# Metal-air batteries: Science and technology

How reversible rusting stores power, and what clogs the air side

Source page: https://www.materialinsights.org/energy/e_metal_air/sg-sci/

What this tab covers The physics and chemistry: how the energy is stored, which materials it needs (scarcity, toxicity, recyclability) and what makes it age.

Metal-air batteries pair a metal anode with oxygen drawn from the air, so half the reactant is free and weightless. Iron-air (Form Energy) targets cheap 100-hour grid storage; zinc-air is a mature primary cell; aluminium-air is a high-energy primary system that is refuelled by swapping plates.

#### Mechanism
Mixed

On discharge the metal oxidises and oxygen from the air is reduced at a porous air electrode; on charge (for rechargeable types) the oxide is turned back into metal. Form Energy describes its iron-air cell as reversible rusting. Theoretical energy is very high (about 1,200 Wh/kg for iron-air and zinc-air, 8,100 Wh/kg for aluminium-air), but practical cells deliver far less: zinc-air reaches under 60% of theory, and aluminium-air runs at 1.2 to 1.6 V against a theoretical 2.7 V. Aluminium-air is not electrically rechargeable in water-based electrolytes; the spent plates are replaced and the used aluminium is recycled, which Hindalco has agreed to do in India.^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/)^[4](https://www.mdpi.com/2313-0105/11/8/315)^[5](https://www.eng.uwo.ca/nanoenergy/publications/2017/PDFs/a-comprehensive-review-on-recent-progress-in-aluminum-air-batteries.pdf)^[7](https://www.hindalco.com/media/press-releases/hindalco-phinergy-and-iop-partner-revolutionary-aluminium-air-batteries-evs-and-energy-storage)
 Theoretical specific energy, iron-air About 1,200 Wh/kg^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/) Theoretical specific energy, zinc-air Up to 1,220 Wh/kg^[4](https://www.mdpi.com/2313-0105/11/8/315) Theoretical specific energy, aluminium-air 8,100 Wh/kg^[4](https://www.mdpi.com/2313-0105/11/8/315)^[5](https://www.eng.uwo.ca/nanoenergy/publications/2017/PDFs/a-comprehensive-review-on-recent-progress-in-aluminum-air-batteries.pdf) Practical zinc-air Below 60% of theoretical, i.e. under about 730 Wh/kg (calculated)^[4](https://www.mdpi.com/2313-0105/11/8/315)

#### Material scarcity and toxicity
Favourable

Iron is among the cheapest and most abundant metals, and Form describes its raw materials as abundant and recyclable. Zinc is plentiful but China mined about 32% (calculated) of world zinc in 2025. China also produced about 61% (calculated) of the world's primary aluminium. The alkaline electrolytes (potassium or sodium hydroxide) are corrosive but not toxic metals. Zinc-air is one of the chemistries that replaced toxic mercury-zinc batteries in the US.^[3](https://www.energy-storage.news/form-energy-signs-definitive-agreement-for-100-hour-iron-air-bess-with-georgia-power/)^[8](https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-zinc.pdf)^[9](https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-aluminum.pdf)^[10](https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-mercury.pdf)
 China share of zinc mine output, 2025 About 32% (calculated from 4,100 of 13,000 thousand t)^[8](https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-zinc.pdf) China share of primary aluminium, 2025 About 61% (calculated from 45,000 of 74,000 thousand t)^[9](https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-aluminum.pdf)

#### Degradation and ageing
Mixed

The air electrode is the weak point: carbon dioxide from the air forms carbonates that clog its pores, and the catalyst degrades. Zinc grows dendrites that can short the cell, and aluminium corrodes in the electrolyte and releases hydrogen even when idle, which alloying with zinc, indium, gallium or tin reduces. For iron-air, Form has reported over 80% capacity retained after more than 1,000 cycles (company figure, reported by Energy-Storage.news); battery experts estimate 25 to 43 cycles a year, or only 500 to 860 cycles over a 20-year life.^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/)^[4](https://www.mdpi.com/2313-0105/11/8/315)^[5](https://www.eng.uwo.ca/nanoenergy/publications/2017/PDFs/a-comprehensive-review-on-recent-progress-in-aluminum-air-batteries.pdf)
 Form iron-air cycling (company figure) Over 80% capacity after more than 1,000 cycles^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/) Expected iron-air use (expert estimate) 25-43 cycles a year^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/) Form system design life (company figure) About 20 years^[2](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/)

