Back in 2015, a team of scientists made a battery breakthrough by using salty water as an electrolyte to offer a potentially safer and greener alternative to commercial lithium-ion batteries, but its voltage left something to be desired. The same team has now powered up its design to a point where it could be used in household appliances, without the risk of fire and explosion that can accompany conventional alternatives.

"In the past, if you wanted high energy, you would choose a non-aqueous lithium-ion battery, but you would have to compromise on safety," says study co-senior author Dr. Kang Xu, a research fellow at the US Army Research Laboratory. "If you preferred safety, you could use an aqueous battery such as nickel/metal hydride, but you would have to settle for lower energy. Now, we are showing that you can simultaneously have access to both high energy and high safety."

Key to the team's design, and many of today's batteries for that matter, is a protective layer called a solid-electrolyte interphase. This forms around the battery's anode when the energy-carrying electrolytes are broken down during the first charge, guarding it from chemical reactions that can cause fire, smoke and explosions and allowing it to operate at higher voltages.

It had been thought that this interphase was unable to form in aqueous batteries with water-based electrolytes, but in 2015 scientists from the University of Maryland and the US Army Research Laboratory did just that hitting just the right mix of salt and water. A high concentration of salt compared to water, six-to-one to be exact, allowed the interphase to form and boost the battery's maximum voltage from 1.23 V to 3 V.

Now the same researchers have raised the bar even further, by developing a new gel polymer electrolyte that coats the anode and better repels water from its surface, allowing it to hit a maximum voltage of 4 V.

"The key innovation here is making the right gel that can block water contact with the anode so that the water doesn't decompose and can also form the right interphase to support high battery performance," says co-senior author Chunsheng Wang, Professor of Chemical & Biomolecular Engineering at the University of Maryland.

While the work so far is promising, the team says further research is needed to explore how well the technology can be scaled up. It notes several aspects of the design that could be improved upon, including reducing material expenses and increasing the number of full-performance cycles it can complete.

"Right now, we are talking about 50–100 cycles, but to compare with organic electrolyte batteries, we want to get to 500 or more," Wang says.

If all goes to plan, the team estimates the technology could be ready for commercialization in around five years. The research is published in the journal Joule.

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