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    Iron-air batteries

    Also known as iron-air battery, multi-day storage, Form Energy battery

    Iron-air batteries store electricity by turning iron into rust while discharging and back into iron while charging, aiming for multi-day storage of up to 100 hours.[1][2] US developer Form Energy began delivering its first commercial batteries in late 2025, reports an 80 GWh backlog including a 300 MW / 30 GWh project tied to a Google data center in Minnesota, and its first system is now expected online in 2027.[3][4][5][6]

    Editor reviewedUpdated Batteries and energy storageEnergy and climateScience
    Key facts

    How it works

    An iron-air battery “breathes” oxygen. When discharging it converts iron metal to rust, and when charging an electric current turns the rust back into iron, a process Form Energy calls reversible rusting.[1] The cell uses iron, water and air with a water-based, non-flammable electrolyte. Form says the design has no mechanism for thermal runaway.[7] The system can store and discharge energy for up to 100 hours.[2] Typical lithium-ion grid batteries provide one to eight hours.[8]

    Why multi-day storage matters

    Lithium-ion batteries cover daily cycles well and have become cheap, with stationary packs at 70 dollars per kilowatt-hour in 2025.[9] Iron-air developers aim at a different job, storage over several days, using iron, water and air.[2][7]

    Projects and timeline

    • Cambridge, Minnesota. Great River Energy and Form Energy broke ground in August 2024 on a 1.5 MW pilot, then expected online by late 2025.[10] Form began deploying its first commercial batteries there in October 2025, and said it had more than 200 MW under contract.[3] In March 2026 Form still expected the full project online during 2026.[11] In October 2026 MIT Technology Review reported that the 150 MWh system was set to come online in 2027.[6]
    • Crusoe. On 24 March 2026 Form and Crusoe announced an agreement for 12 GWh of iron-air storage for AI data centers. Deliveries start in 2027 from Form Factory 1 in Weirton, West Virginia.[12]
    • Xcel Energy and Google. A 2026 Google-Xcel agreement includes a 300 MW / 30 GWh Form system, described as the largest battery project by energy capacity announced to date.[5] Form’s chief executive said the first modules should ship by the end of 2028.[13]
    • Ireland. On 17 March 2026 Form and FuturEnergy Ireland agreed to deploy a 10 MW / 1,000 MWh system in the northwest of Ireland. It is expected online in 2029 and is Form’s first announced project outside the US.[14]

    Funding and manufacturing

    Form raised a 750 million dollar Series G round led by T. Rowe Price in August 2026, taking its total equity raised to over 2 billion dollars. It said its backlog had grown during 2026 from about 20 to 80 gigawatt-hours, including the Xcel-Google, Crusoe and Ireland projects.[4] These are company figures. In September 2026 it closed a 270 million dollar credit facility, part of which advances cash against Section 45X manufacturing tax credits.[15] Those credits are subject to the foreign-entity rules added in 2025.[16]

    Scaling is the open question. Form’s factory can make about 2 gigawatt-hours of batteries a year, a fraction of the 80 gigawatt-hours it has promised customers.[17] The company has not disclosed what its systems cost today. Its target is 20 dollars per kilowatt-hour.[17] Stationary lithium-ion packs averaged 70 dollars per kilowatt-hour in 2025, although packs are only part of a system’s cost.[9]

    How it compares

    Iron-air competes for long-duration work with flow-batteries, which scale energy by enlarging electrolyte tanks and are typically offered with 2 to 18 hours of discharge.[18][19] Iron-air targets much longer discharge, up to 100 hours.[2] Its materials, iron, water and air, avoid the vanadium supply concerns of flow batteries and the lithium price swings of lithium-ion.[7][20][21]

    What to watch

    The key test is whether the Cambridge pilot reaches commercial operation and performs as promised, because the much larger contracts depend on the same technology.[6][5] The pilot has already slipped from its original late-2025 target, and its expected start is now 2027.[10][6] The second test is whether Form can expand its factory fast enough to meet its 80 GWh backlog.[17] Demand from AI data centers is one driver: the Crusoe deal and the Google-Xcel project are both tied to data centers.[12][5] The grid storage explainer covers how multi-day storage fits alongside shorter-duration batteries.

    Questions readers ask

    How does an iron-air battery work?

    When discharging it takes in oxygen from the air and converts iron to rust; charging converts the rust back to iron. Form Energy calls this reversible rusting.[1]

    How long can it supply power?

    Form Energy says up to 100 hours, far longer than typical lithium-ion grid batteries, which provide one to eight hours.[2][8]

    Is any iron-air battery running yet?

    Not commercially as of October 2026. Form began deploying batteries at its first project in Minnesota in late 2025; the system, first expected by late 2025 and then in 2026, was reported in October 2026 to be set to come online in 2027.[3][10][11][6]

    What is the 30 GWh project?

    A 300 MW / 30 GWh Form Energy system in a Google-Xcel Energy agreement, described as the largest battery project by energy capacity announced to date. First modules are expected to ship by the end of 2028.[5][13]

    Sources

    Each numbered claim is a statement we checked against the sources listed with it. Status shows how well established it is.

