The battery that has drawn the most attention in long-duration storage works on a principle so humble it sounds like a joke: it rusts iron on purpose. Form Energy's iron-air chemistry is real engineering, though, and a 2023 grant lets you read the actual electrode behind the headlines.
The mechanism is reversible rust. When the battery discharges, iron reacts with oxygen from the air and turns to rust (iron oxide), releasing energy. When it charges, electricity drives that reaction backward, converting the rust back into metallic iron and giving off oxygen. The 'air' electrode handles the oxygen; the iron electrode is where the storage lives. Charge is stored not in scarce lithium but in the oxidation state of cheap, abundant iron.
Form Energy's grant US11552290B2, "Negative electrodes for electrochemical cells," is exactly the part that matters most: the iron electrode where the reversible rusting happens. The long inventor list — including names long associated with this chemistry — signals that this is core IP, not a peripheral filing. The hard problem an iron electrode must solve is doing this rust-and-un-rust cycle thousands of times without the electrode crumbling or losing capacity.
Why anyone wants a battery made of rust: cost and duration. Lithium-ion batteries are excellent for hours of storage but expensive to scale to days. The grid's hardest problem is the multi-day lull — a windless, cloudy stretch — and bridging that with lithium is economically brutal. Iron is one of the cheapest materials on Earth, so an iron-air battery can be built big and slow: not much power per dollar, but enormous energy per dollar over long durations. That is precisely the niche lithium serves poorly.
The discipline to apply: long-duration storage is genuinely hard, round-trip efficiency for iron-air is modest, and a patent on the electrode is not a fleet of operating plants. Form Energy is a real company with real projects, but the technology's commercial proof is still being written. Read this 2023 grant as the chemistry's foundation — the specific electrode that makes 'store energy by rusting' a buildable claim rather than a slogan.