air energy storage startup which claims that a 100-hour storage system will cost $60/kWh.

The 1 MWh facility Zinc8 is building for the New York Power Authority is quite a bit more expensive, at an estimated $210/kWh. NYPA paid $2.55M for the project.

They’re also doing a 100kW/1.5MWh project in an apartment building in Queens, for a contribution of $460,400, and 8-hour storage capacity.

But they think their design has favorable scaling properties; to store energy for more hours, they don’t need to make more batteries, they can just make the zinc storage tanks larger without adding more of the most expensive components like cathodes and membranes.

So, yes, the low cost figure is a projection, and such projections have been wrong before.

A different zinc-air battery company, EoS, reported higher prices of $263/kWh in 2018 for a 1 MW/4 MWh system. This is well above the present-day cost of lithium-ion batteries.

Back in 2017, EoS was taking orders at $160/kWh, a figure they have since walked back as their technology underperformed expectations.

However, as far as raw materials costs go, zinc-air batteries are consistently lower than lithium-ion raw materials costs (though not as cheap as iron-air.)

Will zinc-air batteries turn out to be cheaper than lithium-ion in practice as their deployments scale up? We don’t know; as with iron-air, it depends largely on whether they can bring down the cost of the air electrodes.

Thanks to Brady Hauth for feedback.