While renewable energy sources like solar and wind have now become cheaper than fossil fuels1, developing long-term energy storage is key to overcome their intermittency. Lithium-ion batteries are the state-of-the-art storage technology but they can only cost-effectively provide energy for about 6 hours.2 So, what if we could extend the battery duration to 100 hours … out of thin air … and rust? Literally!

An old technology for a new storage form

Solar and wind energy adoption is growing, but they’ll never be available 24/7. On top of that, climate change-induced extreme weather events like storms can cause several days of power outages. The booming demand for a more secure and cleaner energy supply is driving the need for storing surplus green energy, so that it can be fed back to the grid on a cloudy day or when the wind isn’t blowing. To quench the thirst for carbon-free power, grid operators are investing in today’s most mature storage technology: lithium-ion batteries. In the US, the scale of these installations is expected to grow from a current level of 1.5 gigawatts up to hundreds of gigawatts by 2030.3

After the last heatwave-induced blackouts, California has been warming up the race to energy storage. Last year, Vistra Energy began developing the world’s largest battery with a 300-megawatt capacity of lithium-ion battery technology.4 Along with another 100-megawatt storage unit scheduled to go online this year, the Californian plant will provide energy to about 300,000 homes for four hours during evenings, or whenever a power outage occurs.

But what if demand outstrips supply for 24 hours or even for a few days? Lithium-ion systems are better suited for delivering short-term bursts of energy. The downside, their charge dissipates over time and you’d need many of them to provide energy over a longer period of time. While lithium-ion batteries are already at commercial scale, their levelized cost of storage (LCOS) doesn’t scale well. We’re talking about $132-245/MWh for large-scale applications according to Lazard.5 You can probably see why a cheaper, long-duration energy system would come in handy.

So, is there anything like that? Form Energy seems to have something in storage for us.6 The Massachusetts-based startup has developed a rechargeable iron-air battery.7 This technology isn’t exactly new. NASA was the first at playing around with it back in 1968,8 but it was never commercialized because of technical challenges with its components, such as the electrodes and electrolytes.9 For example, NASA struggled with the hydrogen formation at the anode. Because of this, the electrode had to be overcharged to reach its full capacity.

Form Energy has spent years fine-tuning the process and said its innovation will let us have low-cost renewable energy every day of the year. Dubbed as a multi-day storage solution, Form’s batteries can store large amounts of energy and release it over more than four days. This storage option could tackle the day-to-day and seasonal variability of renewable production. The company managed to close $240 million of series D funding by attracting big investors like Jeff Bezos, Bill Gates’ Breakthrough Energy Ventures, and ArcelorMittal, which is one of the world’s leading iron ore producers who will help Form Energy generate the raw material.10

But how does Form Energy’s battery work? Think of a box filled with racks of 10 to 20 cells. Every cell is filled with an electrolyte solution, similar to the one used in AA batteries, and two plates: an anode made of iron pellets and an air-breathing cathode. This setup allows an electrochemical redox reaction to happen. Essentially, Form Energy’s battery stores electricity through a reversible rusting cycle … yes, rust. They turn corroded iron pellets into an energy dispenser. The way it works is actually pretty simple. When they expose the tiny iron pellets to air, oxygen will turn them into rust, a.k.a. iron oxide. That oxidation releases electrons, which is the electricity that can be sent to the grid. To charge up the battery, you basically reverse that process. The incoming electrons will eat up the oxygen and reduce the rust layer until it converts it back into metallic pellets. Thanks to this … e-looping … process, the iron-air battery can store energy for much longer than conventional setups.

The best battery?

The benefits of this type of technology could be absolutely massive. No matter if you’re in Pittsburgh, the cloudiest city in the US or in Oak Ridge, Tennessee, where the wind is calmer than anywhere else in America. You could store and have 100% clean energy for at least 4 days. Forget about fossil fuels backup.

