A little rust might make batteries safer, easier to recycle, and a potential competitor to lithium-ion.

By using water in place of the hazardous chemical electrolytes used in commercial batteries, an international team of scientists hailing from Australia to China has proposed a water-based battery technology, a safer and greener alternative to chemical-laden batteries marketed today.

Lithium-ion batteries, used in everything from laptops and phones to electric vehicles, contain volatile chemicals that can overheat and sometimes ignite, resulting in dangerous chemical fires. Though rare, this safety hazard leaves lithium batteries and their cooling systems heavily overengineered at "a high cost, relative lower safety, and negative environmental impact," said lead author and chemical scientist Tianyi Ma of RMIT University in Melbourne, Australia.

To mitigate these kinds of risks, researchers and engineers have been working tirelessly to design batteries that use alternative materials without sacrificing lithium's impressive performance.

Batteries store and distribute energy by creating a flow of electrons that move from the positive cathode to the negative anode and back again.

Aside from the separator between the anode and cathode, all these materials often sit within a medium that facilitates the transference of electrons back and forth between both ends – this is called the electrolyte.

In a water battery (technically termed aqueous metal-ion batteries), the electrolytic fluid is water with a few added ingredients like magnesium or zinc, which are cheaper to assemble and safer than conventional batteries which sport more toxic and less abundant ingredients like sulfuric acid or lithium salt.

In most batteries, due to the nature of the chemicals, primary failure is caused by dendrite formation or dendrites – metallic growths that work through minuscule cracks on the metal anode, degrading compartments and eventually short-circuiting the battery.

A major obstacle to using all types of aqueous metal-ion batteries has been suppressing dendrite growth – in this case, researchers coated the zinc anode of the battery with bismuth metal, which oxidizes to form rust, creating a protective layer that stops dendrites from forming and also helps the prototypes last longer, retaining more than 85% of their capacity after 500 cycles.

Although water batteries aren't yet on their way to replace lithium-ion, in a decade or so, with further R&D, they could provide a cheaper, safer and more easily recycled alternative.

The team has so far developed water-based prototypes of coin-sized button batteries as well as common cylindrical formats and is working to improve their overall energy density to be comparable to standard off-the-shelf batteries.

Magnesium is lighter than zinc, with a greater potential energy density. Ma is optimistic that if magnesium-ion batteries can be commercialized, the technology could replace bulky lead-acid batteries – common in most internal combustion vehicles – within three years and potentially replace lithium-ion batteries in five to ten.

"Addressing end-of-life disposal challenges that consumers, industry, and governments globally face with current energy storage technology, our batteries can be safely disassembled and the materials can be reused or recycled," said Ma.

To test the battery's practical application, the researchers ran a solar panel and 45-watt solar light, which the battery kept illuminated for 12 hours after a day's charge. Although a small-scale demonstration of the massive potential of water batteries to be used for renewable energy storage across several industries, it is a remarkable start for a disruptive portable energy source.