Metal-air batteries: Science and technology

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

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.²⁴⁵⁷

Theoretical specific energy, iron-air About 1,200 Wh/kg² Theoretical specific energy, zinc-air Up to 1,220 Wh/kg⁴ Theoretical specific energy, aluminium-air 8,100 Wh/kg⁴⁵ Practical zinc-air Below 60% of theoretical, i.e. under about 730 Wh/kg (calculated)⁴

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.³⁸⁹¹⁰

China share of zinc mine output, 2025 About 32% (calculated from 4,100 of 13,000 thousand t)⁸ China share of primary aluminium, 2025 About 61% (calculated from 45,000 of 74,000 thousand t)⁹

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.²⁴⁵

Form iron-air cycling (company figure) Over 80% capacity after more than 1,000 cycles² Expected iron-air use (expert estimate) 25-43 cycles a year² Form system design life (company figure) About 20 years²

Metal-air variants compared¹⁴⁶⁷¹⁵

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

15 Climate Tech Companies to Watch 2026: Form Energy (6 Oct 2026) MIT Technology Review, 2026
100-hour LDES battery technologies from Form, Noon and Ore: how do they compare? (cites Form presentations and a LinkedIn commentator) Energy-Storage.news, 2025
Form Energy signs definitive agreement for 100-hour iron-air BESS with Georgia Power Energy-Storage.news, 2023
Breaking the polarization bottleneck: innovative pathways to high-performance metal-air batteries (Ma et al., Batteries 11(8) 315) MDPI, 2025
A comprehensive review on recent progress in aluminum-air batteries (Liu et al., Green Energy and Environment) KeAi / Western University, 2017
Indian Oil's aluminium-air battery manufacturing JV readies itself for manufacturing rollout Autocar Professional, 2023
Hindalco, Phinergy and IOP partner on revolutionary aluminium-air batteries for EVs and energy storage (press release, 18 Jul 2022) Hindalco Industries, 2022
Mineral Commodity Summaries 2026: Zinc US Geological Survey, 2026
Mineral Commodity Summaries 2026: Aluminum US Geological Survey, 2026
Mineral Commodity Summaries 2026: Mercury US Geological Survey, 2026
Cambridge Energy Storage Project (project page) Great River Energy, 2025