But the thought of that probably raises another question in your mind, how do iron-air batteries stack up against other energy storage systems? If you compare them to lithium-ion batteries, the gold standard in power storage, Form Energy’s device boasts a 17x longer duration. Based on a recent study11, a 100+ hour storage system could have a huge impact on reducing the power generation costs.

Although Form hasn’t put a price tag on its invention yet, they claim that technology improvements paired with using extremely abundant materials like Iron and air would make their batteries 10 times cheaper than lithium-based systems. If they managed to go below $20/kWh, the integration of their batteries in our energy systems could save billions in electricity costs.12

Besides being more widely recycled, Iron is the fourth most common element on Earth13, while lithium is about 2,000 times less plentiful.14 You may see why the production of Form Energy’s batteries are more sustainable than lithium-based devices in the long run. And it’s also safer. That’s because the electrolyte solution isn’t flammable, unlike the mixture of lithium salts and organic solvents inside typical lithium-ion batteries.

On the other hand, Form Energy doesn’t see their batteries as a replacement for lithium-ion storage systems, but as a complement to them. Lithium-ion batteries are great for fast-response actions like frequency regulation and grid stabilization. They also have a high power-to-weight ratio or energy density. That means they can hold a lot of energy while keeping a small size, which is ideal for electric vehicles (EVs). In contrast, Iron-air batteries would be best at storing and supplying large amounts of energy at lower power and density.15 Which makes sense given their dishwasher size they’re no where near fitting inside your smartphone and are nearly as large as your EV. Instead, they’re designed to be hooked together in grids.

But Form Energy isn’t the only one working on long-duration batteries. The Oregon-based company ESS built a factory producing iron flow batteries back in 2019.16 One of the company’s key achievements was to invent a Proton Pump that recycles the unwanted hydrogen from the anode into the electrolyte solution. By recirculating protons into the liquid phase, the pump rebalances the chemistry of the electrolyte and maintains its conductivity, which preserves the battery efficiency. ESS has recently launched a new scalable and custom-designed system starting from a power capacity of 3 MW. However, their commercial plant can continuously supply electricity for only 16 hours at most.17

And it’s not just about Iron. The Canadian start-up Zinc818 has developed a Zinc-air battery that can also provide 100+ hours of storage. This works pretty much the same way as the Iron-air system, except you store the energy in Zinc particles which are as big as grains of sand rather than in Iron pellets. When you need electricity, you combine the charged Zinc particles with the oxygen from the air and water to form zincate, an oxidised form of zinc. That probably helps make sense of the companies name. This is then broken down upon recharging to close the loop. Same principle but different price as the capital cost of a 100-hour Zinc8 battery is $60/kWh19, which is above the $20/kWh threshold Form Energy may go under. Nevertheless, Zinc8 has been working towards three pilot projects. A problem with Zinc is that it’s more prone to form dendrites compared to Iron. These crystals pile up onto the anode over time damaging the electrode and affecting its durability.

Besides Iron and Zinc, Aluminium is another active element that could be used in the metal-air battery setup. I’ve actually got a video I put out a few weeks ago on this topic. Aluminium-air units are touted to have high energy density and to be cheaper and safer than lithium-ion batteries. This places them as a potential alternative to lithium packs in EVs. While you can’t recharge the Aluminium-air battery, you could swap it for a new one at the fuel station and recycle the old one.

Looping back to the iron-air system, Form Energy touts flexibility as another plus for their technology. Based on the energy demand, tens of thousands to hundreds of thousands of modules can be grouped into megawatt-scale power blocks. Their modular plants can be built anywhere, including urban areas, targeting utility-scale energy needs.

All this sounds electrifying, right? But will it actually work? It’s a little too early to tell since the US startup isn’t launching its first pilot until 2023. They’ll build a 1MW plant at the Great River Energy utility’s Minnesota facility. It’s going to take reaching a commercial scale before we’ll know the real feasibility of iron-air batteries. According to some researchers, this storage unit should reach an efficiency of 80% and have a cycle life of 5,000 cycles to withstand a commercial operation.20 As of today, Form Energy seems to be keeping the efficiency and durability of their devices behind an iron curtain, however, the company appears to be focusing on tech-savvy work.

