Lithium-ion batteries power everything from smartphones to electric cars, but they rely on materials that raise environmental and ethical concerns.
Metals like nickel and cobalt are costly to mine, difficult to recycle, and linked to toxic processing.
Now, materials scientists are exploring a very different idea: building high-capacity batteries using iron oxide—better known as rust.
Researchers at Saarland University, working with colleagues at University of Salzburg, have developed a promising new battery material made from tiny hollow carbon spheres filled with iron oxide.
Their results, published in Chemistry of Materials, show that this unusual combination can store large amounts of energy using materials that are abundant, inexpensive, and far less harmful to the environment.
To picture the material, the researchers suggest thinking of Mozartkugeln, the famous chocolate balls from Salzburg. Like those sweets, the new structures are spherical and hollow.
On a much smaller scale, these “carbon spherogels” measure about 250 nanometers across and are riddled with tiny pores, giving them a huge surface area.
That structure makes them especially attractive for battery electrodes, where chemical reactions need plenty of room to take place.
The challenge was figuring out what to put inside these hollow spheres. After early tests with titanium dioxide produced modest results, the team turned to iron oxide. Iron has several advantages: it is widely available, easy to recycle, and, at least in theory, capable of storing a large amount of electrical charge.
Using a scalable chemical process, the researchers embedded tiny iron particles evenly throughout the carbon spheres.
Over time, those particles react with oxygen during battery operation, gradually turning into iron oxide. This process turns out to be a key advantage.
Unlike most batteries, which slowly lose capacity as they are used, this system actually improves with age. As the battery goes through repeated charge and discharge cycles, more of the iron becomes activated. After about 300 cycles, the iron inside the spheres has fully converted to iron oxide, and the battery reaches its maximum storage capacity.
This behavior, known as electrochemical activation, means the battery performs better the longer it is used—an unusual and appealing trait. Still, the researchers acknowledge that there are challenges ahead.
The activation process currently takes too long, and future designs will need to reach full performance more quickly. In addition, the new material has only been tested as a battery anode, meaning a compatible cathode still needs to be developed before a complete battery can be built.
Despite these hurdles, the researchers are optimistic. They believe iron oxide-filled carbon spheres could form the basis of environmentally friendly batteries for renewable energy storage. The material is also being explored for sodium-ion batteries, which are attracting growing interest as a low-cost alternative to lithium-based systems.
Beyond battery performance, the project also focuses on recycling. The team is investigating how future batteries can be designed for easy disassembly and material recovery, helping to create a more sustainable, closed-loop energy system.
If successful, “rust-based” batteries could one day help power a cleaner and more resilient energy future.
