A water-based zinc battery that is simultaneously powerful, rechargeable and intrinsically safe has been developed by army scientists, together with a team of researchers from the University of Maryland (UMD) and the National Institute of Standards and Technology.

In prior achievements, these scientists invented a new class of water-based electrolytes that can work under extreme electrochemical conditions that ordinary water cannot. The team explains it successfully applied these electrolytes on different lithium-ion chemistries and adapted the electrolyte to a battery chemistry much cheaper than lithium – Zinc. The team say it was able to demonstrate that an aqueous battery can satisfy the multi-facet goals of high energy, high safety and low cost simultaneously.

The world's very first battery used zinc as anode in 1799. In the following two centuries, many zinc-based batteries were commercialised, some of which are still on market. These batteries used to provide safe and reliable energy, although at moderate energy density, to satisfy our daily needs. But, their presence in our lives has significantly shrunk since the emergence of lithium-ion batteries 28 years ago. Besides energy density, a major reason for the diminishing role of zinc batteries is the poor reversibility of the zinc chemistry in aqueous electrolytes, the team explains. Non-rechargeable batteries already created significant amount of landfill, imposing serious environmental burden on industrialised societies.

"On the other hand,” says Dr. Kang Xu, an Army Research Laboratory (ARL) fellow and team leader, "the safety hazard of lithium-ion batteries are rooted in the highly flammable and toxic non-aqueous electrolytes used therein. The batteries of aqueous nature, thus, become attractive, if they can be made rechargeable with high energy densities. Zinc is a natural candidate."

The researchers say the new aqueous zinc battery could eventually be used not just in consumer electronics, but also in extreme conditions to improve the performance of safety-critical vehicles.

As an example of the aqueous zinc battery's power and safety, Fei Wang, a jointly appointed postdoctoral associate at UMD's Clark School and ARL, cites the numerous battery fire incidents in cell phones, laptops and electric cars highlighted in recent media coverage. The hope is that the new aqueous zinc battery presented in this work could be the answer for safe battery chemistry, whilst still maintaining the comparable or even higher energy densities of conventional lithium-ion batteries.

"Water-based batteries could be crucial to preventing fires in electronics, but their energy storage and capacity have been limited. Until now,” Wang says. “For the first time, we have a battery that could compete with the lithium-ion batteries in energy density, but without the risk of explosion or fire.”

This highly concentrated aqueous zinc battery also overcomes other disadvantages of conventional zinc batteries, the team continues, such as the capacity to endure only limited recharging cycles, dendrite (tree-like structures of crystals) growth during usage and recharging, and sustained water consumption, resulting in the need to regularly replenishing the batteries' electrolyte with water.

"Existing zinc batteries are safe and relatively inexpensive to produce, but they aren't perfect,” adds professor Chunsheng Wang of UMD. “This is due to poor cycle life and low energy density. We overcome these challenges by using a water-in-salt electrolyte.”

The research team believes that this battery technology advance lays the groundwork for further research and they are hopeful for possible future commercialisation.

"The significant discovery made in this work has touched the core problem of aqueous zinc batteries and could impact other aqueous or non-aqueous multivalence cation chemistries that face similar challenges, such as magnesium and aluminum batteries", Xu says. "A much more difficult challenge is, of course, the reversibility of lithium metal, which faces similar but much more difficult challenges."

Xu believes resolution of lithium-metal deposition could unlock the ‘Holy Grail’ of all batteries, which is the area where these scientists are closely working on with the scientists at the Department of Energy.