#### Metal-air variants compared^[1](https://www.technologyreview.com/2026/10/06/1145020/2026-climate-tech-companies-to-watch-form-energy-iron-batteries/amp/)^[4](https://www.mdpi.com/2313-0105/11/8/315)^[6](https://autocarpro.in/news/indian-oils-aluminium-air-battery-manufacturing-jv-readies-itself-for-manufacturing-rollout--113826)^[7](https://www.hindalco.com/media/press-releases/hindalco-phinergy-and-iop-partner-revolutionary-aluminium-air-batteries-evs-and-energy-storage)^[15](https://greatriverenergy.com/what-we-do/cambridge-energy-storage-project)

| Variant | Rechargeable? | Main use | Status (2026) |
|---|---|---|---|
| **Iron-air** | Yes, electrically | Multi-day (100 h) grid storage | Form Energy: 2 GWh a year factory in Weirton, West Virginia; first 1.5 MW / 150 MWh utility system (Great River Energy) still being completed in late 2026 |
| **Zinc-air** | Primary cells; rechargeable versions in development | Hearing aids, navigation lights | Mature primary product |
| **Aluminium-air** | No; plates swapped and aluminium recycled | Backup power, EV range extension, telecom towers | IOC Phinergy JV in India preparing manufacturing (2023); no production start found by late 2026 |

#### Sources

1. [15 Climate Tech Companies to Watch 2026: Form Energy (6 Oct 2026)](https://www.technologyreview.com/2026/10/06/1145020/2026-climate-tech-companies-to-watch-form-energy-iron-batteries/amp/) MIT Technology Review, 2026
2. [100-hour LDES battery technologies from Form, Noon and Ore: how do they compare? (cites Form presentations and a LinkedIn commentator)](https://www.energy-storage.news/100-hour-ldes-battery-technologies-from-form-noon-and-ore-how-do-they-compare/) Energy-Storage.news, 2025
3. [Form Energy signs definitive agreement for 100-hour iron-air BESS with Georgia Power](https://www.energy-storage.news/form-energy-signs-definitive-agreement-for-100-hour-iron-air-bess-with-georgia-power/) Energy-Storage.news, 2023
4. [Breaking the polarization bottleneck: innovative pathways to high-performance metal-air batteries (Ma et al., Batteries 11(8) 315)](https://www.mdpi.com/2313-0105/11/8/315) MDPI, 2025
5. [A comprehensive review on recent progress in aluminum-air batteries (Liu et al., Green Energy and Environment)](https://www.eng.uwo.ca/nanoenergy/publications/2017/PDFs/a-comprehensive-review-on-recent-progress-in-aluminum-air-batteries.pdf) KeAi / Western University, 2017
6. [Indian Oil's aluminium-air battery manufacturing JV readies itself for manufacturing rollout](https://autocarpro.in/news/indian-oils-aluminium-air-battery-manufacturing-jv-readies-itself-for-manufacturing-rollout--113826) Autocar Professional, 2023
7. [Hindalco, Phinergy and IOP partner on revolutionary aluminium-air batteries for EVs and energy storage (press release, 18 Jul 2022)](https://www.hindalco.com/media/press-releases/hindalco-phinergy-and-iop-partner-revolutionary-aluminium-air-batteries-evs-and-energy-storage) Hindalco Industries, 2022
8. [Mineral Commodity Summaries 2026: Zinc](https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-zinc.pdf) US Geological Survey, 2026
9. [Mineral Commodity Summaries 2026: Aluminum](https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-aluminum.pdf) US Geological Survey, 2026
10. [Mineral Commodity Summaries 2026: Mercury](https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-mercury.pdf) US Geological Survey, 2026
11. [Cambridge Energy Storage Project (project page)](https://greatriverenergy.com/what-we-do/cambridge-energy-storage-project) Great River Energy, 2025

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_metal_air/index.md)
- [Overview](https://www.materialinsights.org/energy/e_metal_air/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_metal_air/correlation/index.md)
- [Engineering and performance](https://www.materialinsights.org/energy/e_metal_air/sg-eng/index.md)
- [Economics](https://www.materialinsights.org/energy/e_metal_air/sg-econ/index.md)
- [Impact and additionality](https://www.materialinsights.org/energy/e_metal_air/sg-impact/index.md)
- [What to track](https://www.materialinsights.org/energy/e_metal_air/tracking/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_metal_air/links/index.md)