    1. [1]

      Form Energy's iron-air battery works by reversible rusting - while discharging it breathes in oxygen from the air and converts iron to rust, and while charging an electric current converts the rust back to iron. confirmedas of 2026-10-10

    2. [2]

      Form Energy says its iron-air system can store and discharge energy for up to 100 hours. confirmedas of 2026-10-10

    3. [3]

      In October 2025 Form Energy began deploying its first commercial batteries, for Great River Energy's multi-day storage project in Minnesota, and said it had more than 200 MW of batteries under contract. reportedas of 2025-10-29

    4. [4]

      On 12 August 2026 Form Energy announced a $750 million Series G round led by T. Rowe Price and said its backlog had grown in 2026 from about 20 to 80 gigawatt-hours, including the Xcel-Google, Crusoe and FuturEnergy Ireland projects. confirmedas of 2026-08-12

    5. [5]

      A 2026 agreement between Google and Xcel Energy includes a 300 MW / 30 GWh Form Energy iron-air battery, described as the largest battery project by energy capacity announced to date. confirmedas of 2026-03-04

    6. [6]

      In October 2026 MIT Technology Review reported that Form Energy's first commercial deployment, a 150 megawatt-hour system for Great River Energy in Minnesota, was set to come online in 2027. reportedas of 2026-10-06

    7. [7]

      Form Energy's battery uses iron, water and air with a water-based, non-flammable electrolyte, and the company says it has no mechanism for thermal runaway. confirmedas of 2026-10-10

    8. [8]

      Battery storage provides short-term flexibility to the power system, typically for periods of one to eight hours. confirmedas of 2024-04-25

    9. [9]

      Battery pack prices for stationary storage fell to 70 US dollars per kilowatt-hour in 2025, 45% lower than in 2024. confirmedas of 2025-12-09

    10. [10]

      In August 2024 Great River Energy and Form Energy broke ground on a 1.5 MW multi-day iron-air storage project in Cambridge, Minnesota, then expected to operate by late 2025. confirmedas of 2024-08-15

    11. [11]

      As of March 2026 Form Energy had begun delivering its first commercial pilot system in Minnesota, with the full project expected online in 2026. confirmedas of 2026-03-24

    12. [12]

      On 24 March 2026 Form Energy and Crusoe announced an agreement for 12 gigawatt-hours of iron-air storage for AI data centers, with deliveries from 2027 built at Form Factory 1 in Weirton, West Virginia. confirmedas of 2026-03-24

    13. [13]

      Form Energy's chief executive said the company expects to ship the first modules for the Xcel-Google project by the end of 2028. reportedas of 2026-03-04

    14. [14]

      On 17 March 2026 Form Energy and FuturEnergy Ireland agreed to deploy a 10 MW / 1,000 MWh iron-air battery in the northwest of Ireland, expected online in 2029 and Form's first announced international deployment. confirmedas of 2026-03-17

    15. [15]

      On 21 September 2026 Form Energy closed a $270 million credit facility, including a facility that advances funds against Section 45X manufacturing tax credits. confirmedas of 2026-09-21

    16. [16]

      The One Big Beautiful Bill Act, enacted on 4 July 2025, added prohibited foreign entity rules that restrict access to the 48E investment, 45Y production and 45X manufacturing tax credits, including for battery storage. confirmedas of 2026-07-16

    17. [17]

      Form Energy's production capacity of 2 gigawatt-hours of batteries a year is a fraction of its 80 gigawatt-hour backlog; the company targets a system cost of 20 US dollars per kilowatt-hour but has not disclosed current costs. reportedas of 2026-10-06

    18. [18]

      Flow battery energy capacity is raised by enlarging the electrolyte tanks and power by enlarging the reactor, so the two can be sized separately. confirmedas of 2023-04-07

    19. [19]

      Invinity offers vanadium flow battery configurations with discharge durations of 2 to 18 hours. confirmedas of 2026-10-10

    20. [20]

      Vanadium is widespread but dilute and hard to extract, production is concentrated in Russia, China and South Africa, and its prices are high and volatile. confirmedas of 2023-04-07

    21. [21]

      Lithium prices at the start of 2026 were more than twice their level a year earlier, and cobalt prices also doubled over the year. confirmedas of 2026-10-10

    22. [22]

      CATL says it has ranked first globally in EV battery usage for nine consecutive years and first in storage battery deliveries for five consecutive years. confirmedas of 2026-10-10

    Revision history (2)
    1. Added Form Energy's first deliveries, the Ireland project, Series G, credit facility, factory capacity and cost target; the Minnesota pilot is now reported for 2027.
    2. Page created.

    Created Oct 10, 2026. Last reviewed by an editor on Oct 10, 2026. Next scheduled review: Jan 10, 2027.

    Cite this page

    "Iron-air batteries." ContentLora, updated Oct 11, 2026. https://contentlora.com/wiki/iron-air-batteries

    Spotted an error? Suggest a correction or emailcorrections@contentlora.com.

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