If you’ve ever wondered why Form Energy uses two different air electrodes (giving them three total electrodes in one battery) this post is for you.
Metal–air batteries, including zinc–air, aluminum–air, and lithium–air systems, have attracted enormous interest from scientists and engineers for a simple reason: theoretical energy density.
On paper, they look unbeatable.
Lithium–air: ~13,300 Wh/kg
Aluminum–air: ~8,100 Wh/kg
Zinc–air: ~1,086 Wh/kg
That’s far beyond what conventional lithium-ion batteries can offer.
But theory and reality part ways quickly.
The Core Problem: Energy Efficiency, Not Energy Density
During discharge, the metal anode oxidizes while oxygen is reduced at the air cathode via the oxygen reduction reaction (ORR).
During charge, the process reverses through the oxygen evolution reaction (OER).
The problem is that both reactions are inefficient.
Metal–air batteries suffer from large voltage losses, meaning there’s a big gap between:
the voltage you put in during charging, and
the voltage you get out during discharge.
That gap is called overpotential, and it represents energy that turns into heat instead of useful work.
The result:
astonishing theoretical energy density + disappointing real-world efficiency.
The Bottom Line
Air electrodes aren’t fake.
They’re just much harder than the pitch deck implies.
And once you accept that ORR and OER are fundamentally different problems, using two air electrodes starts to look less like a quirk and more like good engineering.
Some Resources We Love:
A comprehensive review paper on metal–air batteries and air cathode design
A video response to the two people who rated us poorly, now accepting apologies