Besides optimizing the electrodes manufacturing and the cell design, modeling plays a key role. Form Energy appears to be ahead of the simulation game with their grid modeling tool, Formware™.21 By factoring real-world weather variability across the year with hourly resolution, the company toolkit could help design more reliable, cost-effective and clean power systems that deliver around-the-clock electricity. One of the practical applications of their tool was to assess whether long-duration storage units could cost-effectively replace the energy supply of peaker plants in New York State between 2010 and 2019. According to their estimates,22 low-cost multi-day batteries would economically match the operations of 83% of New York’s plants. In addition, using long-duration storage along with lithium-ion batteries would enable replacing 4 times more peaker plants compared to using lithium-based systems alone.

Based on data from one of their partners, Form Energy conducted some case studies to compare the cost-effectiveness of their higher time resolution model with a standard method using a lower number of data points. Results show that Form Energy’s more accurate approach would save grid users $27 million per gigawatt of peak demand per year.23 That’s because Formware captured renewables variability across the year much better than the conventional model.

What lies in storage for the future

Long-lasting energy storage will be essential to increase the utilization of renewables and leave fossil fuels underground. Combined with lithium-ion batteries, the Iron-air systems could accelerate the transition to carbon-free electricity. While its feasibility still needs to be proved out, this technology holds a lot of promise for recharging the planet’s batteries.


  1. “Most new wind and solar projects will be cheaper than coal, report ….” 23 Jun. 2021 ↩︎
  2. “Fact Sheet | Energy Storage (2019) | White Papers | EESI.” 22 Feb. 2019 ↩︎
  3. “How US Grid Operators Plan To Tackle Energy Storage at Gigawatt.” 10 Feb. 2021 ↩︎
  4. “In Boost for Renewables, Grid-Scale Battery Storage Is on the Rise.” 15 Dec. 2020 ↩︎
  5. “Levelized Cost of Energy and of Storage – Lazard.com.” 19 Oct. 2020 ↩︎
  6. “Form Energy: We are Transforming the Grid.” ↩︎
  7. “Battery Technology | Form Energy.” ↩︎
  8. “A Review of the Iron–Air Secondary Battery for Energy Storage.” 26 Sept. 2014 ↩︎
  9. “Metal-Air Batteries: Future Electrochemical Energy Storage of Choice?.” ↩︎
  10. “Iron-air long-duration battery startup Form Energy closes US$240 ….” 25 Aug. 2021 ↩︎
  11. “The design space for long-duration energy storage in decarbonized ….” 25 May. 2021 ↩︎
  12. “The design space for long-duration energy storage in decarbonized ….” 25 May. 2021 ↩︎
  13. “The Eight Most Abundant Elements in the Earth’s Crust – Sciencing.” 4 Mar. 2020 ↩︎
  14. “Iron-air batteries: Huge green-energy breakthrough, or just a lot of ….” 27 Jul. 2021 ↩︎
  15. “Iron-air long-duration battery startup Form Energy closes US$240 ….” 25 Aug. 2021 ↩︎
  16. “ESS is betting the world is ready for a billion-dollar battery disrupter.” 24 Jun. 2021 ↩︎
  17. “‘All-iron’ flow battery maker ESS Inc launches – Energy Storage News.” 15 Feb. 2021 ↩︎
  18. “Zinc8 Energy Solutions: The Leader in Zinc Air Battery Technology.” ↩︎
  19. “New zinc-air battery is ‘cheaper, safer and far longer-lasting than ….” 21 May. 2020 ↩︎
  20. “Materials challenges and technical approaches for realizing ….” 28 May. 2012 ↩︎
  21. “Grid Modeling Toolkit | Form Energy.” ↩︎
  22. “Solving the Clean Energy and Climate Justice Puzzle – Form Energy.” ↩︎
  23. “Best Practice Modeling to Achieve Low Carbon Grids – Form Energy.” 1 Dec. 2020 ↩︎

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